Alpha value in ClearColor

OpenGL OpenGL: Basic Coding
1

Hi!

Somehow I am not able to use ClearColor with transparency while primitives make no problems in this context so far. Here is some example:


void display()
{
	glClearColor(1,0,0,1);
	glClear(GL_COLOR_BUFFER_BIT);
	glClearColor(0,1,0,0);
	glClear(GL_COLOR_BUFFER_BIT);
	glPointSize(20);
	glColor4d(1,1,1,0.5);
	glBegin(GL_POINTS);
	glVertex2d(0,0);
	glEnd();
	glFlush();
}

In the main function I used


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

to enable transparency. Now this code yields a green window with an opaque white square. But the alpha value of the green ClearColor call is 0, so the window should stay red.

Thanks for help!

2

No it shouldnt stay red. The function glClear sets the surface of GL to whatever you asked for. GL_BLEND has no effect on it.

3

Thanks! But I am still a bit confused, in which situation can the alpha value of ClearColor be used then?

4

Its used. glClear sets all the contents of scissored area to specified color (lets ignore color masks for now). If you want to blend, draw a quad.

Something like this should be ok for you (assuming identity matrices & no depth test):


glClearColor(0,1,0,0);
glRectf(-1,-1,+1,+1);

Clear (at least logically) is something entirely different then drawing a quad.