OpenGL glBlendFuncSeparate

Question

I need some help with OpenGL textures masking. I have it working but need to find some other blending function parameters to work in other way. Now I have:

//Background 
...code...
    glBlendFunc(GL_ONE, GL_ZERO);
...code

//Mask
...code...
    glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_DST_COLOR, GL_ZERO);
...code...

//Foreground
...code
    glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA);
...code

Now it sets foreground's opacity to 0 (fills with background texture) where mask is transparent. I need it to react to mask's colors. I mean something like setting foregrounds opacity depending on mask's color. For example if mask is black (0.0,0.0,0.0) then the opacity of that place in foreground is 0 (is filled with background), and if mask is white (1.0,1.0,1.0) then the opacity of foreground is 1 (not filled with background). It can be in reverse consequence (white = opacity 0, black = opacity 1). I just need it to work depending on color.

My current result's visualization bellow.

---

Background:

Mask (circle is transparent):

Foreground:

Result:

---

And I want it to work like this:

Background:

Mask (circle is white, background is black):

Foreground:

Result:

---

So that later it could be used like this:

Background:

Mask (circle is white, background is black):

Foreground:

Result:

---

Attempt with @Gigi solution:

Answer 1

Perhaps this is what you want:

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1) Clear the destination image:

glClearColor(0.0, 0.0, 0.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);

---

2) Draw the background, masking out the alpha channel:

glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE);

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3) Draw the "masking overlay", masking out the color channels:

glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE);

---

4) Draw the foreground, enabling blending:

glEnable(GL_BLEND);
glBlendEquationSeparate(GL_FUNC_ADD, GL_FUNC_ADD);
glBlendFuncSeparate(GL_ONE_MINUS_SRC_ALPHA, GL_SRC_ALPHA, GL_ONE, GL_ZERO);

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Note: The overlay image must have the alpha channel specified.