How to handle the fading out of several circles in openGL? - objective-c

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);
}