Question

I need to draw small circles on a frame and make them to disappear after few frames (fading out). How can I achieve this with OpenGL ES?

This is what I have so far, the code to draw the circle in a frame:

 [EAGLContext setCurrentContext:context];
    glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
    glViewport(0, 0, backingWidth, backingHeight);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);
    glMatrixMode(GL_MODELVIEW);

    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT);

    GLfloat vertices[720];

    for (int i = 0; i < 720; i += 2) {
        vertices[i]   = (cos(DEGREES_TO_RADIANS(i)) * 1);
        vertices[i+1] = (sin(DEGREES_TO_RADIANS(i)) * 1);
    }

    glVertexPointer(2, GL_FLOAT, 0, vertices);
    glEnableClientState(GL_VERTEX_ARRAY);

    glColor4f(1.0f, 1.0f, 0.0f, 1.0f);

    glDrawArrays(GL_TRIANGLE_FAN, 0, 360);

    glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
    [context presentRenderbuffer:GL_RENDERBUFFER_OES];

What I don’t understand is how to keep track of the same circle and fade it in the successive frames.
Thanks

Answer 1

You will need to re-render all objects (circles) on every frame. So you need to keep around a data structure which holds the information about the circles (probably their center, radius, and opacity would be sufficient). Something like this:

typedef struct Circle {
    NSPoint center;
    float radius;
    float opacity;
} Circle;

Circle allCircles [ kMaxNumCircles ];

Then, when you go to draw them, you iterate over the array of circles, updating their opacity and then drawing them:

for (int i = 0; i < kMaxNumCircles; i++)
{
    allCircles [ i ].opacity -= someDelta;
    drawCircle (allCircles [ i ]);
}

The drawCircle() function could just be what you’ve written above:

void drawCircle(Circle c)
{
    GLfloat vertices[720];

    for (int i = 0; i < 720; i += 2) {
        vertices[i]   = (cos(DEGREES_TO_RADIANS(i)) * 1) + c.center.x;
        vertices[i+1] = (sin(DEGREES_TO_RADIANS(i)) * 1) + c.center.y;
    }

    glVertexPointer(2, GL_FLOAT, 0, vertices);
    glEnableClientState(GL_VERTEX_ARRAY);

    glColor4f(1.0f, 1.0f, 0.0f, c.opacity);

    glDrawArrays(GL_TRIANGLE_FAN, 0, 360);

}