From:	Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Allegro won't display more than 1 PCX (properly)!!!
Date:	Mon, 27 Jan 1997 22:29:38 +0000
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To:	djgpp AT delorie DOT com
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Gautam N. Lad writes:
>I modified the example EX15.C source to display two different PCXs.
>So, here's the code.  Now what I don't understand is why
>does Allegro use one palette to display an image, and when another
>palette is created for another image, that original palette 
>gets screwed up, and starts using the palette from the new palette.

It does that because that's how the hardware works...

I'll have a go at describing what is going on, but palettes aren't the
easiest concept to get to grips with. For a better explanation, download
the PCGPE from x2ftp and read Denthor's article in tut2.txt. The
examples are all in Pascal, but he explains it very clearly.

In a 256 color graphics mode, each pixel is stored in video memory as a
single byte, containing a number 0-255. These numbers don't represent
colors directly - they are indexes into a table called the palette,
which lists what color each of the 256 possible pixel values refers to.
So when the video hardware sees a pixel with a value of 10, it goes and
grabs the tenth entry from the palette and displays that color on your
monitor. It could be red, it could be blue, it all depends what you put
in the palette.

This means that displaying an image must be done in two steps. First,
set the palette registers to tell the hardware which 256 colors you want
to use. Then you can copy the pixels to the screen, and they will come
out looking right. When you change the palette it affects all the pixels
on the screen, not just the ones that are drawn after you make the
change.

To display two PCX images side by side, they will have to be using the
same palette. The easiest way to do this is with a paint program or a
conversion tool like Image Alchemy. It can be done in your code, of
course, but generating an optimal palette for merging two bitmaps is a
non-trivial task...

This may seem awkward, but once you get the hang of them palettes can be
used to great effect. Once you have drawn a graphics onto the screen,
you can totally change it's color just by altering a few palette
registers (much faster than redrawing the image itself). Hence you can
get nice smooth fade out effects by altering the entire palette to
gradually make all the entries black. Or you can tint the entire palette
to a different color (how do you think Doom makes the screen go red when
you are about do die?). Or you can rapidly cycle a few of the palette
colors to get very efficient animation effects (waterfalls being the
most common use - I'm not sure I've ever seen a platform game that
doesn't have a palette-cycled waterfall in it somewhere :-)


/*
 *  Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
 *  Ghoti: 'gh' as in 'enough', 'o' as in 'women', and 'ti' as in 'nation'.
 */

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