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OpenGL SDL + OpenGL + Alpha

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artefon    122
Hi folks, im having some problems using OpenGL with SDL. The way i am doing i can blit images on screen but i get a wierd result in the alpha channel. I am using the following code: Initing SDL:
 39     glEnable( GL_TEXTURE_2D );
 40     glEnable( GL_ALPHA_TEST );
 41     glAlphaFunc( GL_GREATER,0.01f);
 43     glClearColor( 0.0f, 0.0f, 0.0f, 0.0f );
 44     glViewport( 0,
 45                 0,
 46                 Config::SCREEN_WIDTH,
 47                 Config::SCREEN_HEIGHT
 48                 );
 50     glClear( GL_COLOR_BUFFER_BIT );
 51     glMatrixMode( GL_PROJECTION );
 52     glLoadIdentity();
 54     glOrtho(0.0f,
 55             Config::SCREEN_WIDTH,
 56             Config::SCREEN_HEIGHT,
 57             0.0f,
 58             -1.0f,
 59             1.0f);
 61     glMatrixMode( GL_MODELVIEW );
 62     glLoadIdentity();
 64     if( glGetError() != GL_NO_ERROR ) { throw SDLError(); }
Screen draw:
 81             glBindTexture(GL_TEXTURE_2D, img->texture);
 82             glBegin( GL_QUADS );
 83             //Top-left vertex (corner)
 84             glTexCoord2i( 0, 0 );
 85             glVertex3f( x, y, 0 );
 86             //Top-right vertex (corner)
 87             glTexCoord2i( 1, 0 );
 88             glVertex3f( x+img->width, y, 0 );
 89             //Bottom-right vertex (corner)
 90             glTexCoord2i( 1, 1 );
 91             glVertex3f( x+img->width, y+img->height, 0 );
 92             //Bottom-left vertex (corner)
 93             glTexCoord2i( 0, 1 );
 94             glVertex3f( x, y+img->height, 0 );
 95             glEnd();
Texture creating:
 68     glGenTextures( 1, &texture );
 69     glBindTexture( GL_TEXTURE_2D, texture );
 71     glTexImage2D( GL_TEXTURE_2D,
 72                   0,
 73                   GL_RGBA,
 74                   surface->w,
 75                   surface->h,
 76                   0,
 77                   GL_RGBA,
 78                   GL_UNSIGNED_BYTE,
 79                   surface->pixels);
 83     SDL_FreeSurface(surface);
And the image used has a power of 2 size and was created by the following code:
 26     image = SDL_CreateRGBSurface(
 27             SDL_SWSURFACE,
 28             w,
 29             h,
 30             Config::SCREEN_BPP,
 31             #if SDL_BYTEORDER == SDL_LIL_ENDIAN /* OpenGL RGBA masks */
 32             0x000000FF,
 33             0x0000FF00,
 34             0x00FF0000,
 35             0xFF000000
 36             #else
 37             0xFF000000,
 38             0x00FF0000,
 39             0x0000FF00,
 40             0x000000FF
 41             #endif
 42             );
if i try using one of the following modes all the screen gets black:

glEnable( GL_ALPHA_TEST );
glAlphaFunc( GL_GREATER,0.1f);
Using only SDL the image is displayed correctly but if i use opengl the alpha channel gets black like this: can someone help me?? thanks in advance!

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bschneid    126
Original post by Split Thumbs
Accessing pixels straight from an SDL surface does not work.

It does, but you must lock the surface first.

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artefon    122
i corrected it! missed this code:

/* Save the alpha blending attributes */
Uint32 savedFlags = surface->flags&(SDL_SRCALPHA|SDL_RLEACCELOK);
Uint8 savedAlpha = surface->format->alpha;
if ( (savedFlags & SDL_SRCALPHA) == SDL_SRCALPHA ) {
SDL_SetAlpha(surface, 0, 0);

area.x = 0;
area.y = 0;
area.w = w;
area.h = h;
SDL_BlitSurface(surface, &area, image, &area);

/* Restore the alpha blending attributes */
if ((savedFlags & SDL_SRCALPHA) == SDL_SRCALPHA) {
SDL_SetAlpha(surface, savedFlags, savedAlpha);

Hope it helps!

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