Re: [Linux-fbdev-users] FB_VISUAL_TRUECOLOR vs. FB_VISUAL_DIRECTCOLOR

[Antonino A. D. <ad...@gm...>]

James Lehman wrote:
> Can someone please explain the difference between these two display types?
> 
> FB_VISUAL_TRUECOLOR
> FB_VISUAL_DIRECTCOLOR
> 
> My frame buffer API, ezfb, works well with cards that are
> FB_VISUAL_TRUECOLOR, but not so well with cards that are
> FB_VISUAL_DIRECTCOLOR.
> 
> http://www.akrobiz.com/ezfb/

Truecolor:

pixel = red << offset | green << offset | blue << offset | alpha << offset;

where red, green, blue, alpha are the actual values of each pixel component

Directcolor:

pixel = red_index << offset | green_index << offset |
	blue_index << offset | alpha_index << offset;

where *_index's are the index to the Hardware LUT, ie:

red = red_map[red_index];

Thusly:  In Directcolor, you have to initialize the LUT first, by
sending an FBIOPUTCMAP ioctl. If you want Directcolor to behave like
Truecolor, then send a linear colormap where:

{red|green|blue|alpha}_map[i] = i;

In Truecolor, the color map need not be initialized.

Tony

Note: Each color component in struct fb_cmap are u16 in size, so the
contents of the cmap must be duplicated in both bytes, ie:

i = red;
red <<= 8;
red_map[i] = red;