OpenGL Programming Guide

Chapter 7Blending, Antialiasing, and Fog

Blending

A Blending Example

Example 7-1 draws four overlapping colored rectangles - each with an alpha of 0.75 - so that the lower left and upper right quadrants of the window are covered twice. In these two quadrants, the colors are blended in different orders using source and destination blending factors of GL_SRC_ALPHA and GL_ONE_MINUS_SRC_ALPHA: In the lower left quadrant, cyan is blended with the original yellow; in the upper right quadrant, yellow is blended with the original cyan. The other quadrants are drawn with unblended colors.

Example 7-1 : A Blending Example: alpha.c

#include <GL/gl.h>
#include <GL/glu.h>
#include "aux.h"

void myinit(void)
{
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glShadeModel(GL_FLAT);
    glClearColor(0.0, 0.0, 0.0, 0.0);
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);

    glColor4f(1.0, 1.0, 0.0, 0.75);
    glRectf(0.0, 0.0, 0.5, 1.0);

    glColor4f(0.0, 1.0, 1.0, 0.75);
    glRectf(0.0, 0.0, 1.0, 0.5);

/* draw colored polygons in reverse order in upper right */
    glColor4f (0.0, 1.0, 1.0, 0.75);
    glRectf (0.5, 0.5, 1.0, 1.0);

    glColor4f (1.0, 1.0, 0.0, 0.75);
    glRectf (0.5, 0.5, 1.0, 1.0);

    glFlush();
}

void myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    if (w <= h) 
        gluOrtho2D (0.0, 1.0, 0.0, 1.0*(GLfloat)h/(GLfloat)w);
    else 
        gluOrtho2D (0.0, 1.0*(GLfloat)w/(GLfloat)h, 0.0, 1.0);
    glMatrixMode(GL_MODELVIEW);
}


int main(int argc, char** argv)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA);
    auxInitPosition (0, 0, 500, 500);
    auxInitWindow (argv[0]);
    myinit();
    auxReshapeFunc (myReshape);
    auxMainLoop(display);
}

As you probably expected, the order in which the rectangles are drawn affects the resulting colors. In the lower left quadrant, the cyan rectangle becomes the source fragment that's blended with the yellow rectangle, which is already in the framebuffer and thus is the destination. In the upper right quadrant, the yellow rectangle is the source and the cyan one the destination. Because the alpha values are all 0.75, the actual blending factors become 0.75 for the source and 1.0 - 0.75 = 0.25 for the destination. In other words, the source rectangle is somewhat translucent, but it has more effect on the final color than the destination rectangle. As a result, the lower left quadrant is light cyan, and the upper left one is light yellow. If you do the arithmetic, you'll find that the lower left RGB color is (0.25, 1.0, 0.75) and the upper right color is (0.75, 1.0, 0.25).

OpenGL Programming Guide