Terry Collins was once rumoured to have said:
> "E. J." wrote:
> >
> > Hello,
> > Im having a problem with my Red Hat 7.0 Distribution concerning the
> > switching and cofiguration of screen resolutions. I am using a Cyrix MII 233
> > mhz with a SiS6326 4mb Video Card and a LG monitor with the heighest
> > resolution of 1240*1080.
>
> Erm, basic maths required.
>
> 1240 x 1080 @ 8 bit/pixel[1] > 4Mb, which is why it doesn't work.
> try 1024x786 @ 8 bit.
Uh, CRACK INVOLVED.
*gets the cluebat out*.
OK, Video System Maths 101:
First, the basics.
8 bits = 1 byte.
1024^2 bytes = 1048576 bytes = 1mb
Now, that aside, there are 4 common modes of video oepration, 8bpp,
16bpp[1], 24bpp packed, and 32bpp[2]. These use 1, 2, 3 and 4 bytes per
pixel respectively.
now that we have that established, we're working within 4mbs, thats
4194304 bytes.
lets work out the video memory required for 1280x1024.
at 8bpp (256 colours), thats one byte per pixel:
1280x1024 = 1310270 bytes.
thats only 31% of the available video memory - that leaves heaps of
space for offscreen buffering.
at 16bpp (65536 colours), thats two bytes per pixel:
1280x1024@16bpp = 2621440 bytes.
Thats only 62% of the available video memory.
at 24bpp (16 million colours):
... = 3932160 bytes.... still OK
at 32bpp (16 million colours):
... = 5242880 bytes. OK, this doesn't work.
however, for 8bpp this is still fine.
IIRC, this a rough table of sane maximums:
800x600 1024x768 1280x1024 1600x1200 2048x1512
1Mb 16bpp 8bpp - - -
2Mb 32bpp 16bpp 8bpp - -
4Mb 32bpp 32bpp 24bpp 16bpp 8bpp
8Mb 32bpp 32bpp 32bpp 32bpp 16bpp
16Mb 32bpp 32bpp 32bpp 32bpp 32bpp
...
etc.
You get the idea.
C.
[1] 15bpp is a special case of 16bpp.
[2] 32bpp is still only 2^24 colours, just its faster to work with 4
bytes, rather than with 3. 32bpp is also used with 3d because it
gives an extra byte of space to use for alpha channel data.
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