Xref: news2.mv.net comp.os.msdos.djgpp:973
From: Hartmut Schirmer <hsc AT techfak DOT uni-kiel DOT d400 DOT de>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: GRXlib question
Date: 11 Feb 1996 13:52:14 GMT
Organization: Technische Fakultaet
Lines: 41
Message-ID: <4fksae$q32@purzel.techfak.uni-kiel.de>
References: <311CE589 DOT 41C67EA6 AT solace DOT mh DOT se>
NNTP-Posting-Host: zora.techfak.uni-kiel.de
To: rocwiz AT solace DOT mh DOT se
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Jurgen Wenzel <rocwiz AT solace DOT mh DOT se> wrote:
[...]
>According to the GRXlib documentation each pixel in 32K RGB mode is 
>represented as  xrrrrrgggggbbbbb  (it never mentiones the actual type).

It's an unsigned short  (-> 16 bit)

[...]
>Then there is the issue of setting the unsigned int colour value into
>my unsigned char screen buffer. (When passing a buffer in graphic mode
>initialization, it is supposed to be a char pointer.) This is easily 
>solved by getting the highbyte and the lowbyte and then set them both
>sequentially in the screen buffer. Thus the the colour values end up
>in xrrrrrgggggbbbbb in the screen buffer. When I plot each character
>pair in the screen buffer using  GrPlot(x, y, (*buf << 8) | *(buf + 1))
>it plots the colours correctly through the entire screen buffer. I 
>therefore draw the conclusion that the colour values have been properly
>set in the screen buffer. Finally I try to blit the screen context the
>same way I did in 256 colour mode only to see that it -fails-, producing
>colours that's not the ones I wanted.

It looks like you just swapped the high and low byte. With Intel x86 series
processors, the memory representation of a multi-byte variable looks like

  base+0  ....  base+n
   low    ....  high byte

If you change the byte order, the color bits will move :

   xrrrrrgg gggbbbbb
-> gggbbbbb xrrrrrgg

This will look quite different ;)

Hartmut
-- 
Hartmut Schirmer                        | Phone: +49-431-77572-709  FAX: -703
Automatisierungs- u. Regelungstechnik,  | hsc AT techfak DOT uni-kiel DOT d400 DOT de
Technische Fakult"at, Universit"at Kiel,| http://www.techfak.uni-kiel.de/~hsc
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