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();