OpenGL Set Transparent Color for textures

Question

My question-> How do i make color 255,200,255 transparent in OpenGL? (by transparent i mean removing the pixels color 255,200,255 or whatever works...)

my texture loading functions are from this tutorial-> http://nehe.gamedev.net/data/lessons/lesson.asp?lesson=33

note that i don't have to use Alpha Channels, i have a set of pre-made images with custom color (255,200,255) which must be transparent/removed pixels..

some .tga loading functions of my program:

Texture AllTextures[1000];
typedef struct                                  
{
    GLubyte * imageData;                                    // Image Data (Up To 32 Bits)
    GLuint  bpp;                                            // Image Color Depth In Bits Per Pixel
    GLuint  width;                                          // Image Width
    GLuint  height;                                         // Image Height
    GLuint  texID;                                          // Texture ID Used To Select A Texture
    GLuint  type;                                           // Image Type (GL_RGB, GL_RGBA)
} Texture;  
bool LoadUncompressedTGA(Texture * texture, char * filename, FILE * fTGA)   // Load an uncompressed TGA (note, much of this code is based on NeHe's 
    {                                                                           // TGA Loading code nehe.gamedev.net)
        if(fread(tga.header, sizeof(tga.header), 1, fTGA) == 0)                 // Read TGA header
        {                                       
            MessageBox(NULL, "Could not read info header", "ERROR", MB_OK);     // Display error
            if(fTGA != NULL)                                                    // if file is still open
            {
                fclose(fTGA);                                                   // Close it
            }
            return false;                                                       // Return failular
        }   

        texture->width  = tga.header[1] * 256 + tga.header[0];                  // Determine The TGA Width  (highbyte*256+lowbyte)
        texture->height = tga.header[3] * 256 + tga.header[2];                  // Determine The TGA Height (highbyte*256+lowbyte)
        texture->bpp    = tga.header[4];                                        // Determine the bits per pixel
        tga.Width       = texture->width;                                       // Copy width into local structure                      
        tga.Height      = texture->height;                                      // Copy height into local structure
        tga.Bpp         = texture->bpp;                                         // Copy BPP into local structure

        if((texture->width <= 0) || (texture->height <= 0) || ((texture->bpp != 24) && (texture->bpp !=32)))    // Make sure all information is valid
        {
            MessageBox(NULL, "Invalid texture information", "ERROR", MB_OK);    // Display Error
            if(fTGA != NULL)                                                    // Check if file is still open
            {
                fclose(fTGA);                                                   // If so, close it
            }
            return false;                                                       // Return failed
        }

        if(texture->bpp == 24)                                                  //If the BPP of the image is 24...
        {
            texture->type   = GL_RGBA;                                          // Set Image type to GL_RGB
        }
        else                                                                    // Else if its 32 BPP
        {
            texture->type   = GL_RGBA;                                          // Set image type to GL_RGBA
        }

        tga.bytesPerPixel   = (tga.Bpp / 8);                                    // Compute the number of BYTES per pixel
        tga.imageSize       = (tga.bytesPerPixel * tga.Width * tga.Height);     // Compute the total amout ofmemory needed to store data
        texture->imageData  = (GLubyte *)malloc(tga.imageSize);                 // Allocate that much memory

        if(texture->imageData == NULL)                                          // If no space was allocated
        {
            MessageBox(NULL, "Could not allocate memory for image", "ERROR", MB_OK);    // Display Error
            fclose(fTGA);                                                       // Close the file
            return false;                                                       // Return failed
        }

        if(fread(texture->imageData, 1, tga.imageSize, fTGA) != tga.imageSize)  // Attempt to read image data
        {
            MessageBox(NULL, "Could not read image data", "ERROR", MB_OK);      // Display Error
            if(texture->imageData != NULL)                                      // If imagedata has data in it
            {
                free(texture->imageData);                                       // Delete data from memory
            }
            fclose(fTGA);                                                       // Close file
            return false;                                                       // Return failed
        }

        // Byte Swapping Optimized By Steve Thomas
        for(GLuint cswap = 0; cswap < (int)tga.imageSize; cswap += tga.bytesPerPixel)
        {
            texture->imageData[cswap] ^= texture->imageData[cswap+2] ^=texture->imageData[cswap] ^= texture->imageData[cswap+2];
        }

        fclose(fTGA);                                                           // Close file
        return true;                                                            // Return success
    }
    void LoadMyTextureTGA(int id,char* texturename)
    {
         //texturename ex: "Data/Uncompressed.tga"
        if(LoadTGA(&AllTextures[id], texturename))
        {
            //success      

                glGenTextures(1, &AllTextures[id].texID);               // Create The Texture ( CHANGE )
                glBindTexture(GL_TEXTURE_2D, AllTextures[id].texID);
                glTexImage2D(GL_TEXTURE_2D, 0, 3, AllTextures[id].width, AllTextures[id].height, 0, GL_RGB, GL_UNSIGNED_BYTE, AllTextures[id].imageData);
                glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
                glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);


                if (AllTextures[id].imageData)                      // If Texture Image Exists ( CHANGE )
                {
                    free(AllTextures[id].imageData);                    // Free The Texture Image Memory ( CHANGE )
                }        
        }
        else
        {
           MessageBoxA(0,"Textures Loading Fail! Game will close now","Game Problem",0);
           exit(1);
        }

    }

Answer 1

if texture->bpp == 24 instead of 32 (which means there is no built-in alpha channel), you have to generate a 32-bit openGL texture, and set the alpha value of each texel to 255 iif the tga pixel is 255,200,255.

Answer 2

Well, if you're using textures, then I'm assuming you already have a fragment shader. It's pretty easy actually.

First in the main program turn on alpha blending: glEnable (GL_BLEND); glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Then in the fragment shader add this code:

vec3 chromaKeyColor = texture(myTextureSampler,UV.xy).xyz;
float alpha;
if ((chromaKeyColor.x <= 0.01) && (chromaKeyColor.y <= 0.01) && (chromaKeyColor.z <= 0.01)){
    alpha = 0.;
}
else
{
    alpha = 1.0;
}
color = vec4(texture(myTextureSampler,VertexOut.texCoord.xy).xyz,alpha);

The above code will set black as the chroma key. (within a threshold of 0.01) You can choose another colour by looking up its RGB value. If it's not the chroma key colour, then set the alpha to full (1.0).