Occlusion Query

Our window has 800 x 600 = 480.000 pixels, if we draw a triangle that looks like this:

That means only 1 / 8 of the window is covered by the triangle, that would be about 60.000 pixels.

In Main.cpp

void Render()

{

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

/* start query */

glBeginQuery(GL_SAMPLES_PASSED, query);

glUseProgram(program);

glBindVertexArray(vertexarray);

glDrawArrays(GL_TRIANGLES, 0, 3);

glBindVertexArray(0);

glUseProgram(0);

/* stop query */

glEndQuery(GL_SAMPLES_PASSED);

/* wait until OpenGL has the result */

GLint result = GL_FALSE;

while (result == GL_FALSE)

glGetQueryObjectiv(query, GL_QUERY_RESULT_AVAILABLE, &result);

/* get the result */

glGetQueryObjectiv(query, GL_QUERY_RESULT, &result);

cout << "sample count for 1 triangle: " << result << endl;

CheckForGLError();

}

Try to resize the window so that fewer pixels are draws, minimize the window to see how the number changes.