Transform domain inter-block interleaving schemes for robust image and video transmission in ATM networks

J. Vis. Commun. Image R. 15 (2004) 522–547 www.elsevier.com/locate/jvci

Transform domain inter-block interleaving schemes for robust image and video transmission in ATM networksq Rogelio Hasimoto-Beltr
an,a Shahab Baqai,b and Ashfaq Khokharc,* a

c

Center for Research in Mathematics, Guanajuato, Gto., Mexico b Lahore University of Management Sciences, Pakistan University of Illinois at Chicago, Departments of CS and ECE, 851 South Morgan Street, Chicago, IL 60607, USA Received 10 December 2001; accepted 26 November 2003 Available online 27 February 2004

Abstract Data interleaving schemes have proven to be an important mechanism in reducing the impact of correlated network errors on image/video transmission. Current interleaving schemes fall into two main categories: (a) schemes that interleave pixel intensity values and (b) schemes that interleave JPEG/MPEG transform blocks. The schemes in the first category suffer in terms of lower compression ratio since highly correlated information in the spatial domain is de-correlated prior to compression. The schemes in the second category interleave DCT transformed blocks. In this case, in the absence of ARQ, if a packet is lost, an entire block may be lost thus yielding poor image quality and making the error concealment task difficult. Interleaving transform coefficients is tricky and error concealment in the presence of lost coefficients is challenging. In this paper, we develop three different interleaving schemes, namely Triangular, Quadrant, and Coefficient, that interleave frequency domain transform coefficients. The transform coefficients within each block are divided into small groups and groups are interleaved with the groups from other blocks in the image, hence they are referred to as inter-block interleaving schemes. The proposed schemes differ in terms of group size. In the Triangular interleaving scheme AC coefficients in each block are divided into two triangles and interleaving

q

This work was supported in part by NSF Grant CCR–0196365 and the Mexican Council of Science and Technology under Grant 42523. * Corresponding author. Fax: 1-312-996-6465. E-mail address: [email protected] (A. Khokhar). 1047-3203/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jvcir.2003.11.001

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is performed among triangles from different blocks. In the Quadrant interleaving scheme, coefficients in each block are divided into four quadrants and quadrants are interleaved. In the Coefficient interleaving scheme, each coefficient in a block is a group and it is interleaved with the coefficients in other blocks. The compression ratio 3 of the proposed interleaving schemes is impressive ranging from 90 to 98% of the JPEG standard compression while providing much higher robustness in the presence of correlated losses. We also propose two new variable end-of-block (VEOB) techniques, one based on the number of AC coefficients per block (VAC-EOB) and the other based on the number of bits per block (VB–EOB). Our proposed interleaving techniques combined with VEOB schemes yield significantly better compression ratios compared to JPEG (2–11%) and MPEG-2 (3–6.7%) standards while at the same time improve the resilience of the coded data in the presence of transmission errors. Ó 2003 Elsevier Inc. All rights reserved.

1. Introduction Advances in computer technology such as computer architecture, networks, and data compression have transformed the nature of the data transmitted in current computer networks. It is rich in multimedia content (image, audio, video, graphics, etc.) compared to text dominant data a few years ago. An important characteristic of multimedia content communication (in particular image/video communication) is that it requires quality of service in terms of low latency, better PSNR, and low jitter, under the pretext of high bandwidth bursty traffic, that is, too much information is being generated in a small period of time. For the Internet, which is based on besteffort datagram delivery, no bandwidth is reserved for specific connections. The capacity may be exceeded and queues within the network may grow until eventually they are full. Under such a scenario, the network starts dropping packets. Once a packet has been lost (because of the buffer overflow), it is likely that the next packet will also be lost, especially if the inter-arrival time is less than the service time (Bolot, 1993). Therefore, packet losses are correlated and might occur in bursts. The side effect of correlated losses is the decrease in the effectiveness of Forward Error Control (FEC) and Error Concealment (EC) schemes (Cidon et al., 1993). Similar behavior of packet losses is also seen in ATM networks (Lee et al., 1995). Another important factor to be considered in compressed image/video data is that packet losses and/or bit errors can easily propagate to contiguous areas due to the loss of synchronization in the bit stream (up to a sequence marker or start code). Data loss along with error propagation have severe consequences in the visual quality of the transmitted information (Bolot, 1993). For correlated losses, several approaches have been proposed to ameliorate its effect in multimedia communication (DeBrunner et al., 1999; Kinoshita et al., 1993; Tom et al., 1991; Turner and Peterson, 1992), among them is the Spatial Data interleaving (interleaving data within the same frame or picture). This scheme has been widely used to increase the robustness of image and video communications in lossy environments by spreading the potential effect of network errors over the whole image. User controlled interleaving can be performed at different resolution levels: Packet (Chin and Glynn, 2000; Liang et al., 2002), Slice (Varsa and

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Karczewicz, 2000), Block (Posnak et al., 1995), and Inter-Block (DeBrunner et al., 1999; Tom et al., 1991; Turner and Peterson, 1992). The Inter-Block Interleaving (IBI) can be performed in the spatial domain (DeBrunner et al., 1999; Turner and Peterson, 1992) or in the frequency domain (Zhu et al., 1993; Turner and Peterson, 1992), with a negative side effect on compression ratio (CR). The main advantage of interleaving in the spatial domain is that in general it requires much simpler EC schemes to recover from losses. The IBI in the spatial domain is accomplished by grouping one or several pixels inside a block and packetizing them in such a way that they are sent far apart from other groups in the same block. The IBI in the frequency domain is tricky and error concealment in the presence of lost coefficients is challenging. In this paper we introduce three novel frequency domain IBI schemes, along with a new variable end-of-block technique based on the number of AC coefficients per block (VAC-EOB) and study their performance in the presence of network losses. Contrary to the existing interleaving schemes, our interleaving schemes combined with VEOB yield significantly better compression ratios when compared to JPEG and MPEG-2 standards. At the same time the proposed schemes improve the resilience of the coded data to network errors. The problem of error propagation due to a loss in synchronization has been addressed by introducing Synchronization Markers (SM) (ISO/IEC IS, xxxx; ISO/ IEC JTC 1/SC 29/WG, 1995) and Fixed-Length Code words (FLC) (Llados, 1998; Redmill and Kingsbury, 1996). The former involves the addition of redundant information (
28 bits plus an 8-bit vertical position information in MPEG-2) placed at some points in the image/video layer structure, while the latter requires additional complexity over the standard schemes and often results in lower compression efficiency (Redmill and Kingsbury, 1996). Another method that has been proposed to alleviate the synchronization problem is Reversible Variable-Length Code (RVLC) along with SM (Wen and Villase~ nor, 1999). The advantage of RVLC over standard VLCs is that the bit stream can be decoded in the backward direction as well. An alternative scheme called Error Resilient Entropy Code (EREC) has been proposed in Redmill and Kingsbury (1996), which protects encoded data against synchronization errors at a block level (that is in the case of errors the decoder can find the start of the next block). In addition to providing good resilience to channel errors, it has the advantage that compression ratio is unaffected. For error synchronization in MPEG-2 frames we propose, as an extension to our VAC-EOB technique, a variable end of block based on the number of bits per unit of block (VB-EOB), which is scalable in terms of providing protection according to the network error conditions during transmission. Our scheme is compression dependent, and it works better (in terms of gain in CR) for high to medium compression rates (low to good/very good picture quality). The rest of the paper is organized as follows. In Section 2, we review the recent related work. In Section 3, we present the proposed interleaving schemes. Section 4 analyzes the effect of proposed interleaving schemes on compression ratio, their performance under network errors, and presents the error concealment results. In Section 5 we describe the effect of VEOB on MPEG-2 standard and propose a scalable error propagation protection using a modified VEOB based on the number of bits per block. Conclusions and future work are presented in Section 6.

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2. Related work There are only a handful of schemes in the literature that deal with interleaving in the spatial domain. This may be related to the fact that any scheme based on interleaving spatial data in terms of pixel intensities is likely to be inefficient in terms of compression since it de-correlates spatial information. However, one important advantage of interleaving in the spatial domain is that in case of losses, error concealment is easier compared to error concealment in the transform domain. In the following we review only a few representative interleaving schemes developed for spatial and frequency domains. Turner and Peterson (1992) proposed a pixel-level interleaving scheme in the spatial domain, where pixels within a packet are taken according to an offset, denoted by ByteOffset that specifies the distance between two pixels in the same packet. This ensures that no two bytes within the same packet are adjacent in the image space. To cater for bursty packet losses, PacketOffset specifies the number of bytes between starting pixels of adjacent packets. This parameter controls how far apart temporally adjacent packets are in image space. Fig. 1, shows the construction of a packet and packet offset using ByteOffset and PacketOffset as (RowLength +2) and PacketLength, respectively. RowLength represents the width of the image in pixels and PacketLength is the packet size in bytes. In the example a pixel is assumed to be one byte for simplicity. In this scheme since the interleaving step is performed in the spatial domain, it destroys spatial correlation yielding very poor compression ratios. Tom et al. (1991), on the other hand, proposed a two-step scheme for the interleaving at the pixel level. In the first step, input image is sub-sampled by a factor of 2, and divided into 4 sub-images, which are then transformed into frequency domain using DCT and quantized following the JPEG standard specifications except for the variable length coding. In the second step (packetization), AC coefficients with the same index are pseudo-randomly selected from different blocks within each sub-image to avoid correlated losses. As in TurnerÕs work, this technique also yields poor compression ratios. Posnak et al. (1995) proposed a block-based interleaving scheme in the frequency domain for layered JPEG video stream over ATM. In this scheme, assuming DCT has been performed in all the blocks, DC coefficients from all the blocks are grouped as essential layer and encoded. The AC coefficients from all the blocks are put in the enhancement layer. The essential layer is packetized and transmitted with QoS guarantees to avoid packet losses. The enhancement layer, however, is transmitted at a lower priority using best-effort service and is subject to random as well as bursty packet losses that may occur due to network congestion or buffer overflow. In order to minimize the effects of bursty losses adjacent blocks in the enhancement layer are vertically and horizontally de-correlated according to a scalar parameter, referred to as STEP, which determines the distance of the next block to be packetized. If (xi ; yi ) denotes the most recent block of AC coefficients included in the transmission sequence for an image containing NCOL  NROW blocks then the next block, represented by (xiþ1 ; yiþ1 ), in the sequence is given by:

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Fig. 1. Inter-block interleaving in the spatial domain (Turner and Peterson, 1992).

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xiþ1 ¼ jxi þ STEPjNCOL ; yiþ1 ¼

   xi þ STEP yi þ : NCOL

The advantage of a block-based interleaving scheme is that it has very low compression penalty assuming standard JPEG as the reference point. However, in the case of network errors, if a packet is lost then the entire block is lost. There is no

Fig. 2. Interleaving scheme in the frequency domain (Zhu et al., 1993).

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simple way to conceal the errors resulting due to a lost block, especially if it contains high frequency contents. Zhu et al. (1993) incorporated a layered interleaving scheme to conceal errors from packet losses in JPEG/MPEG compressed image/video data. In this scheme contiguous blocks are indexed in an even/odd indexing order (see Fig. 2). Within each block DCT coefficients are segmented into four layers (L0 , L1 , L2 , and L3 ). Each layer is packetized separately and the packets are first filled with even indexed blocks followed by odd-indexed blocks in the layer. If an even-indexed packet is lost, their adjacent odd-indexed blocks are likely available, i.e., entire blocks are hardly lost. The missing regions are adaptively interpolated in the spatial and frequency domain. The problem with this scheme is that it needs a different coding table per layer, breaking the rules of standard image and video coders. Another and even more important observation is that this scheme does not consider bursty packet losses.

3. Proposed inter-block interleaving schemes While most of the existing block-based interleaving schemes do a good job of decorrelating block as well packet information; however, once a packet is lost then at the very least the AC information of the block is also lost. Conversely, inter-block interleaving (IBI) schemes dealing with pixels in the spatial domain implicitly succeed in protecting some of the AC information. However, this is at the expense of low compression ratio. Also, most of the interleaving schemes proposed in the literature do not make any distinction among the information contents. If the lost information contains low frequency components, such as smooth surfaces, it is easier to reconstruct using neighboring blocks. On the other hand, high frequency information, such as small edges or unique features not found in the neighboring blocks, can hardly be reconstructed by the EC techniques. When these unique and distinctive features are part of the main object in an image (such as eyes, mouth in a videophone) the effect of their loss is pronounced. In order to preserve some of the unrecoverable features of the transmitted information at the receiver end, and to provide supporting environment for low as well as high frequency component reconstruction, we develop several inter-block interleaving schemes in this section. Our interleaving schemes work in the frequency (transform) domain. The motivation for the proposed schemes is drawn from the spatial distribution of DCT coefficients shown in Fig. 3. We group coefficients in a way that in case of a packet loss, partial frequency domain information is still available and thus helpful in error concealment. In these schemes, information in each block (low and high frequency components) is divided into n disjoint components, which are then exchanged among different blocks. Note that n ¼ 1 corresponds to block-level interleaving and n ¼ 63 corresponds to single coefficient level interleaving. After interleaving, the newly created blocks are organized in such a way that highly correlated blocks do not share the same packet nor l consecutive number of packets (block de-correlation). The value of l is the expected size (in number of packets) of the bursty loss and depends on the packet loss behavior of the network (the heavier the congestion,

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Fig. 3. The spatial distribution of frequency DCT coefficients in an 8  8 block image as suggested in (Liang et al., 2002). Coefficients labeled as: ‘‘0’’ correspond to low frequency, ‘‘1’’ correspond to horizontal frequency, ‘‘2’’ correspond to vertical frequency, ‘‘3’’ correspond to diagonal texture, and ‘‘4’’ correspond to high frequency information in an image block.

the bigger the number l). Furthermore, we do not interleave DC coefficients. They are coded according to the JPEG standard (DPCM and VLC) and transmitted independent of the AC information. An important characteristic to point out here is that the proposed interleaving schemes can be applied to any block-based image and video (intra-frames only) coders such as JPEG, MPEG-x, and H.26x standards. Interleaving is an additional step of the block based compression scheme (after quantization). In the following, we first describe three new coefficient-based interleaving schemes, namely Triangular, Quadrant, and Coefficient representing n ¼ 2, 4, and 63, respectively. 3.1. Triangular interleaving scheme The Triangular Interleaving scheme (TRII) scheme decomposes an 8  8 block of DCT coefficients into three components, DC term, lower triangle, and upper triangle, as shown in Fig. 4. The coefficients are divided in such a way that both low and high frequency coefficients are available in both the triangular components. Furthermore, if any triangular component is damaged or lost, it does provide some pointers as to which neighbors (horizontal/vertical) are likely to be candidates in the reconstruction process. Different steps in the TRII scheme are described in the following: (a) An 8  8 block (after the application of DCT and quantization) is diagonally divided in order to obtain two triangular or complementary components. We refer

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Fig. 4. Triangular-based interleaving (TRII) scheme.

to these as upper (U) and lower (L) components. The coefficients on the main diagonal are added to the U component, except for the DC term, which is treated as an independent component. The transmission of the DC components is assumed lossless (which is achievable in both ATM and IP networks). ATM networks have the advantage that priority levels can be assigned to different parts of the transmitted information. The highest priority is given to the DC component, and U and L components are given low priority. In the case of IP based networks, the transmission of DC component can make use of FEC and/or ARQ techniques. (b) A new set of blocks is created by bringing together the L component of the Xi;j block and the U component of the Xh;k block for all j 6¼ k and/or i 6¼ h, and 1 6 i; h 6 M, 1 6 j; k 6 N , where M and N represent the number of blocks in a row and a column, respectively. The new blocks are grouped and are referred to as Layer L1 . Next, we combine the U component of the Xi;j block and the L component of the Xh;k block for j 6¼ k and/or i 6¼ h. This second set of new blocks is referred to as L2 . All the DC component are grouped together and

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are referred to as L0 . Fig. 4 shows an example TRII interleaving for h ¼ i þ 1 and k ¼ j þ 1. The U and L components can be combined using (ji  hj, jj  kj P 2) for better error dispersion (at the expense of reducing compression efficiency), or using different scanning order such as Spiral, Hilbert, Morton, etc. (c) New blocks are zig–zag scanned and entropy encoded. (d) The encoded information is packetized to ensure that no two triangular components belonging to the same block or same type of triangular components (L or U) of contiguous blocks are present in the same packet. Note that in the case of bursty packet losses ( 650%), if U (L) is lost then L (U) is not lost. Furthermore, when L (or U) is lost there are at least four neighbors with error free L (or U) components. This information can be effectively used by an error concealment algorithm. It is important to point out that U and L components belonging to the same block bi are approximately separated by Tc =2 packets, where Tc is the total number of packets to be transmitted. For a block to lose both U and L components, the packet containing U and the packets containing L both need to be lost. If we consider for instance uniform random loss, then the probability to lose one block in the triangu2 lar scheme is 4=ðTc Þ compared to 1=Tc for the traditional block-level interleaving scheme. 3.2. Quadrant interleaving scheme The Quadrant interleaving (QI) increases the number of interleaved components in a block to 4 instead of 2 as in TRII. This scheme is described as follows: (a) Every block after the application of DCT and quantization is divided into four sub-blocks, namely L, HD, VD, and HI, representing different spatial frequency information present in the block. The L component represents the low frequency information in the vertical and horizontal directions, HD represents medium to high Horizontal–Diagonal frequency information, VD represents Vertical–Diagonal frequency information, and HI represents High frequency information. As shown in Fig. 5A. (b) Four spatially contiguous blocks are grouped together to form a 16  16 size macro-block, and are numbered from 0 to 3 in clockwise direction (see Fig. 5B). A new macro-block is formed by allowing each block to exchange 3 of its sub-blocks (HD, HI, and UD) with its neighboring blocks within the same macro-block. The ith block in the new macro-block is formed by taking the Li , HDiþ1 , HIiþ2 , and UDiþ3 components, where the sub-index value i þ n is computed in module 4, for 0 6 n 6 3. For example, block 0 is formed by the L0 , HD1 , VD2 , and HI3 , block 1 is formed by L1 , HD2 , VD3 , and HI0 , and so on. This process is continued for all macro-blocks until the end of data is reached following the row-prime order scanning. (c) New blocks are zig–zag scanned and VLC encoded. (d) The packetization is performed by putting together all the blocks in the first quadrant of each new macro-block (as numbered in step (b)), followed by all

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Fig. 5. Quadrant-based interleaving scheme (QUAD). (A) Represents block subdivision and (B) macroblock subdivision and interleaving.

the blocks in the second, third, and fourth quadrants. By using this format, subblocks belonging to the same original block are separated by a distance of Tc =4 packets. 3.3. Coefficient interleaving scheme Before the interleaving process AC coefficients in each block are scanned in the zig–zag order and are numbered 1–63 based on the scanning order. The main idea in the Coefficient interleaving (CI) is to consider a group of k blocks, where k is the number of AC components in a block (in an 8  8 block, the group is made of 63 blocks) and perform a k-way shuffle over all the AC coefficients in the group by taking the first element from block 1 (first AC coefficient visited during the zig–zag scanning), second element from block 2 (second AC coefficient in the zig– zag order), and so on until we reach the kth block (see Fig. 6 for a 4-way shuffle). At this point we have created the first new block. We now take the second element from block 1, and continue in order until we reach the kth block again. Note that after the kth element has been taken from a block Bm for 1 6 m < k, the first AC

Fig. 6. Four-way perfect shuffle for coefficient-based interleaving scheme (CI).

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Fig. 7. Packetization scheme for CI after k-way shuffling.

coefficient of the next block is taken next. The above procedure is executed until all sub-groups of size k in the image have been shuffled. It is important to note that in order to avoid penalties in the compression ratio (CR), the zig–zag position of an AC coefficient in its original block is kept the same in the new block. After this procedure, the new blocks are Huffman encoded and packetized. This technique totally de-correlates the elements within the blocks, and each new block formed after interleaving has only one previously owned element. In order to de-correlate blocks within a group, we use a variable block_offset during the packetization process (Fig. 7). All the blocks in the first position of a group are packetized first, followed by all blocks that are block_offset apart of the previously taken block, and so on until the end of the k-group is reached. We continue in the same way, but now the starting point is block number 2, then 3; 4; . . ., (block_offset-1). One way to approximate the value of block_offset is by computing the average position of the last non-zero DCT-coefficient among all blocks and add a confident interval such that their sum is less than the maximum position of the last non-zero coefficient. For instance, we found that under the compression ratio of 20 Barbara keeps an average number of coefficients of 2 per block, and a maximum non-zero coefficient of 13, so a good block_offset might be equal to 7.

4. Experimental results for JPEG data In this section the proposed interleaving schemes, TRII, QI, and CI are analyzed according to their compression efficiency using the constant End-of-Block (CEOB) as prescribed in the JPEG standard. We further augment the proposed interleaving schemes by replacing the CEOB by a novel Variable EOB (VEOB) based on the number of AC coefficients per block. This VEOB is aimed at increasing the compression efficiency of the previously described interleaving schemes. The resilience of the interleaving schemes is also analyzed. Note that in all our experiments involving the transmission of data we follow the ATM packetization format, which

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consist of a header of 5 bytes and a payload of 48 bytes (called a cell in the ATM nomenclature). 4.1. Compression performance Five images with different spectral characteristics were used for the analysis including Lenna, Barbara, Boats, Bridge, and Sussie, assuming 0.5, 0.45, 0.4, 0.35, 0.3, 0.25 bits per pixel (bpp) representation. The quantization matrices were computed as given in Nelson (1992): Qði; jÞ ¼ 1 þ ½ð1 þ i þ jÞ  Quality; where Quality specifies the quality factor (1 6 Quality 6 25), larger the value of Quality, higher the compression ratio. Table 1 shows the average bit rate increase in percentage with respect to the baseline-JPEG, using CEOB. Considering standard JPEG as reference point, TRII and QI have very low effect on the final compression ratio, with an average performance of 1.2% and 0.88% below JPEG, respectively. The effect of CI on the compression ratio is higher than TRII and QI (as expected, since we are creating one block out of 64 uncorrelated blocks), with an average performance of 5.7%. In general, the effect of coefficient interleaving on the compression ratio decreases as the value of bpp is reduced. For lower bpp, QI achieved the best CR values, because it does not interleave low frequency components as in the case of TRII and PI, and thus behaves like a block-based interleaving scheme. Based on the overall average of Table 1, our interleaving schemes in the transform domain performed 6–13 times better than the scheme reported in Turner and Peterson (1992). Tom et al. (1991) does not report numerical values of CR performance, but based on the facts that their interleaving schemes involves a sub-sampling in the spatial domain (increasing the block high frequency content), scrambling in the frequency domain, and skipping the entropy coding process, it is fair to assume that Tom et al. (1991) would have much lower CR values with respect to JPEG.

Table 1 Minimum, maximum, and average bit rate increase with respect to the baseline JPEG Bpp

0.5 0.45 0.4 0.35 0.3 0.25

TRII (%)

QI (%)

CI (%)

(min,max)

Avg.

(min,max)

Avg.

(min,max)

Avg.

(0.6,2.0) (0.5,2.1) (0.5,2.1) (0.3,2.0) (0.4,1.9) (0.3,1.9)

1.3 1.2 1.3 1.1 1.1 1.1

(1.0,1.9) (0.8,1.8) (0.8,1.6) (0.4,1.5) (0.4,1.4) (0.2,1.1)

1.3 1.2 1.0 0.8 0.7 0.6

(3.1,11.0) (2.8,10.0) (2.5,9.5) (1.8,8.1) (1.5,6.8) (1.1,4.4)

7.9 7.3 6.7 5.1 4.3 3.1

Simulations were run using Lenna, Barbara, Boat, Bridge, and Sussie images.

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4.2. Variable end-of-block based on AC coefficients In the JPEG standard, a fixed size 4-bit EOB marker is placed at the end of each block regardless of the number of bits inside the block. This contributes a significant overhead in the overall compressed bit stream. While in an encoded image block, the number of AC coefficients per block is relatively small, we have noticed this number is pretty repetitive. For example, in the case of Lenna at 0.5 bpp, there are 20 different AC frequencies (with an average of 3.5 coefficients per bock), where 40% of the blocks contain zero or one AC coefficients and 60% of the blocks contain between two and 19 coefficients. Based on this information, we encode the number of AC coefficients in each block using Huffman Table and use this code as the EOB marker. We refer to this EOB as variable end-of-block based on AC coefficients (VAC-EOB). The effectiveness of VAC-EOB is first analyzed for JPEG compressed images without interleaving (Table 2), and then using the proposed interleaving schemes (Table 3). In the first case, we found that the number of AC coefficients among contiguous blocks is correlated, so we DPCM and entropy coded this value using a Huffman Table based on the image data. The results in Table 2 (second column) show that VAC-EOB achieved much better performance than JPEG for all values of bpp, with a CR gain fluctuating between 2 and 11.1% above JPEG with a total average gain of 5.5%. The same experiment was performed using the Huffman tables specified in the JPEG standard for coding the DC coefficients. The purpose was to evaluate the feasibility of using this predefined Huffman table instead of creating new ones for each image. Our results show that, even though there is always a gain Table 2 Average performance of VAC-EOB coded using the JPEG specified table and the image dependent Huffman table Bpp

Image dependent table (%)

JPEG table (%)

0.5 0.45 0.4 0.35 0.3 0.25

)2.0 )2.7 )3.8 )5.5 )7.6 )11.1

)0.3 )0.8 )1.2 )2.0 )3.0 )5.0

Negative values represent a gain in CR with respect to baseline JPEG. Table 3 Average performance of the proposed IBI schemes with variable end-of-block Bpp

ITRII (%)

QI (%)

CI (%)

0.5 0.45 0.4 0.35 0.3 0.25

)3.6 )4.2 )5.0 )6.7 )8.0 )10.4

)0.5 )1.6 )2.7 )4.4 )6.7 )10.5

5.5 4.1 2.4 )0.4 )3.0 )7.4

Negative numbers represent a gain in CR with respect to the JPEG.

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in CR with respect to JPEG (right column in Table 2), the creation of an image dependent table outperforms the efficiency of the JPEG specified table by 2.2–6.6 times, with an average of 3.5. Table 3 shows the CR of the proposed interleaving schemes using VAC-EOB and image dependent tables with respect to the baseline JPEG (CEOB). Since interleaving destroys correlation among neighbors, the best results were achieved without using prediction. Negative numbers mean better performance (a gain in CR), while positive numbers represent a decrement in CR for the interleaving schemes with respect to the baseline JPEG. Interleaving with VAC-EOB coding, achieves better performance than interleaving with CEOB (for all CRs and all images) and baseline JPEG (except for CI at 0.5–0.4 bpp). Since the main reason for interleaving is to protect information against correlated losses, we notice that in the case of TRII the best compression ratios were obtained by creating new interleaved blocks out of two consecutive blocks. In order to increase the de-correlation among consecutive blocks (and avoid contiguous partially damaged blocks) we encoded each triangular region independently (we refer to this as ITC—Independent Triangular Coding), so that they could be sent far apart from each other and only one half-block in every neighborhood might be damaged. The problem is that in order to code each triangular region independently an additional EOB is needed per block. In order to avoid confusions in our nomenclature, the combination of ITC with VEOB will be called Variable Independent Triangular Coding (V-ITC), and the combination of V-ITC with interleaving will be called Independent Triangular Interleaving (ITRII). As shown in Table 3, ITRII achieved superior CR for all values of bpp compared to TRII (Table 1) as well as standard JPEG. The reason is that zig–zag scanning is performed independently on each triangular region (rather than in a square block), which reduces the distance between two non-zero AC coefficients as well as the resulting encoded number of bits. Tables 1–3 do not consider the extra overhead needed for the transmission of the Huffman table along the encoded data. It was found that this extra overhead is less than 0.1% at 0.5 bpp, and this value decreases as the bpp value is reduced. Therefore, the impact of adding this overhead on the CR values is insignificant. 4.3. Performance under network errors: resilience to bursty packet loss In the previous section, we showed that the proposed frequency domain IBI schemes have relatively low impact on the compression ratio with respect to JPEG. When these schemes are combined with VAC-EOB the CR is improved considerably, having a gain for all compression ratios (except in the case of CI when bpp is between 0.4 and 0.5). In this section we present the performance results of the proposed interleaving schemes in the presence of bursty packet losses. During the packetization process, the encoded bit stream is split into 2 components or layers, the DC and VAC-EOB (with its corresponding Huffman table), and the AC information. These components are sent independently, assuming lossless transmission for the DC components and lossy transmission for the AC compo-

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nents. In ATM networks, important information (such as DC components) is marked as high priority data. Figs. 8 and 9 compare the performance (in terms of visual quality) of the proposed interleaving schemes with the block based interleaving scheme of Posnak et al. (1995), assuming 25% bursty loss. For comparison purposes, Fig. 8 shows the results when the DC coefficients are also lost. Fig. 9 shows the results when the loss is encountered only during the transmission of AC coefficients. Comparing PSNR would not help in this case as it would be practically the same for all interleaving schemes. This is because we are eliminating the same amount of information in all cases, and the only difference is how data loss is distributed over the whole image. However, visual quality may be different particularly over the regions with high frequency contents. The reason is that the proposed IBI schemes spread out the errors over larger regions of the image, so that each block maintains some of its original AC information (see Fig. 10). In particular, 1 lost block in the block interleaving corresponds to 2, 4, and 64 partially damaged blocks in ITRII, QI, and CI schemes, respectively. This characteristic is very important from the error concealment point of view, since it is difficult to reconstruct high frequency components.

Fig. 8. Effect of 25% consecutive losses in the presence of: (A) block-based ([Posnak et al., 1995), (B) QI, (C) ITRII, and (D) CI interleaving schemes, assuming DC coefficient in each block is also lost.

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Fig. 9. Effect of 25% consecutive losses over (A) block-based, (B) QI, (C) TRII, and (D) CI interleaving schemes, assuming DC coefficient in each block is present.

4.4. Error concealment for partially damaged blocks The error concealment (EC) problem in the case of proposed interleaving schemes amounts to reconstruction of information corresponding to a few frequency components. Also, we do have some information as to which frequency components need reconstruction and where to look for them. For example, the EC problem for ITRII (EC-ITRII) is the reconstruction of either horizontal–diagonal (U) or vertical (L) edges, and for QI (EC-QI) it is the reconstruction of one of its 4  4 pixels square region. In the case of CI (EC-CI), the number of missing coefficients is variable, with a minimum of 1. An important point is that in all cases, the lost frequencies in a specific block are present in the neighboring blocks, which is valuable information for the EC process. The error concealment algorithms used to reconstruct the lost information are very simple. In order to keep the focus of this paper on interleaving we are skipping the details of error concealment techniques. In general, for ITRII and QI, the missing information was replaced by considering the partial information available in the damaged block and interpolating the missing information from the neighboring blocks. In the case of CI, the EC scheme is just a simple de-blocking operation around the damaged block. Our intent here is to show that with little extra post-processing on partially damaged blocks we can significantly improve the quality

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Fig. 10. Zoom of the top right portion of (A)–(D): from top left (clock-wise), block based (Posnak et al., 1995), ITRII, QI, and CI.

of the received image/video information (for more EC schemes in the frequency domain see Ancis et al., 2000; Chung et al., 1998; Zhu et al., 1998). Fig. 11 (left column) shows the images with the damaged blocks corresponding to QI, TRII, and CI interleaving schemes, and their corresponding reconstructed images (right column). The reconstructed images have high visual quality since we could use partial information from within the damaged blocks to conceal errors.

5. MPEG-2 video transmission using IBI and error propagation control In the previous section, we have shown the effectiveness of the proposed IBI schemes and VAC-EOB for improving the quality and compression performance of transmitted JPEG images. We will now apply these concepts to SNR (Signalto-Noise Ratio) scalable MPEG-2 transmission over ATM. Since digital video transmission is more sensitive to processing delays than still image communication, we use only triangular interleaving in our experimental results. In addition to providing

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Fig. 11. The results of error concealment for the proposed interleaving schemes. The left column from top to bottom, shows images with damaged blocks corresponding to QI, TRII, and CI schemes, respectively. The right column presents corresponding reconstructed images.

better CR, processing overhead for Triangular interleaving is insignificant compared to QI and CI. We also present VB-EOB, a variable EOB based on the number of bits per unit of Macroblock (MB) for controlling the propagation of errors in the bit stream beyond the physically affected area (see Section 5.2). 5.1. SNR scalability SNR scalable video coding is a feature of the MPEG-2 standard that generates two layers of equal spatial resolution but at different qualities: the Base Layer (BL) and the Enhancement Layer (EL) (Rao and Hwang, 1996). The base layer contains header information and a coarse version of the DCT coefficients, while the enhancement layer is the refinement step of the base layer. High priority level (better error performance) can be given to the base layer for providing the most important contribution to the decoded quality, and low priority to the enhancement layers. In ATM networks, priority levels are used by the network protocol to decide which layer has to be dropped out in the case of congestion (thus maintaining the most important information on its way to the receiver). At the encoder, each output layer is

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independently entropy encoded and transmitted over its corresponding priority channel. At the decoder, the inverse operation is performed on both layers and the DCT coefficients are added together. One of the drawbacks of scalable coding is its relative lower compression performance with respect the one-layered encoding. There is an extra overhead in the enhancement layer at both the Macroblock (MB) header level and data level. We are particularly interested in the overhead produced by the EOB at the data level. An n-layered coding will produce n  ðnumber of blocks  EOBÞ bits, where EOB ¼ 4 bits for MPEG-2 intra coding. In our experiments we use n ¼ 2 for both low and main profiles in order to follow the specification of the scalable MPEG-2 standard. 5.2. Proposed interleaving scheme for MPEG data Enhancement layer is first interleaved using either TRII or ITRII (see Section 3). In order to provide better protection against synchronization errors (errors spreading beyond the physically affected area) produced by a cell loss or a simple bit error, we propose a scalable protection using a variable end of block based on the number of encoded bits (VB-EOB) per block, 1/2-, 1-, 2-, and 4-MB. Fig. 12 shows our VBEOB scheme at a MB level. We use VAC-EOB based on number of AC coefficients to mark the end of a block and VB-EOB to mark the end of a Macroblock. Note that

Fig. 12. Scalable VB-EOB protection at MB level. Gray rectangles at the beginning of a block represent a VAC-EOB. VB-EOBi is the variable end of block using the number of pixels in the MB, including VACEOBs.

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for blocks the variable end of block is moved at the beginning of each block, in order to simplify the decoding process. VB-EOB is computed over all bits in an MB including inner VAC-EOB (as explained in Section 5.3). We now know the number of bits in each MB, so in the case of errors we can find the start of the next MB and continue with the decoding process. The above process is performed only for intra frames; the small overhead specified in the enhancement layer can be omitted if we use a constant quantizer _scale_code. 5.3. Experimental results for MPEG data Before we present the results of the proposed SNR scalable scheme, let us first analyze the compression performance of VAC-EOB applied to both base and enhancement layers independently. The CIF frames (frame 1) from Akiyo and Flower-Garden (FG) sequences at different compression ratios were used in the analysis. Once a compression level for the base layer was fixed, we varied the quantization level of the enhancement layer in order to provide good to very good subjective image quality (end quality considering both the BL and EL), and calculated the average compression gain at each level. In computing the average compression gain, we used the best result between VAC-EOB and VAC-EOB with Independent Triangular Coding (V-ITC) in both layers. We also varied the base layer quality from low (highest compression level allowed by MPEG-2) to good subjective image quality,
0.55 bpp for Akiyo and
0.80 bpp for Flower-Garden, and varying again the enhancement layer quality in order to have a end quality between bpp range as shown in Table 4a and b. As in the case of JPEG images (Table 3), the performance of variable end of block (VAC-EOB or V-ITC) is better at high compression ratios. The average gain in compression ratio over the MPEG-2 standard fluctuates between 3.6 and 6.7% for Akiyo (Table 4a) and 3.0–3.5% for Flower-Garden (Table 4b). The highest values were obtained by setting the base layer at the highest compression level allowed by MPEG-2 and varying the end image quality (base + enhancement layer) from good to very good. In general, the compression gain in the case of FG video is relatively lower than Akiyo; this is because FG does not yield high compression due to its high frequency content. The use of VAC-EOB or V-ITC improves compression, but it is not robust to transmission errors. For this, we provide a scalable bit-protection against synchronization error propagation, as shown in Fig. 12. The base layer is left unmodified (same as in the previous analysis), and the enhancement layer can receive scalable protection at any of the following levels: block, 1/2-, 1-, 2-, and 4-MB level. Table 5a and b show that at a given protection level (block, 2- or 4-MB), the average CR gain is proportional to the CR in the base layer. On the other hand, for a fixed CR in the base layer, the average CR gain increases as the protection level decreases. In general, as compression ratio decreases, so does the protection level; this is to avoid penalties in the compression performance. In protecting the enhancement layer against propagation errors due to a synchronization loss in the bit-stream, we are creating additional information such as Huffman tables and end of blocks codes VAC-EOB and VB-EOB (see Fig. 12), which are

Bpp

Base layer AC + DC (bits)

Enhancement layer V-EOB gain (%)

Avg. AC coeff. (bits)

SNR performance V-EOB Avg. gain (%)

Bpp range (min–max)

Avg. bpp increment

Avg. CR gain

a. Simulation was run for frame 1 of Akiyo 0.2616 3625 14.7 0.3089 8412 9.6 0.4062 18,281 5.6 0.5073 28,532 4.3

46,623 37,524 21,584 28,470

3.0 3.7 3.1 3.6

(0.58–1.00) (0.56–1.00) (0.61–1.01) (0.70–1.02)

0.5219 0.4298 0.3700 0.3400

6.7 6.0 4.1 3.6

b. Simulation was run for frame 1 of FG 0.4977 25,259 4.0 0.5922 34,737 3.2 0.7089 46,568 1.7 0.8087 56,685 1.3

38,461 34,720 28,270 29,461

3.5 4.7 11.0 10.5

(0.76–1.11) (0.84–1.13) (0.98–1.20) (1.03–1.28)

0.4406 0.4038 0.3436 0.3520

3.3 3.1 3.5 3.0

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Table 4 Average compression gain for VAC-EOB with respect to the SNR scalable MPEG-2 standard

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Table 5 Average CR gain and bpp increment at block, 2- and 4-MB protection levels. . b: Same as Table 5a except that simulation was run for frame 1 of FG Base layer

Avg. CR gain (%), bpp increment

Bpp

Block

2-MB

4-MB

a. Simulation was run for frame 1 of Akiyo 0.2616 (1.8,0.30) 0.3089 (2.4,0.25) 0.4062 (1.3,0.26) 0.5073 (1.6,0.27)

(1.8,0.33) (1.5,0.27) (1.7,0.26) (1.5,0.22)

(3.8,0.43) (2.9,0.30) (2.6,0.27) (2.3,0.24)

Total (gain/increment)

(1.6,0.27)

(2.9,0.31)

b. Simulation was run for frame 1 of FG 0.4977 (1.5,0.24) 0.5922 (0.8,0.29) 0.7089 (1.0,0.21) 0.8087 (0.4,0.23)

(1.7,0.24) (0.8,0.25) (1.2,0.21) (0.5,0.23)

(2.5,0.27) (1.6,0.29) (1.6,0.24) (1.1,0.27)

Total gain

(1.1,0.23)

(1.7,0.27)

(1.7,0.27)

(0.9,0.24)

Bpp increment represents the average increment in quality provided by the enhancement layer for a fixed base layer quality.

very sensitive to network errors. The most important information in considering transmission errors and its effect in the decoded image quality is in the Huffman tables and the variable EOB code stream based on the number of bits per protection level (VB-EOB). This information should reach the receiver error free. There are several ways to accomplish this, the easiest one would be to include this information as part of the base layer and send it over the error free channel, causing an increase of 2–15% in the base layer data (but at the same way causing a decrease in the enhancement layer data by the same magnitude). A second solution and most plausible one is to negotiate the Minimum Cell Loss Rate (MCR) (Rosdiana et al., 2001) such that at least (under heavy traffic condition) we can transmit the Huffman tables and the EOB codes error free over the lossy channel. These two solutions keep the compression gain obtained in our proposed scheme unchanged. In our scheme, the enhancement layer is naturally byte aligned according to a specified level of protection (block, 1/2-MB,. . ., or 4-MB), therefore the use of the start codes are not useful anymore. If we consider start codes at the end of each row (MPEG-2 allows a variable slice size), we have an additional gain of 576, 1408, and 2816 bits for CIF, 4CIF, and 16CIF intra-frames, respectively. 5.4. Performance of VB-EOB under network errors We simulated the scalable transmission of Akiyo and Flower-Garden over ATM, under 1, 5, 10, 15, 20, and 25% cell loss rate (CLR) on the enhancement layer. Five simulations were performed and averaged at each error percentage. Different scal-

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able protection levels were applied; block and 4-MB for Akiyo and only 4-MB for FG and compared against the MPEG-2 standard with start code at every slice and at every 4 slices (4-slice). A quantizer scale code of 31 for the base layer and 13 for the enhancement layer were used for Akiyo to get a combined scalable quality of 0.6369 bpp. For FG we use a quantizer scale code of 29 for the base layer and 25 for the enhancement layer for a combined scalable quality of 0.8262 bpp. For Akiyo (Fig. 13) the improvement of variable end of block based on the number of bits per block in the average is 3.6 and 6.4 db with respect to MPEG-2 with start code at slice and 4-slice, respectively. With 4-MB protection, the average gain is 3.1 and 5.3 db over the MPEG-2 standard with start code at slice and 4-slice, respectively. With block protection for a constant 37 db output PSNR, our scheme withstand 3.5 times as many lost cells than the MPEG-2 with start code at every slice and 8 times when the start code is set to every 4 slices. For FG (Fig. 14) at 4-MB protection level an average improvement of 3.1 and 6.4 db with respect to MPEG2 with start code at slice and 4-slice, respectively, were obtained (this is close to the values obtained for Akiyo at block level protection). At a constant 41 db output

Fig. 13. Degradation of signal-to-noise ratio vs. channel cell loss rate (CLR) for frame 1 of Akiyo.

Fig. 14. Degradation of signal to noise ratio vs. channel cell loss rate (CLR) for frame 1 of Flower Garden.

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PSNR, our scheme withstands 2.5 times as many lost cells as in MPEG-2 with start code at every slice and 8.3 times when the start code is set to every 4 slices. Redmill and Kingsbury (1996) presented an Error Resilient Entropy Code (EREC) for block de-synchronization that reorganizes the variable-length code such that each block starts at a known position within the code. Under channel error conditions, the decoder can find the start of each block automatically. The goal of EREC and our scheme is the same, i.e., stop de-synchronization. However, EREC does it at one level only (block level), whereas our proposed VB-EOB can be applied at multiple levels (depending on the degree of bit errors in the channel), such as 1/2-Macroblocks, Macroblocks, 2-Macroblocks, 4-Macroblocks, Slice, etc. EREC is independent of the compression ratio and requires a minimal data overhead; VB-EOB is effective under high CRs in non-scalable coding and under medium to high CRs if SNR scalability is used. EREC implies significant memory requirements and delay (Redmill and Kingsbury, 1996), while VB-EOB is simple and only requires the computation of at the most two Huffman tables with average size of 70 elements.

6. Conclusions and future work We have presented three novel interleaving schemes TRII, QI, and CI in the frequency domain along with a variable end of block for robust image/video transmission over lossy networks. The proposed schemes interleave DCT coefficients at the inter-block levels in such a way that it reduces the effect of correlated losses and consequently improves the visual quality of the received data. Interleaving schemes coupled with a variable end of block provides better compression performance than JPEG and MPEG-2 standards at medium to high compression rates. We have presented experimental results for different images and network scenarios. We have shown that despite of the de-correlation of DCT coefficients the compression ratio gain of the proposed interleaving with variable end of block schemes is 10% higher than the standard JPEG and up to 6% higher than the MPEG-2 SNR scalable profile. We applied the concept of variable end of block based on the number of bits per unit of block to MPEG-2 SNR scalability for the purpose of protecting the transmitted information against propagation of synchronization errors (caused by cell loss and/or bit errors) beyond the physically affected area. Our proposed protection is scalable to block, 1/2-, 1-, 2-, and 4-MB levels, with the highest protection provided at block level. Under network error conditions from 1 to 25%, the scalable protection provided better PSNR than MPEG-2, going from 2.2 to 5.5 db when using block level protection and MPEG-2 slice start codes, and from 4.2 to 10.8 db when using block level protection and start codes every 4 slices in MPEG-2. One of the main drawbacks in our scalable error protection scheme is the creation and transmission Huffman tables and end of block codes, which end up reducing the performance of the scheme in addition to be very sensitive to network errors. One solution for a future work is the creation of static Huffman tables, which can be known by both the encoder and decoder. In this way, we increase the applicability of the scalable error

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protection to lower compression ratios in scalable and non-scalable coding. The results of error concealment algorithms are also very encouraging.

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