[SDL] glSDL backend

Stephane Marchesin stephane.marchesin at wanadoo.fr
Tue Jan 11 10:11:18 PST 2005


Gaetan de Menten wrote:

>>You don't seem to have 3d acceleration correctly setup. You should load
>>the dri module in your X configuration file.
>>
>
>I got the Nvidia closed source drivers and they explicitly say not to
>load the dri module, but I guess this is pretty much off topic...
>
Hmm, I assumed that was the DRI drivers. Anyway as you said, it's off topic.

>
>>The semantics when mixing colorkey and alpha is explained in large
>>detail in the SDL_SetAlpha manpage.
>>It depends on the format of the source and destination surfaces, not
>>only on the flags that are set.
>>
>
>I think you misunderstood what I said... I was probably unclear but I
>really think there is a problem... The semantics are what I thought
>they were, and even if I got the semantics wrong (I don't think I do),
>you'll have to explain me how a call to "SDL_SetAlpha(Surface, 0,
>255)" can result in an alpha blended sprite...
>
>Again, it looks like the call is totally ignored (the previous alpha
>value is used) when
>the SDL_SRCALPHA is not present, instead of IN THIS CASE (alpha = 255)
>"setting up" the surface as opaque.
>
>Btw: this problem is, of course, only present with the glSDL backend
>and not in any  other backend I tried (otherwise I wouldn't have
>reported it as an answer to your mail).
>

Ok, Id' like to have the full source code reproducing the problem 
(including the surface creation code, that's important).
It would be nice if it could compile out of the box, because I don't 
have too much free time currently :)

>>I think we have most of the pitfalls described there :
>>http://icps.u-strasbg.fr/~marchesin/sdl/glsdl.html
>>And yes, most of the advice there also applies to non-glSDL situations.
>>
>
>I had already read that page since you mentioned it in your first
>mail... (it's a pretty nice page, btw). But I think some pitfalls of
>hardware surfaces are not mentioned.   Isn't there a problem with lost
>surfaces on the DirectDraw backend (or something like that)?
>
Well, DirectDraw surely causes problems ;) but :
- if the surface is lost, it can very well be recreated exactly as it 
was before (in particular, it can be recreated as a hardware surface) by 
reloading the artwork. This is clearly stated in the directdraw docs.
- losing surfaces only happens during long program inactivity or when 
you alt-tab a fullscreen apps and such...

So in short, that's not a performance problem (we're talking about 
performace, aren't we ?).

Stephane







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