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POProblem with SDL_DisplayFormatAlpha (c++)
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copied!<p>As I stated in <a href="https://stackoverflow.com/questions/341192/segmentation-fault-using-sdl-with-c-trying-to-blit-images">this question</a>, I am using SDL for a small game I'm developing. Now I am having problems with SDL_DisplayFormatAlpha. I am trying to create a surface with an alpha channel from a PNG image. It was working before, but now that I've done some slight refactoring something got broken. I've narrowed it down to this constructor:</p> <pre><code> Surface::Surface( tfilename file ) { // initialize the surface data member to the image indicated by filename SDL_Surface *tempSurface; tempSurface = IMG_Load( file.c_str() ); if ( !tempSurface ) { surface = NULL; exit(1); } else { surface = SDL_DisplayFormatAlpha( tempSurface ); //surface = tempSurface; } SDL_FreeSurface( tempSurface ); } </code></pre> <p>This compiles just fine, but when I run it I get a Segmentation fault. The error reported by gdb:</p> <pre> Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 0xb79c16c0 (LWP 8089)] 0xb7e8b9a3 in SDL_DisplayFormatAlpha () from /usr/lib/libSDL-1.2.so.0 </pre> <p>The stack trace is as follows:</p> <pre> #0 0xb7e8b9a3 in SDL_DisplayFormatAlpha () from /usr/lib/libSDL-1.2.so.0 #1 0x0804987e in Surface (this=0x804d060, file=@0xbfb20760) at Surface.cpp:16 #2 0x0804a159 in Image (this=0x804d038, x=0, y=0, file=@0xbfb207a0) at Image.cpp:16 #3 0x0804a3de in Object (this=0x804d028, imageFile=@0xbfb207dc) at Object.cpp:4 #4 0x080491cb in Application (this=0xbfb20810) at Application.cpp:8 #5 0x08048e0d in main () at main.cpp:5 </pre> <p>If I comment out <code>surface = SDL_DisplayFormatAlpha( tempSurface );</code> and <code>SDL_FreeSurface( tempSurface );</code> and uncomment <code>surface = tempSurface;</code> like so:</p> <pre><code> Surface::Surface( tfilename file ) { // initialize the surface data member to the image indicated by filename SDL_Surface *tempSurface; tempSurface = IMG_Load( file.c_str() ); if ( !tempSurface ) { surface = NULL; exit(1); } else { //surface = SDL_DisplayFormatAlpha( tempSurface ); surface = tempSurface; } //SDL_FreeSurface( tempSurface ); }</code></pre> <p>Then it seems to work just fine. Can anyone tell me what's going on? Actually, the transparency seems to work, too when I comment out SDL_DisplayFormatAlpha. Is that function only meant to be used with images that do not already have an alpha channel?</p>

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