OpenGL Set Transparent Color for textures - opengl

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).

Answer 3

OpenGL and color-keying can be a little tricky. Using 1-bit alpha is probably easier, but I've seen SDL used for color-keying like you describe. This thread might help you out.