Ignore certain colour alpha blending

Question

I load two textures using a function like below:

GLuint LoadImage(std::string ImageTitle, GLenum Target)
{
    Image Img(ImageTitle.c_str());
    std::vector<std::uint8_t> Buffer, Out(((Img.Width() * Img.GetBitsPerPixel() + 31) / 32) * 4 * Img.Height());
    ImageToBuffer(Buffer, Img);
    FlipImageBytes(Buffer.data(), Out.data(), Img.Width(), Img.Height(), Img.GetBitsPerPixel());

    GLuint ID;
    glGenTextures(1, &ID);
    glBindTexture(Target, ID);
    glTexParameteri(Target, GL_TEXTURE_WRAP_S, Target == GL_TEXTURE_2D ? GL_REPEAT : GL_CLAMP_TO_EDGE);
    glTexParameteri(Target, GL_TEXTURE_WRAP_T, Target == GL_TEXTURE_2D ? GL_REPEAT : GL_CLAMP_TO_EDGE);
    glTexParameteri(Target, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(Target, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexImage2D(Target, 0, Img.GetBitsPerPixel() == 32 ? GL_RGBA : GL_RGB, Img.Width(), Img.Height(), 0, Img.GetBitsPerPixel() == 32 ? GL_BGRA : GL_BGR, GL_UNSIGNED_BYTE, Out.data());
    return ID;
}

The first texture is a Map and I draw it on screen normally. The second texture is Black with Red text on it and that's the one I want to draw on top of the Map texture. However, I want to ignore the black pixels so that it only draws the red text.

I draw using the following:

void DrawTexture(GLenum Target, GLuint ID, float X1, float Y1, float X2, float Y2, int Width = 1, int Height = 1)
{
    //glEnable(Target);
    //glBindTexture(Target, ID);
    glBegin(GL_QUADS);
        glTexCoord2f(0, 0);
        glVertex2f(X1, Y1);

        glTexCoord2f(0, Height);
        glVertex2f(X1, Y2);

        glTexCoord2f(Width, Height);
        glVertex2f(X2, Y2);

        glTexCoord2f(Width, 0);
        glVertex2f(X2, Y1);
    glEnd();
    glDisable(Target);
}


int X1 = 0, Y1 = 0, X2 = 512, Y2 = 512;
            glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
            glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
            glPushMatrix();


            //Draw MAP.
            glEnable(GL_TEXTURE_2D);
            glBindTexture(GL_TEXTURE_2D, ID);
            glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
            glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
            glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

            glEnable(GL_BLEND);
            glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
            DrawTexture(GL_TEXTURE_2D, MAPID, 0, 0, 512, 512);
            glFlush();


            //Draw TEXT..
            glBindTexture(GL_TEXTURE_2D, ID2);
            glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
            glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
            glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
            glBlendFunc(GL_ONE, GL_DST_ALPHA);
            DrawTexture(GL_TEXTURE_2D, TEXTID, 0, 0, 512, 512);

            glDisable(GL_BLEND);
            glDisable(GL_TEXTURE_2D);

            glPopMatrix();
            SwapBuffers(DC);
            Sleep(1);

What I'm trying to do:

std::vector<std::uint8_t> Buffer; 
    //Fill Buffer with Pixels. 

    glDrawPixels(Width, Height, .........);

enter image description here

However, it does not make the black pixels on the second image transparent. How can I make it draw every pixel except for Black only in the second texture? So draw the first texture normally and draw the second ignoring black.

I tried using glDrawPixels (I'd prefer to use this for drawing the black Text pixels if possible)

Any ideas?

emartel emartel · Accepted Answer · 2013-06-19T00:43:06

The proper way to do this is to use alpha blending, with an alpha channel. Your texture is probably 32bits anyway, with a fully opaque alpha channel at 255. Using your favorite picture editor, you can probably convert your picture to copy the red channel to your alpha channel, which will give you exactly what you need. It also will support semi-transparent textures as well.

Otherwise, what you're looking for is alpha test which would also require the usage of the alpha channel, unless you're using indexed colors.

Ignore certain colour alpha blending

2 Answers