[SDL] SDL_BlitSurface on MacOSX 10.3.9

[Michael Ryan Bannon]

So, is this a bug in SDL or am I doing something wrong?  Has anybody tested 
the code?

Thanks,

Ryan

"Michael Ryan Bannon" <mrbannon at swen.uwaterloo.ca> wrote in message 
news:e8jdmu$8j8$1 at sea.gmane.org...
> Hello,
>
> I finally found out what was causing the problem.  Now my question is the 
> following: is it an SDL problem or a problem with my code?
> First, here's a sample:
>
> #include <SDL/SDL.h>
>
> #include <SDL_image/SDL_image.h>
>
>
>
> int main(int argc, char*argv[])
>
> {
>
>     // Initialize SDL with the given driverName.  This driver is specified 
> by setting the
>
>     // environment variable, "SDL_VIDEODRIVER" to driverName.
>
>    SDL_Init(SDL_INIT_VIDEO);
>
>     SDL_putenv("SDL_VIDEODRIVER=Quartz");
>
>
>
>    // Create surfaces.
>
>    SDL_Surface* pSurfaceOne = IMG_Load("/1.jpg");
>
>     SDL_Surface *pSurfaceTwo = IMG_Load("/2.jpg");
>
>
>
>    // Convert surfaces.
>
>    SDL_Surface* pConversionSurface = SDL_CreateRGBSurface(SDL_SWSURFACE, 
> 1, 1, 32, 0xff000000, 0x00ff0000, 0x0000ff00, 0x000000ff);
>
>    SDL_Surface* pSurfaceOneConverted = SDL_ConvertSurface(pSurfaceOne, 
> pConversionSurface->format, SDL_SWSURFACE);
>
>    SDL_Surface* pSurfaceTwoConverted = SDL_ConvertSurface(pSurfaceTwo, 
> pConversionSurface->format, SDL_SWSURFACE);
>
>
>
>    // Create window and display surface.
>
>     SDL_Surface* pDisplaySurface = SDL_SetVideoMode(800, 600, 32, 
> SDL_SWSURFACE);
>
>
>
>    // Display
>
>    int alpha = 0;
>
>    while(true)
>
>    {
>
>        // Set alpha.
>
>        alpha += 10;
>
>        if(alpha > 255)
>
>        {
>
>            alpha = 0;
>
>            SDL_Surface* temp = pSurfaceOneConverted;
>
>            pSurfaceOneConverted = pSurfaceTwoConverted;
>
>            pSurfaceTwoConverted = temp;
>
>        }
>
>
>
>        // Apply to surfaces and blit.
>
>        pSurfaceOneConverted->format->Amask = 0;
>
>        pSurfaceTwoConverted->format->Amask = 0;
>
>        SDL_SetAlpha(pSurfaceTwoConverted, SDL_SRCALPHA, 255 - alpha);
>
>        SDL_BlitSurface(pSurfaceTwoConverted, NULL, pDisplaySurface, NULL);
>
>        SDL_SetAlpha(pSurfaceOneConverted, SDL_SRCALPHA, alpha);
>
>        SDL_BlitSurface(pSurfaceOneConverted, NULL, pDisplaySurface, NULL);
>
>
>
>        // Update the surface.
>
>        SDL_Flip(pDisplaySurface);
>
>    }
>
>    return 0;
>
> }
>
>
> So, to restate the issue...I'm running this code on two different 
> machines. The first is a Mac OS 10.4.6 2GHz Intel Duo running XCode 2.3. 
> I'm using SDL 1.2.11 UB.
> The second machine is a Mac OS 10.3.9 PowerPC...no XCode (I was doing a 
> remote debug).
>
> The above sample (with the SDLMain stuff used, of course) would run as 
> expected on the 10.4.6 machine.  That is, one background would fade out 
> while the other faded in.  When they had finished their fades, they would 
> fade backwards...and so on.  This sample did NOT run as expected on the 
> 10.3.9 machine.  There seems to be an extra rendering every time an alpha 
> value (not 0 or 255) is present and this extra rendering is put about 5 
> pixels to the right and on top of the proper rendering.
>
> The cause of this is the surface conversion.  If you get rid of the 
> conversion code, it will run as expected on both machines.  Should this 
> code always work as expected, regardless of machine?  I would assume not, 
> but I don't understand the surface conversion that well.
>
> I'm more than happy to provide any other information.  And any help is 
> greatly appreciated.
>
> Thanks,
>
> Ryan
>
> ps - I just put 1.jpg and 2.jpg in the root for convenience.  They're 
> 800x600 jpegs...that's all.
>
> "Torsten Giebl" <wizard at syntheticsw.com> wrote in message 
> news:3617.141.99.122.11.1151530170.squirrel at mail.syntheticsw.com...
>> Hello !
>>
>>
>>> Some further information (I'm really hoping somebody might be able to
>>> help me with this...) The problem, I believe, has something to do with 
>>> the
>>> alpha blending.  Each surface has the following done before the blit:
>>>
>>> surface->format->Amask = 0; // turns off per-pixel alpha; note that
>>> surface is an SDL_Surface SDL_SetAlpha(surface, SDL_SRCALPHA,
>>> constantAlpha);
>>>
>>> "cosntantAlpha" is a number between 0 and 255.  Now, if I set the Amask
>>> parameter to 1, the SDL_SetAlpha call is ignored (I assume) because SDL
>>> thinks that each pixel now has an individual alpha.  When I do this,
>>> there's no more 2nd background, which is what I want, but at the same 
>>> time
>>> there's obviously no alpha blending.
>>>
>>> So, I've determined that per-surface alpha blending has something to do
>>> with it (along with Mac OS 10.3.9).  Aside from that I'm stumped.
>>
>> Please put together a minimal example that
>> shows the problem.
>>
>>
>> CU