Thanks for this. I know a little about Jpeg compression but I learned a lot
from reading this.

"Peter J. Holzer" wrote:

> [Crossposted to povray.general and Followup-To set]
>
> On Sun, 05 Nov 2000 19:44:14 -0800, Thomas Lake wrote:
> >Some may remember the fish tank. Sorry for the file size but anything
> >bellow 2% compression and artifacts started to eat away at it.
>
> Since this problem comes up so often in this group, I think this calls
> for inclusion in Warp's VFAQ. Here is a draft entry:
>
>     How can I avoid artifacts and still get good JPEG compression?
>
>     First, you have to know a little bit about how a picture is stored
>     in JPEG format.
>
>     Unlike most image formats it doesn't store RGB values, but YUV
>     values (1 grayscale value and two "color difference" values) just
>     like they are used in a color TV signal. Since the human eye uses
>     mostly the gray values to detect edges, one can usually get away
>     with storing the color information at a lower resolution - an
>     800x600 JPEG typically has only grayscale information at 800x600,
>     but color information at 400x300. This is called supersampling.
>
>     For each color channel separately, the picture is then divided into
>     little squares and the cosine transform of each square is computed.
>     A neat feature of this transformation is that if you throw away only
>     a few of the values, the quality will degrade very little, but the
>     image will compress a lot better. The percentage of values stored is
>     called the quality.
>
>     Finally, the data is compressed.
>
>     Most programs only let you change the quality setting. This is fine
>     for photos and photorealistic renderings of "natural" scenes.
>     Generally, quality values around 75% give be best compromise between
>     quality and image size.
>
>     However, for images which contain very saturated colors, the lower
>     resolution of the color channels causes visible artifacts which are
>     very similar to those caused by low quality settings. They can be
>     minimized by setting an extremely high quality (close to 100%), but
>     this will dramatically increase the file size, and often the
>     artifacts are still visible.
>
>     A better method is to turn off supersampling. The higher resolution
>     will cause only a modest increase in file size, which is more than
>     offset by the ability to use a lower quality setting.
>
>     The cjpeg command line utility (which should be available for all
>     systems which have a command line, e.g., Linux, MS-DOS, Unix, ...)
>     has an "-sample" to set the sampling factors for all passes.
>
>         cjpeg -sample 1x1,1x1,1x1 -quality 75
>
>     should be good default values which have to be changed only rarely.
>
> Comments? Improvements? Recommendations for Windows or Mac tools?
>
>         hp
>
> --


> | |   | hjp### [at] wsracat      |    -- Lutz Donnerhacke in dasr.
> __/   | http://www.hjp.at/ |

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