Re: trying to get 16bpp under XFree86

["John H. Kim" <jokim@MIT.EDU>]

On Tue, 10 Sep 1996, mykle hansen wrote:

> hi.  i now have XFree86 3.1.2G and i am futzing around
> trying to find the right settings for 16-bit-per-pixel 
> mode.  it occurs to me that LCD screens are so different
> than normal screens that i might be making some incorrect
> assumptions.  for instance:
> 
> the liturature on my 760EL claims that it does 16 bits per
> pixel.  however, from what i know about how these screens work,
> i would expect the real bit depth to be a multiple of three.
> (that is, i don't think they use indexed color -- i think
> they just assign brightness values to the red, green and
> blue subpixels.)  am i wrong about this? 

It depends.  :)  Unlike a desktop system, there are two video
hardware systems at work here -- the computer's and the screen's.
The screen hardware can have some funny requirements and limitations.
I've seen TFTs that can do 256 colors from a palette of 32k or
4096 colors simultaneously, but not 32k colors simultaneously.
This is regardless of what the computer's video hardware is doing.
The computer may be sending video signals for a 32k color picture
(verifyable if you hook up a monitor), but the screen may only be
able to display 4096 colors (4bpp).

The *screen's* bit depth is a multiple of three.  On my 701 it's
5 bpp, or 32k colors.  I dunno about the 760 although since it's
the flagship, I expect it's either 5 bpp or 6 bpp.  I don't think
there are any LCDs/TFTs in outside of research labs which can do
8 bpp yet.

The *computer's video* bit depth is what you normally run across
in hardware specs.  In 32k color mode it's 5 bpp.  In 64k color mode,
it's usually 5 bpp for RB and 6 bpp for G. Both of these modes are
"16-bit", with the extra bit going unused in the 32k color mode.

There is a web site that explains all this in full gory detail:
http://cal003109.student.utwente.nl/stefan/el/misc/lcd_faq.htm

> really 15, or 18 or maybe only a measley 12 bits per pixel?
> i'm sure the X server would love to know, and so would i.

Ahh, the real question.  As I understand it, the beta XFree server
doesn't yet support 15/16 bpp on the 760's video chipset.  It's
planned, and there may be some alpha work on it.  Then again, that
was what I read two beta version ago so things may have changed.

> and if that is the case, then is the video ram really as
> large as advertised?  i read that i'm supposed to have
> 2 megs of vram, but i wonder if it's really all there.

In all likelihood it's all there.  The last thing IBM needs is a slew
of false advertising lawsuits from a bunch of fortune 500 CEOs.

Keep in mind some of the VRAM is used for the video acceleration
featuress, so when you finally do get everything working, don't pick
the biggest virtual desktop your VRAM will support.
--
John H. Kim
jokim@mit.edu