Question

I have a simple iPhone application which uses OpenGL ES (v1) to draw a line based on the touches of the user. In the XCode Simulator, the code works perfectly. However, when I install the app onto an iPod or iPhone, the OpenGL ES view “flashes” when drawing the line. If I disable the line drawing, the flash disappears. By “flash”, I mean that the background image (which is an OpenGL texture) disappears momentarily, and then reappears. It appears as if the entire scene is completely erased and redrawn.

The code which handles the line drawing is the following:

renderLineFromPoint:(CGPoint)start toPoint:(CGPoint)end
{
    static GLfloat*       vertexBuffer = NULL;
    static NSUInteger vertexMax = 64;
    NSUInteger            vertexCount = 0,
    count,
    i;

    //Allocate vertex array buffer
    if(vertexBuffer == NULL)
        vertexBuffer = malloc(vertexMax * 2 * sizeof(GLfloat));

    //Add points to the buffer so there are drawing points every X pixels
    count = MAX(ceilf(sqrtf((end.x - start.x) * (end.x - start.x) + (end.y - start.y) * (end.y - start.y)) / kBrushPixelStep), 1);
    for(i = 0; i < count; ++i) {
        if(vertexCount == vertexMax) {
            vertexMax = 2 * vertexMax;
            vertexBuffer = realloc(vertexBuffer, vertexMax * 2 * sizeof(GLfloat));
        }

        vertexBuffer[2 * vertexCount + 0] = start.x + (end.x - start.x) * ((GLfloat)i / (GLfloat)count);
        vertexBuffer[2 * vertexCount + 1] = start.y + (end.y - start.y) * ((GLfloat)i / (GLfloat)count);
        vertexCount += 1;
    }

    //Render the vertex array
    glVertexPointer(2, GL_FLOAT, 0, vertexBuffer);
    glDrawArrays(GL_POINTS, 0, vertexCount);

    //Display the buffer
    [context presentRenderbuffer:GL_RENDERBUFFER_OES];
}

(This function is based on the function of the same name from the GLPaint sample application.)
For the life of me, I can not figure out why this causes the screen to flash. The line is drawn properly (both in the Simulator and in the iPod). But, the flash makes it unusable.

Anyone have ideas on how to prevent the “flash”?

Answer 1

After much testing, I have found a solution to this issue, although I’m not sure if it’s a best-practice.

The solution was changing the kEAGLDrawablePropertyRetainedBacking from NO to YES. This stopped the flashing. Specifically, the code to setup the OpenGL view now looks like this:

CAEAGLLayer *eaglLayer = (CAEAGLLayer*) self.layer;
eaglLayer.opaque = YES;
eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
                                [NSNumber numberWithBool:YES], 
                                kEAGLDrawablePropertyRetainedBacking, 
                                kEAGLColorFormatRGB565, kEAGLDrawablePropertyColorFormat, nil];

Again, I’m not sure if it’s a best practice to use the RetainedBacking strategy, but it stopped the flashing. According to http://developer.apple.com/iphone/library/documentation/iPhone/Reference/EAGLDrawable_Ref/EAGLDrawable/EAGLDrawable.html#jumpTo_6 one should set RetainedBacking to YES only in certain circumstances.

Setting the value to YES is recommended only when you need the content to remain unchanged, as using it can result in both reduced performance and additional memory usage.

It’s not clear how the performance or memory is impacted, but this one change completely stopped all “flashing” that I was seeing.