Question

i’m trying to get a full screen “trail” effect similar to the one here: http://elenzil.com/progs/js1k

the traditional way to do this is to not clear the image at the beginning of rendering,
but instead blend on a big polygon that’s the color of the background and has low alpha.

drawing the polygon is pretty straight-forward in openGL ES:

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

GLfloat clearRectPts[] = {
                          -0.5f, -0.5f,
                          -0.5f,  0.5f, 
                           0.5f, -0.5f,
                           0.5f,  0.5f,
                         };
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glColor4f(0.0f, 0.0f, 0.0f, 30.0f / 255.0f);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_FLOAT , 0, clearRectPts);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glDisableClientState(GL_VERTEX_ARRAY);
glDisable(GL_BLEND);

but there’s a Gotcha,
which is that current gl operations happen to the backbuffer,
which is stale in comparison to the front buffer.
so you end up with “stuttering” images.

before i go down the road of rendering to an offscreen texture and such,
is there a known way of doing this ?

the obvious solutions i’d love to find are any of these:

• copy the front buffer to the back buffer at the start of rendering.
• instead of swapping buffers, copy the backbuffer to the front.
• run single-buffered.

Answer 1

figured it. kEAGLDrawablePropertyRetainedBacking needs to be TRUE, as in the following:

        eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [NSNumber numberWithBool:TRUE],
                                    kEAGLDrawablePropertyRetainedBacking,
                                    kEAGLColorFormatRGBA8,
                                    kEAGLDrawablePropertyColorFormat,
                                    nil];