I can't get the transparency in my images to work.

Question

Stemming from this question of mine: http://stackoverflow.com/questions/1191093/im-seeing-artifacts-when-i-attempt-to-rotate-an-image

In the source code there, I am loading a TIF because I can't for the life of me get any other image format to load the transparency parts correctly. I've tried PNG, GIF, & TGA. I'd would like to be able to load PNGs. I hope the source code given in the question above will be enough, if not, then let me know.

For a better description of what happens when I attempt to load some other format -- One of the images I was attempting was a 128*128 orange triangle. Depending on the format, it would either make the entire 128*128 square orange, or make the transparent parts of the image white.

Answer 1

I'm not familiar with SDL, but since it's SDL that loading the image, I would look closer at their docs. I use .png in my own work along with OpenGL, and transparency works with no problem. (I use a .png parser called LightZPng.)

Also, I just noticed your linked post has:

glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

instead of:

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

This would have the affect of adding the pixels that should be transparent to whatever is in the background (assuming the alpha is 0 in those texels).

Answer 2

Make sure that you have alpha blending enabled with

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

otherwise primitives will draw solid colors where there should be transparency. You may need a different blendfunc. This is a common setup.

Answer 3

OK, I'm new at OpenGL + SDL but here is what I have.. Loads all? formats SDL_image supports except I can't get .xcf to work and don't have a .lbm to test with.

//called earlier..
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

//load texture
SDL_Surface* tex = IMG_Load(file.c_str());
if (tex == 0) {
 std::cout << "Could not load " << file << std::endl;
 return false;
}

glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);

//nearest works but linear is best when scaled?
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

width = tex->w;
height = tex->h;

//IMG_is* doesn't seem to work right, esp for TGA, so use extension instead..
std::string ext = file.substr(file.length() - 4);
bool isBMP = (ext.compare(".bmp") == 0) || (ext.compare(".BMP") == 0);
bool isPNG = (ext.compare(".png") == 0) || (ext.compare(".PNG") == 0);
bool isTGA = (ext.compare(".tga") == 0) || (ext.compare(".TGA") == 0);
bool isTIF = ((ext.compare(".tif") == 0) || (ext.compare(".TIF") == 0) ||
     (ext.compare("tiff") == 0) || (ext.compare("TIFF") == 0));

//default is RGBA but bmp and tga use BGR/A
GLenum format = GL_RGBA;
if(isBMP || isTGA)
 format = (tex->format->BytesPerPixel == 4 ? GL_BGRA : GL_BGR);

//every image except png and bmp need to be converted
if (!(isPNG || isBMP || isTGA || isTIF)) {
 SDL_Surface* fixedSurface = SDL_CreateRGBSurface(SDL_SWSURFACE, width, height, 32, 0x000000ff, 0x0000ff00, 0x00ff0000, 0xff000000);
 SDL_BlitSurface(tex, 0, fixedSurface, 0);
 glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, format, GL_UNSIGNED_BYTE, fixedSurface->pixels);
 SDL_FreeSurface(fixedSurface);
} else {
 glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, format, GL_UNSIGNED_BYTE, tex->pixels);
}

SDL_FreeSurface(tex);

list = glGenLists(1);
glNewList(list, GL_COMPILE);
 GLint vertices[] = {
   0,0, 0,0,
   0,1, 0,height,
   1,1, width,height,
   1,0, width,0
  };

 glEnableClientState(GL_TEXTURE_COORD_ARRAY);
 glEnableClientState(GL_VERTEX_ARRAY);

 glBindTexture(GL_TEXTURE_2D, texture);
 glTexCoordPointer(2, GL_INT, 4*sizeof(GLint), &vertices[0]);
 glVertexPointer(2, GL_INT, 4*sizeof(GLint), &vertices[2]);
 glDrawArrays(GL_POLYGON, 0, 4);

 glDisableClientState(GL_VERTEX_ARRAY);
 glDisableClientState(GL_TEXTURE_COORD_ARRAY);
glEndList();

And then to draw I set the color to opaque white (doesn't affect transparency?) then just call the list..

glColor4f(1,1,1,1);
glCallList(list);

And of course, any help for my code would be much appreciated too! :)