Re: Colour bleeding...

[mjmahon@aol.com (Michael J. Mahon)]

Jeff Thomas asked:

>I am using "Koala Pad" (graphics program). Whenever, I tried to fill
>with one colour and another, next to it, in a clearly defined areas (to
>prevent flooding the whole screen into one colour), the picture seems to
>bleed in a completely different colour. Why is it doing this? How can I
>prevent it from happening? Or is it the graphics program?

You are encountering the "feature" of Apple II hi-res graphics that
there are two "color sets", and any display memory byte (with seven
pixels and a high-order bit that selects the color set) can contain
only the colors from a single set (since the high order bit is the
same for all seven pixels).

Color set 1 consists of colors 0, 1, 2, 3, and color set 2 is
4, 5, 6, 7.  You must keep adjacent colors in the same color
set for intuitive results.  If pixels from one color set "intrude"
into a byte containing pixels from the other color set, then
all pixels "switch" to the last written color set.

There are other effects of adjacent colors, even in the same
color set, as you may have noticed.  For example, adjacent
pixels of purple and green make two white pixels.  This color
"artifacting" is a natural consequence of the way that color
video is generated by the Apple II.

Many interesting mixtures or "dithered" colors can also be
produced, but some practice is required, and other drawing
programs may provide more versatile support for dithered
color than the Koala program.

-michael

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