Re: 24 bit-per-pixel thread

[Peter Finderup Lund <firefly diku dk>]

On Wed, 19 Feb 2003, Juan Pablo wrote:

> Hi
> I have been folowing the thread, and all i can do is post what i know,
> wich is not the last word, of course...
>
> There is two kind of "visuals", truecolor and indexed (indexed are out
> of the scope)

StaticGray, StaticColor, TrueColor, GrayScale, PseudoColor, or
DirectColor.

PseudoColor is like the old mode 13h palette indexed mode: the whole pixel
value is looked up inside the palette chip.

DirectColor takes each component (R/G/B) and looks it up in a palette
(colormap)  individually.

StaticGray, StaticColor and TrueColor doesn't have such a palette (=
read-only colormap).

TrueColor has sensible mappings from the R/G/B numbers to what you get on
the screen.  StaticColor is like PseudoColor with a read-only,
server-dependent palette that doesn't have to make sense.

Traditional visuals don't have an alpha channel.

With the EVI (Extended Visual Information) extension your X server can
also "sort of" an alpha channel on some visuals (e.g. tell you that pixel
value xxx or pixel value mask xxx is transparent).

As far as I know both visuals and pixmaps (called pictformats) do support
an alpha channel with the Render extension.

> Human eye can distinguish more luma(grayscale) than croma(colors), that
> is why, when the light is very low we start to see in grayscale.

the "grayscale" sensors are more sensitive to light, is what you are
saying here.  They /also/ have a higher spatial resolution frequency + the
eyes have great edge detection circuitry.  Those are two /different/
things, the latter of which is why 24-bit colours aren't always enough --
but ordinary output devices (monitors) are not really good enough that it
would matter much.

> So i recommend you to use all the bits you want per pixel in an internal
> data structure (array) and then shift to 8bpp to display it :)

My suggestion all along...

-Peter

"Of course, I'm not unbiased, but in my humble opinion, I've
 gotten close to something that I can be really proud of."
 -- Knuth on The Art of Computer Programming.