Greetings,

I have tone mapping working in my app, and I'd like to add multisampling. So that I may perform tone mapping before the MSAA resolve, I'm doing my own custom MSAA resolve by a) binding the multisampled texture and rendering a full-screen quad, b) using texelFetch to grab the individual samples, tone map them, and then blend them.

When I do this, it looks great, but it takes a big wet bite out of my render times (about 3ms). I don't think it's the tone mapping operator, because even if I only grab one sample instead of all 4, the performance seems about the same. This is for a 1920x1080 buffer on my GTX 680.

If 3ms is typical, then I'm okay with it, but since it's a good 20% of my frame time at 60Hz, I want to make sure it's not excessive. Obviously I can't afford to give everything 3ms. I'm using OpenSceneGraph, and so it can be difficult to tell what exactly is going on in terms of the underlying OpenGL, but I'm currently running it through gDEBugger to see what I can find. If you can think of anything I should be looking for, let me know.

Thanks, guys.

If it legitimately does take ~3ms, is that a price you would pay for doing (say) tone mapping before the resolve?

I have a little more data. Today, I set it up to run normal (non-explicit) MSAA that just blits the multisampled texture to a texture, and then renders that texture to the back buffer, so that I could do a performance comparison. With a scene of perhaps medium-low complexity, running simple shaders (forward-rendered), tone mapped, but no other post-processed effects, I have the following render times:

No MSAA: 1.89ms

4x MSAA (standard): 2.67ms

4x MSAA (explicit): 4.69ms

Again, this is at 1080p with a GTX 680.

So, about a 0.77ms difference with just plain MSAA and a 2.81ms difference for explicit MSAA, which means that the explicit MSAA is costing me an extra 2ms. Oddly, this number goes down slightly (1.79ms) if I reduce the scene complexity a bit. This is somewhat alarming, because I don't see how scene complexity can affect the MSAA resolve (I'm blending all four pixels regardless of whether they're on an edge or not; maybe the default resolve does something smarter).

So, I don't know. I don't have much of a choice, it seems. If I want to do tone mapping and gamma correction correctly, explicit multisampling seems to be the way to go. The best I can do is profile and make sure that I'm optimizing my app/shader code. *shrug*

I have a little more data. Today, I set it up to run normal (non-explicit) MSAA that just blits the multisampled texture to a texture, and then renders that texture to the back buffer, so that I could do a performance comparison. With a scene of perhaps medium-low complexity, running simple shaders (forward-rendered), tone mapped, but no other post-processed effects, I have the following render times:

No MSAA: 1.89ms

4x MSAA (standard): 2.67ms

4x MSAA (explicit): 4.69ms

Again, this is at 1080p with a GTX 680.

So, about a 0.77ms difference with just plain MSAA and a 2.81ms difference for explicit MSAA, which means that the explicit MSAA is costing me an extra 2ms. Oddly, this number goes down slightly (1.79ms) if I reduce the scene complexity a bit. This is somewhat alarming, because I don't see how scene complexity can affect the MSAA resolve (I'm blending all four pixels regardless of whether they're on an edge or not; maybe the default resolve does something smarter).

So, I don't know. I don't have much of a choice, it seems. If I want to do tone mapping and gamma correction correctly, explicit multisampling seems to be the way to go. The best I can do is profile and make sure that I'm optimizing my app/shader code. *shrug*

Can always just ditch MSAA. Give alternate AA techniques a look, personally I'm a fan of Crytek's implementation of SMAA: http://www.crytek.com/download/Sousa_Graphics_Gems_CryENGINE3.pdf

Shouldn't take much more than your standard MSAA results on a 680, and looks about as good I'd say. Plus if you're using deferred you don't have to worry about fiddling with transparencies, as it's essentially all post.

That makes a lot of sense, thank you. It might be worth playing with some edge detection, so that I'm only doing the full blend on edge pixels. I'm not sure that the performance savings will be worth the overhead, but it will be a fun experiment, anyway.