Draw from a separate thread with NSOpenGLLayer

Question

I'm working on an app which needs to draw with OpengGL at a refresh rate at least equal to the refresh rate of the monitor. And I need to perform the drawing in a separate thread so that drawing is never locked by intense UI actions.

Actually I'm using a NSOpenGLView in combination with CVDisplayLink and I'm able to achive 60-80FPS without any problem.

Since I need also to display some cocoa controls on top of this view I tried to subclass NSOpenGLView and make it layer-backed, following LayerBackedOpenGLView Apple example.

The result isn't satisfactory and I get a lot of artifacts.

Therefore I've solved the problem using a separate NSWindow to host the cocoa controls and adding this window as a child window of the main window containing the NSOpenGLView. It works fine and I'm able to get quite the same FPS as the initial implementation.

Since I consider this solution quite like a dirty hack, I'm looking for an alternative and more clean way of achiving what I need.

Few days ago I came across NSOpenGLLayer and I thought that it could be used as a viable solution for my problem.

So finally, after all this preamble, here comes my question: is it possible to draw to a NSOpenGLLayer from a separate thread using CVDisplayLink callback?.

So far I've tried to implement this but I'm not able to draw from the CVDisplayLink callback. I can only -setNeedsDisplay:TRUE on the NSOpenGLLayer from the CVDisplayLink callback and then perform the drawing in -drawInOpenGLContext:pixelFormat:forLayerTime:displayTime: when it gets automatically called by cocoa. But I suppose that this way I'm drawing from the main thread, isn't it?

After googling for this I've even found this post in which the user claims that under Lion drawing can occur only inside -drawInOpenGLContext:pixelFormat:forLayerTime:displayTime:.

I'm on Snow Leopard at the moment but the app should run flawlessly even on Lion.

Am I missing something?

Answer 1

Yes, it is possible, though not recommend. Call display on the layer from within your CVDisplayLink. This will cause canDrawInContext:... to be called and if it returns YES, drawInContext:... will be called and all this on whatever thread called display. To make the rendered image visible on screen, you have to call [CATransaction flush]. This method has been suggested on the Apple mailing list, though it is not completely problem free (the display method of other view may get called on your background thread as well and not all views support rendering from a background thread).

The recommend way is to make the layer asynchronous and render the OpenGL context on main thread. If you cannot achieve a good framerate that way, since your main thread is busy elsewhere, it is recommend to rather move everything else (pretty much your whole application logic) to other threads (e.g. using Grand Central Dispatch) and only keep user input and drawing code on the main thread. If your window is very big, you may still not get anything better than 30 FPS (one frame ever two screen refreshes), yet that comes from the fact, that CALayer composition seems a rather expensive process and it has been optimized for more or less static layers (e.g. layers containing a picture) and not for layers updating themselves 60 FPS.

E.g. if you are writing a 3D game, you are advised not to mix CALayers with OpenGL content at all. If you need Cocoa UI elements, either keep them separated from your OpenGL content (e.g. split the window horizontally into a part that displays only OpenGL and a part that only displays controls) or draw all controls yourself (which is pretty common for games).

Last but not least, the two window approach is not as exotic as you may think, that's how VLC (the video player) draws its controls over the video image (which is also rendered by OpenGL on Mac).