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On the WWW, you often you see the statement “Best viewed with Netscape v2.0” or some other similar statement. For digital images (not text), this is largely a content-free statement since both Netscape’s and Microsoft’s browsers render images similarly at this time and these two browsers command at least 90 percent of all distributed browsers. However, both browsers must contend with representing WWW images in the user’s current display system mode, which is the third rub.
If a user system is in a direct color mode, a browser’s job is easy. JPEG images naturally decompress to 24-bit images and any necessary conversion to 15- or 16-bit color modes is trivial. With indexed color GIF images, the image file color palette describes each allowed pixel value using 24 bits. So, it is trivial to render any indexed color pixel. Hence, there is no problem with GIF images, though the number of colors are reduced and the image may exhibit dithering that occurred during image compression. The bad news, however, is that it’s a rare display system that operates in direct color mode. Hence, the fourth and final rub is that the overwhelming majority of user systems operate in an indexed color mode.
Consider now what happens when rendering a JPEG image in indexed color mode. In this situation, a 24-bit image usually contains thousands of colors not represented in the paltry 256-color palette. This requires a reconciliation procedure. Depending on the user’s preferences settings, the browser either dithers the image using the current palette and a computationally expensive (slow) algorithm or uses a closest color match procrustean fit and ignores any chromatic error introduced. The same approach is used for GIF images that have a palette that does not match the display system palette. This means that images that were dithered during compression may be dithered a second time. Hence it is important to understand what the system palette looks like when a browser runs.
When Netscape and Microsoft browsers begin executing, they load 216 entries in the system palette, leaving 40 for other system uses. These 216 entries specify all possible combinations of six different intensities for red, green, and blue—0, 51, 102, 153, 204, and 255. You will see these collective entries referred to as the dither-free, standard browser, or browser safe palette. From the previous discussion, it follows that specifying these 216 colors as the palette for indexed color image compression or artistic graphics development prevents dithering during browser rendering— though with images, dithering may still occur during image compression. So, it follows that it is perhaps best to avoid custom or adaptive palettes during indexed color image compression because that virtually guarantees browser dithering and potential visual surprises from system to system. Avoid browser dithering. This is easily achieved by using a small number of the colors in the browser safe palette.
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