Re: In what "dimension" does JPEG compression loss occur for color images

Richard Owlett wrote:
I understand that JPEG is a "lossy" compression method.

For color images is loss in:
1. resolution
2. color depth
a. would that include contrast
3. resolution AND color depth

I intuitively suspect #3.
If so is it possible trade off resolution for color depth and vice versa?

If not, are there "high" [definition of "high" to be specified by responder] compression algorithms which allow selecting preferred trade off? I would *presume* something defined as a "quality factor" [~proportional to resolution times color depth] would be held constant.

secondary questions
Have I properly framed the question?
Have I framed it well enough to demonstrate that I understand what DSP is about even if I can't do it?



My understanding is that loss in JPEG is primarily in resolution. JPEG (at least before JPEG-2000) only specifies the decoding -- so the encoding can vary from one vendor to another. The idea behind JPEG encoding is that you want to analyze the image for the amount of content and allow resolution loss where the image has fairly low contrast, but retain high resolution where the image has sharp edges or other areas of high spatial detail.

JPEG itself allows you to choose the amount of compression you allow -- so you can choose anything from totally lossless compression to an image that has a header and three bytes specifying the tone and tint of a single large pixel.

From both a user-space and a technical perspective JPEG-2000 is a Really Cool Algorithm. It doesn't seem to have caught on because there are some royalty issues holding it back, but it has some really nice features that'll make it unbeatable if the business end ever gets sorted out.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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