[SDL] 12bpp mode

Joshua Oreman oremanj at gmail.com
Tue Jun 21 22:31:49 PDT 2005


On 6/20/05, Neil Bryden <Neil.Bryden at greyinnovation.com> wrote:
> Hello all,
> 
> My first post to the list, so I apologise if I get any local etiquette
> wrong ;-)
> 
> I would like to use SDL on an embedded device which has an LCD which
> only supports 12 bit/pixel (12bpp) color.
> 
> I have got everything running, (testsprite produces somewhat expected
> results) however (I assume) because the chosen video mode uses 2
> bytes/pixel, I'm missing some vertical lines and the aspect ratio and
> colors are wrong.
> 
> Has anyone successfully got SDL to run at 12bpp?  If not, and I was to
> add this support, can anyone offer any pointers as to where to start and
> how it should be done to best fit in with the current codebase so we can
> contribute it back?

Write a video driver.  The SDL framework is set up well to accept new
ones.  One approach -- which I used for the iPod to get 2-bpp working
-- is to tell SDL that your display is, indeed, 16-bit.  Then, in
FOO_UpdateRects, copy the screen to a private 12-bit buffer, and
update the screen with that.

You could also, in a method like that, set
screen->format->BitsPerPixel 16 but have e.g.
Rmask = 0xF00
Gmask = 0x0F0
Bmask = 0x00F
and just copy the low 12 bits of each 2 bytes -- that would be faster.

SDL doesn't support packed (non-whole number of bytes per pixel)
surfaces because (a) the implementation of them is very
endian-dependent (see 24bpp) and (b) nobody has put in the
(significant) amount of work to make it work.  It's easier to just do
something like the above.

HTH
-- Josh

> 
> Thanks,
> Neil.
> 
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