Question

I fallowed @Stefan Monov tutorial in this question. And everything works, but I need to make it working with my brush. I need to do so:

// Next, we want a blendfunc that doesn't change the color of any pixels,
// but rather replaces the framebuffer alpha values with values based
// on the whiteness of the mask. In other words, if a pixel is white in the mask,
// then the corresponding framebuffer pixel's alpha will be set to 1.
glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_SRC_COLOR, GL_ZERO);
// Now "draw" the mask (again, this doesn't produce a visible result, it just
// changes the alpha values in the framebuffer)
drawQuad(maskTexture);

would be not a static texture, but dynamic shape. I mean, i’m trying to implement brush witch is doing such a thing that is written by @Stefan Monov. So I need that this place could be implemented in other – (void) function, so that it could be called, when coordinates changes (when user draws). I tried a variety of ways to change the sequence of your code, but it does not work correctly then. Now my bursh code is:

glEnable(GL_BLEND);
glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_SRC_COLOR, GL_ZERO);
glPointSize(100);
glBegin(GL_POINTS);
glColorMask(1.0,1.0,1.0, 1.0);
glVertex2f(loc.x, loc.y);
glEnd();
glDisable(GL_BLEND);

It is called when mouse is dragged. “loc” is dragged mouse coordinates. Afcourse its not working now at all, because of blendFunc and code’s sequence. When I leave sequence as @Stefan Monov described, it works, but it draws one point and drags it when mouse is dragged. Becouse after drawing point, other textures is being redrawed too. Any, at least similar, solution for it?

---

To make it more clear i’ll show how I want my APP to work.

Here is the original code:

glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ZERO);
drawQuad(backgroundTexture);

glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_SRC_COLOR, GL_ZERO);
drawQuad(maskTexture);

glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA);
drawQuad(foregroundTexture);

Now its working like this:

1. Draws background
2. Draws mask
3. Draws foreground

I need it to work like this:

1. Draws bacground
2. Draws foreground
3. Draws mask

But if i change the order of drawings it stops working. Ignores mask.

Expected result foto:

Answer 1

Your attempt to draw the mask last makes no sense really.

Drawing the mask only modifies what’s in the alpha channel, and the alpha channel itself is not visible in any way in the final image.

The only use of the mask is to modify what’s drawn after it.