Re: difference in maximum color depth in Debian vs Mac OS?
On Mon, Jan 31, 2005 at 03:04:35PM +0100, Hans Ekbrand wrote:
> On Sat, Jan 29, 2005 at 02:31:40AM -0300, Fernan Aguero wrote:
> > The VRAM of this Macintosh is 1MB (expandable to 4MB I
> > believe). However what I fail to understand is why the Mac
> > OS is able to get a higher color depth than debian/linux on
> > the same hardware.
>
> I have similar problems, but I do have understanding of the issues at
> hand and I think I know why the following failed for you.
>
> > debian:~# fbset -fb /dev/fb0 -depth 16 "832x624-75"
>
> The problem is the speed of the graphic chip, not lack of RAM. I have
> a Macintosh Performa 5400 with the valkyrie graphic chip and if it the
> same as yours, MacOS cannot give 16 bpp at that mode, it only
> handles 16 bpp at 800x600@60HZ.
If you look at the code for platinumfb, it believes that 1M isn't enough
to do 832x624 in 16 bit color. There is a formula coded in to calculate
how much VRAM any particular mode takes. It's something like this:
vres*(hres*bpp + 32) + 4096
Run the numbers, and it adds up to more than 1M. I have no idea where the
constants are from, but the 32 appears to be the difference between the
hres and the line length in the framebuffer. I don't know enough about the
driver to figure out anything more. Perhaps some of the people who worked
on the code are still around to answer these questions.
Brad Boyer
flar@allandria.com
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