Alpha transparency with particle effects in OpenGL

Question

I have a simple particle effect in OpenGL using GL_POINTS. The following is called, being passed particles in order from the particle furthest from the camera first to the one nearest the camera last:

void draw_particle(particle* part) {
    /* The following is how the distance is calculated when ordering.
     * GLfloat distance = sqrt(pow(get_camera_pos_x() - part->pos_x, 2) + pow(get_camera_pos_y() - part->pos_y, 2) + pow(get_camera_pos_z() - part->pos_z, 2));
     */

    static GLfloat quadratic[] = {0.005, 0.01, 1/600.0};
    glPointParameterfvARB(GL_POINT_DISTANCE_ATTENUATION_ARB, quadratic);
    glPointSize(part->size);
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable(GL_POINT_SPRITE_ARB);
    glEnable(GL_TEXTURE_2D);

    glBegin(GL_POINTS);
        glColor4f(part->r, part->g, part->b, part->a);
        glVertex3f(part->pos_x, part->pos_y, part->pos_z);
    glEnd();

    glDisable(GL_BLEND);
    glDisable(GL_POINT_SPRITE_ARB);
}

However, there is some artifacting when rendering as can be seen in the following effect:artifacted image http://img199.imageshack.us/img199/9574/particleeffect.png

The problems go away if I disable depth testing, but I need the effects to be able to interact with other elements of the scene, appearing in front of and behind elements of the same GL_TRIANGLE_STRIP depending on depth.

Answer 1

If your particules are already sorted you can render like this :

• Render particule with GL WRITE DEPTH but no depth testing (I don't remember exactly the constants)
• Render the rest of the scene with depth test.

This way you are sure to get scene interaction with nice-looking particules.