View Full Version : SDL_Image and OpenGL (C program)

March 29th, 2007, 02:59 PM
Im using SDL for OpenGL in C.

Im just learning SDL (I already know a bit of OpenGL) and Im using the IMG_Load function (in SDL_image) to load a PNG. That was fine, but the color is strange (because of different color encoding?). So I looked on Nehe and hacked around with some BMP loader and made a PNG loader with OK color.

However I cant get transparency which I really need. I learn best from examples - if anyone could be really kind and show me some simple SDL/ OpenGL PNG load code that would be great!

Otherwise any ideas?

Thanks all

March 29th, 2007, 03:26 PM
You have to check the surface for a "Amask", then you have to use the right GLU function to load it (with GL_RGBA).

This function will handle both, although I suppose it would be better to add a function parameter for alpha being on/off (if an image has any alpha).

GLuint load_texture(const char* file)
SDL_Surface* surface = IMG_Load(file);
GLuint texture;
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
SDL_PixelFormat *format = surface->format;
if (format->Amask)
gluBuild2DMipmaps(GL_TEXTURE_2D, 4,
surface->w, surface->h, GL_RGBA,GL_UNSIGNED_BYTE, surface->pixels);
gluBuild2DMipmaps(GL_TEXTURE_2D, 3,
surface->w, surface->h, GL_RGB, GL_UNSIGNED_BYTE, surface->pixels);
return texture;

March 29th, 2007, 03:43 PM
The new code still doesnt do alpha. It loads the image and displays it perfectly but no alpha. The image really does have alpha (both GIMP and F-Spot show it with alpha).

Maybe I setup OpenGL wrong? Any more ideas - youve been really helpful :).


March 29th, 2007, 04:34 PM
That loads alpha textures for me. Maybe your OpenGL isn't set up.

To render things with alpha enables you have to enable GL_ALPHA_TEST

Then, you have to set the glAlphaFunc to the right alpha function.

Something like this should work (in your OpenGL initialization):

glAlphaFunc(GL_GREATER, 0.5);

This will ignore drawing any pixels with alpha less then 50%.

Check this page out:


March 29th, 2007, 05:40 PM
Thank you sooooooooooooooooooooooooooooooooooooo much Wybiral :). All perfect!