Re: color map vs. pseudo-palette vs. RAMDAC registers

[Rolf Offermanns <rolf@offermanns.de>]

On Monday 24 October 2005 09:57, Geert Uytterhoeven wrote:
> > --------------------------------------------------------------------
> > Pseudocolor:
> > * cmap is not used
>
> That should be `pseudo palette is not used'.
>
> > * DAC[X] is programmed to (red, green, blue)
> > --------------------------------------------------------------------
> > Does that mean, I do not need to allocate a color map, if my driver would
> > only
>
> It means you don't have to fill in the pseudo palette in this mode.
OK.

> > support pseudocolor visual? Is pseudocolor always used for 8bpp or are
> > there situations where 16bpp pseudocolor makes sense?
>
> Pseudocolor is not always 8 bpp (1, 2, 4 are common values as well), but
> large values are very uncommon, since 16 bpp pseudocolor would mean a
> 65536-entries colormap.
OK.

> > -------------------------------------------------------------------------
> >---- Truecolor:
> >      *    does not use RAMDAC (usually has 3 of them).
> >      *    var->{color}.offset contains start of bitfield
> >      *    var->{color}.length contains length of bitfield
> >      *    cmap is programmed to (red << red.offset) |
> > 				(green << green.offset) |
> >     				(blue << blue.offset) |
> > 				(transp << transp.offset)
> >      *    RAMDAC does not exist
> > -------------------------------------------------------------------------
> >---- Now I am lost:
> > -> does not use RAMDAC.
> > I understand this. It states the same in my chip reference. But what
> > means "usually has 3 of them" in this context?
>
> Red, green, and blue.
OK. But this is for H/W with DirectColor support where each color has its own 
CLUT, right?

> > -> cmap is programmed to...
> > In the code that follows this comment, not the cmap is programmed, but
> > the pseudo palette.
>
> That should be `pseudo palette' again.
OK.
>
> > 1. Why do I need a pseudo palette and where does its size 16 comes from?
>
> It's used to translate from the 16 text console colors to actual pixel
> values.
OK, and these 16 color can be set/defined by text mode apps, right?


Thanks for these explanations Gerd. Shall I provide a patch for the skeletonfb 
driver comments?

The one thing, I still do not understand is:
What are color maps used for in the different visuals? In pseudo mode they are 
not used at all. In TrueColor mode there should be no need to map anything?
I checked the documentation again, but there must be something I have 
overread.

Could you please explain this?

Thanks,
Rolf



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