Background Colors

[Hillary Handwerger]

Thank you for that explanation.  I intuitively knew what was going on, 
but it was good to see a description of the phenomena.


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Hillary Handwerger
hillaryh at sme.org
Society of Manufacturing Engineers-- CoNDUIT Project
313 271-1500 ext 597 
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On Tue, 12 Mar 1996, JQ Johnson wrote:

> kjustie at mgk.nslsilus.org (Kevin Justie) asks why his background colors
> (that look fine on a Mac) appear dithered when viewed with Netscape on a
> PC.  The answer is slightly complex.  Basically, the problem is that not
> all colors an author might want are available; the display hardware is
> typically capable of showing only 256 colors on the screen (not just in the
> window) at a time, and the system reserves some colors, making the set of
> colors Netscape has available less than 256. Netscape allocates a color
> cube, normally 6x6x6 (216 colors in all), then if it needs to display a
> color not in its color cube dithers the image to approximate it using
> nearby colors in its color cube.  The bottom line is that the only colors
> likely to be available in Netscape on a PC are those with 00, 33, 66, 99,
> CC, or FF in each of the red, green, and blue fields of the color triplet
> (#rrggbb).
> 
> Note that this set of 216 colors is a subset of the Mac 8-bit "system
> colors", so using these colors is also good on a Mac.  I believe that on a
> Mac Netscape normally uses the system color palette.
> 
> However, there is additional complexity.  Netscape can't always allocate a
> 6x6x6 color cube.  The user's PC hardware might not allow 256 colors, or
> too many colors might be reserved for other purposes.  In that case, older
> versions of Netscape for the PC (I haven't checked 2.0) reserved a smaller
> color cube.  And some versions of Netscape (old versions for X, for
> instance) didn't use a color cube at all, but instead used a first-fit
> algorithm, allocating colors as needed.
> 
> Finally, the kicker:   Netscape 1.1 for the PC appears to use a different
> color cube for backgrounds than what it uses for images.  Netscape 1.1 on
> the PC doesn't render background colors with "99" in triplets correctly
> except within images (e.g. as the transparent color).  Instead, it
> apparently thinks it has, e.g., A0-based triplets available, and dithers
> other nearby values.  This bug does NOT exist in Netscape 2.0 for the PC.
> 
> To return to Justie's posting, #009999 should work fine on most PCs running
> 2.0, just as it does on most Macs; #BBBBBB may get dithered.  Justie's
> pages looked fine to me when viewed on a PC running 2.0.  However, sure
> enough, both #009999 and #BBBBBB look wrong as the background color on PCs
> running Netscape 1.1.  Given the rapidity that 2.0 is replacing 1.1, I
> wouldn't worry about the problem if I were Justie.
> 
>  JQ Johnson                      office: 115F Knight Library
>  Academic Education Coordinator  e-mail: jqj at darkwing.uoregon.edu
>  1299 University of Oregon       voice: 541-346-1746; fax: 541-346-3485
>  Eugene, OR  97403-1299          <URL:http://darkwing.uoregon.edu/~jqj/>
> 
> 
>