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I've been experimenting with using multisampling to do full-scene anti-aliasing on the iPhone and iPad on iOS 4. The general mechanism uses Apple's APPLE_framebuffer_multisample extension (http://www.khronos.org/registry/gles/extensions/APPLE/APPLE_framebuffer_multisample.txt) and is described in the answer to this question: How do you activate multisampling in OpenGL ES on the iPhone? and documented by Apple in their OpenGL ES Programming Guide.

It works as described, but the drawing performance of my test application suffers by about 50% when I set the number of samples to be 2. I'm primarily testing on an iPhone 4, using a non-retina-enabled application. I am using the other performance suggestions offered by Apple in their documentation (using glDiscardFramebufferEXT to discard the renderbuffers attached to the multisample framebuffer, using glClear to clear the entire framebuffer at the start of the frame, etc.).

The performance overhead of enabling multisampling in this manner seems surprisingly large to me. Are you guys seeing similar results or does this suggest that I'm doing something incorrectly?

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BinaryStar
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2 Answers2

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You mentioned that you're running this on an iPhone 4. Is your OpenGL ES layer rendering at the full 2X Retina display scale factor? That is, have you set the contentScaleFactor on the OpenGL ES hosting layer to [[UIScreen mainScreen] scale]? If so, you're pushing a large number of pixels to start with.

Are you fill rate limited before you apply the multisampled antialiasing? To check, use the OpenGL ES instrument in Instruments against your running application and enable the Tiler Utilization and Renderer Utilization statistics. If your application shows a high Renderer Utilization without MSAA enabled, you are fill rate limited to begin with. Adding MSAA on top of that could significantly reduce your framerates because of this bottleneck.

In an application that I had which was geometry limited, not fill rate limited, I didn't see that great of a slowdown when using 4X MSAA in it on an iPhone 4. I'm guessing that the bottleneck in your application is in pushing pixels to the screen.

Brad Larson
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It is not surprising that your performance suffers by about 50% when you set the # of samples to 2: you're drawing twice the samples! Multisampling means you essentially draw your scene at a higher resolution than the screen to an off-screen buffer, and then you use filtering algorithms to reduce the higher resolution multi-sampled buffer to the display screen resolution, hopefully with fewer aliasing artifacts because the final picture actually includes more detail (filtered higher resolution output) than the single-sampled version.

It is a very common (if not the most common) performance problem in graphics: the more samples you draw, the slower you go.

Bogatyr
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