Re: 24-bit-per-pixel limit to Gdk RGB ?

[individual <individual mi cl>]

On Tuesday, February 18, 2003, at 02:49 PM, Peter Finderup Lund wrote:

> On Tue, 18 Feb 2003, individual wrote:
>
> > Ok, last attempt: I made a 1000 pixel wide gradient going from 255 red
> > to 254 red. In the colour selection panel, I can clearly see the
> > difference between the two colours. When I draw the gradient, there is
> > no dividing line. HA! I have therefore proved that The Gimp and my X
> > server  are capable of displaying colour at more than 8 bits per
> > channel. Or have I?
>
> Did you do this experiment with the gimp?

Yes

>  In that case you proved that
> the gimp does use dithering to avoid Mach banding.

Ah. I see.

> Almost no X servers on Earth will currently give you more than 8 bits 
> per
> colour channel.  I don't know which X server you are using and what
> graphics card you are using.

XFree86 running inside XDarwin on Mac OS X 10.2.3. Here's my card info:

8 MB video RAM
640x480 32-bit
1024x768 32-bit

and, for what it's worth (pardon the lack of sophistication of this 
bit) my display menu allows me to choose from "thousands" and 
"millions".


> A few very new consumer cards will give you
> 10-bits per colour channel (or is it only 10-bits for the green 
> channel?)
> and I think XFree86 can be hacked/is being hacked to support this.  It
> does not, however, have the infrastructure in place for more than 32 
> bits
> for the colour channels + alpha combined.  If you need that, you will 
> have
> to get special equipment, probably from Sun or SGI (or Evans & 
> Sutherland,
> if they still exist -- btw. look up their names in citeseer).

No, I'm in no need of special equipment, thanks for the info though.

> There is no point in putting support for dithering 48-bit to 24-bit 
> pixels
> into GdkRgb since it is only needed in very specialized situations.  
> And
> if you really need it that much you can probably afford to implement it
> yourself.
>
> So, do you have a normal photographic image in 48-bit colour that you 
> want
> to display or do you have some specialized purpose?

Yes! See, I want to add a display functionality to a scientific C++ 
class that I made that is supposed to make dealing with high-quality, 
high resolution scientific data/images really easy. I use 48 bpp in the 
data, and that is because the image doubles as data storage, which can 
be read in at a later time to continue the procedure (images are 
written out as PNG images, with lossless comprsession).
I felt that dithering the image data in order to display it would be a 
non-ideal situation for this intended use.



> In the first case, you can throw away (more than) half the bits with 
> out
> anyone being the wiser.  In the second, you will have to be creative.
>
> google for something like error diffusion dithering + some more of Raph
> Levien's work.


Thanks!

Paul