Drawing to the alpha channel of the frame buffer on with OpenGL while ignoring RGB values?

Question

The opaque property is set to NO on my OpenGL view's CAEAGLLayer. There is a UIImageView behind the OpenGL view (the checkboard). A texture is drawn initially on the OpenGL view (the Emu bird photo). Now I want to draw another texture in the middle of the fame buffer. The new texture is completely black in color and the alpha changes from 0 to 255, from top to bottom.

This is my 2nd texture...

Checkboard indicates transparency

This is what I want...

What I want

This is what I get...

What I get

The checkboard texture is an UIImage in a UIImageView behind the EAGLView.

I do not want to disturb the RGB values in the frame buffer, I only want to write into the Alpha channel. I tried...

glDisable(GL_BLEND) and glColorMask(0, 0, 0, 1)
glEnable(GL_BLEND) and glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_ONE, GL_ZERO)

Nothing seems to work. The RGB values are always modified, the pixels become brighter.

If source RGBA is (r2, g2, b2, a2) and destination is (r1, g1, b1, a1). I want the final value to be (r1, g1, b1, a2) or preferably (r1, g1, b1, some_function_of(a1, a2)). How do I achieve this?

Here is my code...

Objective C code for drawing the second texture...

- (void) drawTexture {
    GLfloat textureCoordinates[] = {
        0.0f, 1.0f,
        1.0f, 1.0f,
        0.0f,  0.0f,
        1.0f,  0.0f,
    };

    static const GLfloat imageVertices[] = {
        -0.5f, -0.5f,
        0.5f, -0.5f,
        -0.5f,  0.5f,
        0.5f,  0.5f,
    };

    [EAGLContext setCurrentContext:context];

    glViewport(0, 0, backingWidth, backingHeight);

    glUseProgram(program);

    glActiveTexture(GL_TEXTURE2);
    glBindTexture(GL_TEXTURE_2D, texture);

    glEnable (GL_BLEND);
    glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_ONE, GL_ZERO);

    glUniform1i(inputImageTexture, 2);

    glVertexAttribPointer(position, 2, GL_FLOAT, 0, 0, imageVertices);
    glVertexAttribPointer(inputTextureCoordinate, 2, GL_FLOAT, 0, 0, textureCoordinates);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
    [context presentRenderbuffer:GL_RENDERBUFFER];
}

Vertex shader...

 attribute vec4 position;
 attribute vec4 inputTextureCoordinate;

 varying vec2 textureCoordinate;

 void main()
 {
    gl_Position = position;
    textureCoordinate = inputTextureCoordinate.xy;
 }

Fragment shader...

 varying highp vec2 textureCoordinate;

 uniform sampler2D inputImageTexture;

 void main()
 {
     gl_FragColor = texture2D(inputImageTexture, textureCoordinate);
 }

I think you have the right idea, but you should try opengl.org/sdk/docs/man/xhtml/glBlendFuncSeparate.xml using the other parameters GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA also for the rgb channels. — Pochi

brigadir brigadir · Accepted Answer · 2012-08-20T06:29:47

glColorMask must work - it worked for me in my case. Also, as far as I know glBlendFuncSeparate works buggy on iOS.

(In theory) you can do the following:

set blending to multiply mode (GL_DST_COLOR, GL_ONE_MINUS_SRC_ALPHA)
make your texture white
set glColorMask to all channels enabled

If you have iDevice with iOS 5+ you can look at framebuffer alpha in OpenGL Capture window in XCode.

Drawing to the alpha channel of the frame buffer on with OpenGL while ignoring RGB values?

1 Answers