Opengl-es 1.1 greyscale the FBO

Question

I have the FBO, I just load it to a texture and an ortho (2D) mesh. How can I greyscale down my FBO texture?

Answer 1

Apple uses the following OpenGL ES 1.1 code for converting an image to grayscale in their GLImageProcessing sample:

static void greyscale(V2fT2f *quad, float t)    // t = 1 for standard perceptual weighting
{
    GLfloat lerp[4] = { 1.0, 1.0, 1.0, 0.5 };
    GLfloat avrg[4] = { .667, .667, .667, 0.5 };    // average
    GLfloat prcp[4] = { .646, .794, .557, 0.5 };    // perceptual NTSC
    GLfloat dot3[4] = { prcp[0]*t+avrg[0]*(1-t), prcp[1]*t+avrg[1]*(1-t), prcp[2]*t+avrg[2]*(1-t), 0.5 };

    // One pass using two units:
    // Unit 0 scales and biases into [0.5..1.0]
    // Unit 1 dot products with perceptual weights

    glVertexPointer  (2, GL_FLOAT, sizeof(V2fT2f), &quad[0].x);
    glTexCoordPointer(2, GL_FLOAT, sizeof(V2fT2f), &quad[0].s);
    glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
    glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB,      GL_INTERPOLATE);
    glTexEnvi(GL_TEXTURE_ENV, GL_SRC0_RGB,         GL_TEXTURE);
    glTexEnvi(GL_TEXTURE_ENV, GL_SRC1_RGB,         GL_CONSTANT);
    glTexEnvi(GL_TEXTURE_ENV, GL_SRC2_RGB,         GL_CONSTANT);
    glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA,    GL_REPLACE);
    glTexEnvi(GL_TEXTURE_ENV, GL_SRC0_ALPHA,       GL_TEXTURE);
    glTexEnvfv(GL_TEXTURE_ENV,GL_TEXTURE_ENV_COLOR, lerp);

    // Note: we prefer to dot product with primary color, because
    // the constant color is stored in limited precision on MBX
    glActiveTexture(GL_TEXTURE1);
    glEnable(GL_TEXTURE_2D);
    glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
    glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB,      GL_DOT3_RGB);
    glTexEnvi(GL_TEXTURE_ENV, GL_SRC0_RGB,         GL_PREVIOUS);
    glTexEnvi(GL_TEXTURE_ENV, GL_SRC1_RGB,         GL_PRIMARY_COLOR);
    glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_ALPHA,    GL_REPLACE);
    glTexEnvi(GL_TEXTURE_ENV, GL_SRC0_ALPHA,       GL_PREVIOUS);

    glColor4f(dot3[0], dot3[1], dot3[2], dot3[3]);
    validateTexEnv();
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    // Restore state
    glDisable(GL_TEXTURE_2D);
    glActiveTexture(GL_TEXTURE0);
}

I've completely given up 1.1 now, particularly for image processing, but greyscale conversion is one of the few effects you can pull off in 1.1.