Re: Fix 8bpp RGB fields length

["Ville Syrjälä" <syrjala@sci.fi>]

On Mon, Mar 02, 2009 at 06:02:39PM +0100, Krzysztof Helt wrote:
> On Mon, 2 Mar 2009 15:02:02 +0200
> Ville Syrjälä <syrjala@sci.fi> wrote:
> 
> > On Sun, Mar 01, 2009 at 09:55:56PM +0100, Krzysztof Helt wrote:
> > > Hi,
> > > 
> > > I found that some fbdev drivers set RGB field's lengths incorrectly
> > > to 6 bits. This is incorrect as the field's length says how many bits
> > > has a color index when using color palette (6 bits = 64 colors).
> > 
> > A slight omission in the fbdev API I suppose since the LUT entries are
> > nearly always 3*8bits wide. VGA being the exception.
> > 
> 
> If offsets of all RGB components are the same the length field says
> how lone the pallete index is. It does not say anything how long
> the LUT entries are. This is the same misunderstanding as done
> inside the driver.

Yes. I meant that the change is correct but the API doesn't provide any
means of conveying the LUT entry size to userspace which is a bit
unfortunate.

> > > Fix this error in uvesafb by dropping DAC switching to 8 bits 
> > > completely. Advantage of this approach is making the driver shorter,
> > > disadvantage is that some color fidelity is lost.
> > 
> > I don't see much point in dropping this. It would reduce the image
> > quality and it looks like after your first fix this code would amount
> > to less than a dozen lines. But not my call anyway.
> > 
> 
> I wonder if it affects quality at all as with 256 different pixel values
> selected from over 262000 is not much more better quality that
>  if these 256 values are taken from another set of values which
> is only about 3% different.

I suppose it doesn't matter much unless the palette is tuned to produce
nice gradients.

-- 
Ville Syrjälä
syrjala@sci.fi
http://www.sci.fi/~syrjala/

------------------------------------------------------------------------------
Open Source Business Conference (OSBC), March 24-25, 2009, San Francisco, CA
-OSBC tackles the biggest issue in open source: Open Sourcing the Enterprise
-Strategies to boost innovation and cut costs with open source participation
-Receive a $600 discount off the registration fee with the source code: SFAD
http://p.sf.net/sfu/XcvMzF8H