Question

I’m making an iPhone game that involves the use of GL_POINT to render a point. However, when the center of the point is off screen, I still want to draw whatever portion of the point that is still onscreen. Here is my code that I’m using to render the point.

-(void)render {
    if (!fill || !outline || !active || dead) 
        return;

    NSLog(@"rendering");

    glPushMatrix();
    glLoadIdentity();
    glMultMatrixf(matrix);

    glEnable(GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable(GL_SMOOTH);
    glEnable(GL_POINT_SMOOTH);

    glPointSize(scale.x*2);

    [outline render];

    glPointSize(2*(scale.x-kLineWidth));

    [fill render];

    glPopMatrix();
}

note that it logs "rendering" when it should be rendering, so this method is getting called properly.

and the [outline render] and [fill render] methods look like this

-(void)render {
    // load arrays into the engine
    glVertexPointer(vertexSize, GL_FLOAT, 0, vertexes);
    glEnableClientState(GL_VERTEX_ARRAY);
    glColorPointer(colorSize, GL_FLOAT, 0, colors); 
    glEnableClientState(GL_COLOR_ARRAY);

    //render
    glDrawArrays(renderStyle, 0, vertexCount);

    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);
}

and I’m using a “panning” effect using this code

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrthof(-kScreenWidth/2.0 + xPan, kScreenWidth/2.0 + xPan, -kScreenHeight/2.0 + yPan, kScreenHeight/2.0 + yPan, -1.0f, 1.0f);

glMatrixMode(GL_MODELVIEW);

but when the point’s center is not on the screen (after panning with glOrthof), the whole point is not drawn. How can I have the point still render even when the center is not on the screen?

Answer 1

I don’t believe there is anything you can do for an easy fix. Primitives are clipped before rasterization, so if that point lies outside the view frustum, it’s not going to be rasterized, even if the rasterization would create fragments that do lie inside the view frustum.

Either switch to real quads with GL_TRIANGLES/GL_QUADS, or if you really don’t want to do that, you can render your points to an offscreen buffer with size slightly larger than the viewport, and then blit the center of that image back onto the main frame.