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Skin colour in digital images
Burns 27 (2001) 297– 298 www.elsevier.com/locate/burns
Letter to the Editor 1
Skin colour in digital images We were interested to read the article by Roa et al. [1] ‘Digital imaging in remote diagnosis of burns’. While we agree that highly compressed images still contain useful information for diagnostic purposes, we think that it was unfortunate that the authors began the Abstract with the statement ‘‘Images are capable of giving an accurate representation of skin colour…’’. The JPEG image compression method significantly alters colours in the image. As the authors state, JPEG compression involves the loss of some image information, and the greatest losses are in the colours. The strategy behind JPEG compression is to separate the spatial and colour components of the image, and compress them by different amounts. The colour is compressed more than the spatial information, so that in the resulting image features such as edges still appear distinct, but colours may be changed and may blur over their original boundaries. Human observers still find the images acceptable and as Roa et al. [1] have shown, useful. More about JPEG compression, including many example images, can be found in Russ [2] (pp. 126 – 145). For the study of the colour of burns, JPEG compression can cause problems. We took images with the same camera as Roa et al., and converted them to JPEG images using the same software and a Q index of 50. We then, extracted the hue components of the original and compressed images and compared them on a pixel by pixel basis. Compression changed the hue of 13.9% of the pixels by more than 5% and 10.4% by more than 10%. These are average figures for the whole image; the proportion of pixels changed was often greater in areas of the image such as burn scars, where there were large colour gradients. A 5% change in the hue value is readily noticeable if it occurs in a large slab of plain colour, and two
1 The Editor regrets that due to an oversight, publication of these letters was delayed.
areas differing in hue by 10% will be perceived by most observers as frankly different colours. However, when these changes are spread throughout a complex image, the human brain is remarkably good at adapting to them, as the designers of the JPEG method knew and as Roa et al. have shown. In summary, we completely agree with Roa et al. that a highly compressed image can be a valuable aid to remote diagnosis. But we would like to add a note of caution for readers who might be interested in other uses for the image, particularly if these involve detailed analysis or measurement of the image colours. In such cases, extreme JPEG compression will probably be unsuitable.
References [1] Roa L, Gomez-Cia T, Acha B, Serrano C. Digital imaging in remote diagnosis of bums. Bums 1999;25:617– 23. [2] Russ JC. The Image Processing Handbook, second ed. Boca Raton, FL: CRC Press, 1995.
T.O. Neild, R.B. Davey PII: S 0 3 0 5 - 4 1 7 9 ( 0 0 ) 0 0 0 2 3 - 1
Author’s response to: Skin colour in digital images With regard to your comments, we would like to state the following items: First, we do not doubt that the JPEG image compression method significantly alters colours in the image. Second, a 5% change in the hue value is readily noticeable if it occurs under the conditions stated above, which are quite different from those of a burns image. Third, we do not intend to propose the JPEG method as a general algorithm for the compression of medical images.
0305-4179/01/$20.00 © 2001 Elsevier Science Ltd and ISBI. All rights reserved.
Letter to the Editor 1
Skin colour in digital images We were interested to read the article by Roa et al. [1] ‘Digital imaging in remote diagnosis of burns’. While we agree that highly compressed images still contain useful information for diagnostic purposes, we think that it was unfortunate that the authors began the Abstract with the statement ‘‘Images are capable of giving an accurate representation of skin colour…’’. The JPEG image compression method significantly alters colours in the image. As the authors state, JPEG compression involves the loss of some image information, and the greatest losses are in the colours. The strategy behind JPEG compression is to separate the spatial and colour components of the image, and compress them by different amounts. The colour is compressed more than the spatial information, so that in the resulting image features such as edges still appear distinct, but colours may be changed and may blur over their original boundaries. Human observers still find the images acceptable and as Roa et al. [1] have shown, useful. More about JPEG compression, including many example images, can be found in Russ [2] (pp. 126 – 145). For the study of the colour of burns, JPEG compression can cause problems. We took images with the same camera as Roa et al., and converted them to JPEG images using the same software and a Q index of 50. We then, extracted the hue components of the original and compressed images and compared them on a pixel by pixel basis. Compression changed the hue of 13.9% of the pixels by more than 5% and 10.4% by more than 10%. These are average figures for the whole image; the proportion of pixels changed was often greater in areas of the image such as burn scars, where there were large colour gradients. A 5% change in the hue value is readily noticeable if it occurs in a large slab of plain colour, and two
1 The Editor regrets that due to an oversight, publication of these letters was delayed.
areas differing in hue by 10% will be perceived by most observers as frankly different colours. However, when these changes are spread throughout a complex image, the human brain is remarkably good at adapting to them, as the designers of the JPEG method knew and as Roa et al. have shown. In summary, we completely agree with Roa et al. that a highly compressed image can be a valuable aid to remote diagnosis. But we would like to add a note of caution for readers who might be interested in other uses for the image, particularly if these involve detailed analysis or measurement of the image colours. In such cases, extreme JPEG compression will probably be unsuitable.
References [1] Roa L, Gomez-Cia T, Acha B, Serrano C. Digital imaging in remote diagnosis of bums. Bums 1999;25:617– 23. [2] Russ JC. The Image Processing Handbook, second ed. Boca Raton, FL: CRC Press, 1995.
T.O. Neild, R.B. Davey PII: S 0 3 0 5 - 4 1 7 9 ( 0 0 ) 0 0 0 2 3 - 1
Author’s response to: Skin colour in digital images With regard to your comments, we would like to state the following items: First, we do not doubt that the JPEG image compression method significantly alters colours in the image. Second, a 5% change in the hue value is readily noticeable if it occurs under the conditions stated above, which are quite different from those of a burns image. Third, we do not intend to propose the JPEG method as a general algorithm for the compression of medical images.
0305-4179/01/$20.00 © 2001 Elsevier Science Ltd and ISBI. All rights reserved.