Other JPEG artifacts

Everyone knows about effects that JPEG compression can have on

<B>DETAIL</B> in an image and all the various artifacts that can be

produced by too much compression, such as "ripples" along the edges of

high (spatial) frequency transitions.<P>

 

Most of us, when converting an image to JPEG from TIFF or raw, will

examine the output image under 200% or 400% magnification along such

high frequency edges and adjust the compression setting until such

artifacts have gone away. I find that with the Photoshop "Save for

web" JPEG converter I can't see any detail artifacts about 80 or 90. <P>

 

So my question is: are there OTHER artifacts resulting from JPEG

compression I should look for, that DON'T have to do with detail

reproduction? For example, distortions of color gamut, contrast,

density range, etc. Thanks in advance.

I can not think of any artifact that affects the attributes you mentioned. Bear in mind that JPEG compression typically subsamples the chroma channel, so you can get <a href="http://www.photo.net/learn/jpeg/#chrom">color smearing</a>. There is a technical article <A href="http://www.photo.net/learn/jpeg/">here</a>.

OTHER artifacts include 8x8 color blocking visible after adjusting

color balance, brightness, contrast, HSV, etc.

Bill is of course correct about the 8X8 blocks (he helped me write the above article). When the chroma subsampling of 2X2 is used then the blocks are 16X16 pixels in size (or 16X8 for 2X1 subsampling).

 

Note that even at the highest quality settings there is a small loss of quality due to the YCbCr subsampling - this is certainly not visible, but can be seen by sharpening edges, etc.