Xref: news2.mv.net comp.os.msdos.djgpp:2489
From: vicari AT mercurio (Marco Barbieri)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: [Q] Homemade 16-bits-per-pixel gfx mode?
Date: 5 Apr 1996 13:20:12 GMT
Organization: Computer Science Dep. - Milan University
Lines: 50
Message-ID: <4k36mc$5or@tic.sm.dsi.unimi.it>
References: <4jtkgr$mfv AT news DOT euro DOT net>
NNTP-Posting-Host: mercurio.cefriel.it
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Bram Stolk (bram AT gig DOT nl) wrote:

: So I mean: by just using standard VGA stuff?
: I already found out the following trick:

:   - read the port 3c6 four times
:   - write 3c6 once with value 128

: What it seems to do, is halve the horiz resolution, and make it
: 15 bpp.
: So if you do this in 320x200x8bpp,
: you end up in 160x200x15bpp.

I tried this out on my Cirrus VGA, and indeed something happened.. but
it doesn't seem to be exactly like that. After some experiments, I
found out that my VGA had entered a mode like this:

- resolution: unchanged (320x200)
- bpp:        unchanged (8bpp)
- write mode: 'direct', that is, what you write into video ram goes
  directly to the DAC, regardless of the palette settings. Actually,
  the bits in each video ram byte are organized as follows:

  IRRBBBGG
  ^      ^
 msb    lsb

  I   = 1-bit intensity (2 levels)
  RR  = 2-bit red       (4 levels)
  BBB = 3-bit blue      (8 levels)
  GG  = 2-bit green     (4 levels)

I also noticed that the thing works the same in 'tweaked' modes
(i.e. (360x480)). However, I think that the same could be obtained
by setting the palette colours with the previous bit-allocation
scheme.

P.S. Hicolor-Lowres modes are available in VESA only from release 1.2
on, I think. My 1.09 VESA bios only supports Hicolor-Hires modes
(640x480x15bpp).

						Good fun and
						especially...
						MERRY EASTER! :)

						Paolo

		-----------------------------------------
		 Paolo Vicari - vicari AT mailer DOT cefriel DOT it
		-----------------------------------------