How do I set the opacity of a vertex in OpenGL?

Question

The following snippet draws a gray square.

glColor3b(50, 50, 50);

glBegin(GL_QUADS);
glVertex3f(-1.0, +1.0, 0.0); // top left
glVertex3f(-1.0, -1.0, 0.0); // bottom left
glVertex3f(+1.0, -1.0, 0.0); // bottom right
glVertex3f(+1.0, +1.0, 0.0); // top right
glEnd();

In my application, behind this single square exists a colored cube.

What function should I use to make square (and only this square) opaque?

Answer 1

In the init function, use these two lines:

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

And in your render function, ensure that glColor4f is used instead of glColor3f, and set the 4th argument to the level of opacity required.

glColor4f(1.0, 1.0, 1.0, 0.5);

glBegin(GL_QUADS);
glVertex3f(-1.0, +1.0, 0.0); // top left
glVertex3f(-1.0, -1.0, 0.0); // bottom left
glVertex3f(+1.0, -1.0, 0.0); // bottom right
glVertex3f(+1.0, +1.0, 0.0); // top right
glEnd();