OpenGL Masking via Alpha Blending - c++

Question

I am Trying to Render 3 textures,
-Background
-Black/White Foreground Mask
-Foreground
I have used this OpenGL - mask with multiple textures
because it acurately descirbes my problem. But i can not get it to work. I only get the Last rendererd Texture, in this case the Foreground. I have called glutInitDisplayMode(GLUT_ALPHA); to get Alpha rendering as sugested in the Answer.
Can anyone spot errors from my side?
My code is as follows:
double stretch = ((double)m_videoResY * (double)m_depthResX) / ((double)m_videoResX * (double)m_depthResY);
glEnable(GL_BLEND);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(+0.5, -0.5, +0.5, -0.5, 0.001f, 1.0);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glTranslatef(0.0f, 0.0f, -0.5f);
glEnable(GL_TEXTURE_2D);
glActiveTexture(GL_TEXTURE0);
glDisable(GL_DEPTH_TEST);
glClearColor(0, 0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBlendFunc(GL_ONE, GL_ZERO);
glBindTexture(GL_TEXTURE_2D, m_backgroundTexture);//Draw BGTexture
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
glBegin(GL_QUADS);
glTexCoord2f(1.0f, 1.0f);
glVertex3f(-0.5f, -0.5f, 0.0f);
glTexCoord2f(1.0f, 0.0f);
glVertex3f(-0.5f, 0.5f, 0.0f);
glTexCoord2f(0.0f, 0.0f);
glVertex3f(0.5f, 0.5f, 0.0f);
glTexCoord2f(0.0f, 1.0f);
glVertex3f(0.5f, -0.5f, 0.0f);
glEnd();
glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_SRC_COLOR, GL_ZERO);
//mask with userID
glBindTexture(GL_TEXTURE_2D, m_userIDTexture);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
glBegin(GL_QUADS);
glTexCoord2f(1.0f, 0.0f);
glVertex3f(-0.5f, -0.5f, 0.0f);
glTexCoord2f(1.0f, 1.0f * stretch);
glVertex3f(-0.5f, 0.5f, 0.0f);
glTexCoord2f(0.0f, 1.0f * stretch);
glVertex3f(0.5f, 0.5f, 0.0f);
glTexCoord2f(0.0f, 0.0f);
glVertex3f(0.5f, -0.5f, 0.0f);
glEnd();
//blend with Video of User
glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA);
glBindTexture(GL_TEXTURE_2D, m_videoTexture);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
glBegin(GL_QUADS);
glTexCoord2f(1.0f, 0.0f);
glVertex3f(-0.5f, -0.5f, 0.0f);
glTexCoord2f(1.0f, 1.0f);
glVertex3f(-0.5f, 0.5f, 0.0f);
glTexCoord2f(0.0f, 1.0f);
glVertex3f(0.5f, 0.5f, 0.0f);
glTexCoord2f(0.0f, 0.0f);
glVertex3f(0.5f, -0.5f, 0.0f);
glEnd();

Answer 1

I suppose your mistakes are:
When you drawing your background with glBlendFunc(GL_ONE, GL_ZERO);
As result you normaly draw it in framebuffer, but providing no needed blending operation, more effective on this pass is don't use blending at all. So, more effective is glDisable(GL_BLEND), but your pass work here like you expect.
At second pass you drawing with glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_SRC_COLOR, GL_ZERO); I don't know why you using so sofisticated function here and separatively blend colors and alpha values.
So, looking on third pass, I suppose you want to modify your background alpha value by your foreground black/white color mask. If It's true, you must use glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_ZERO, GL_SRC_COLOR); - Yep, little mistake.
And at third pass when you drawing foreground you have glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA), what means you wanna draw those regions, where your black/white mask was white and blending with attenuation for more darker mask regions.
If you have any questions about glBlendFunc, I can help you.