Drastic slowdown using layer backed NSOpenGLView

问题

I needed to display some Cocoa widgets on top of an NSOpenGLView in an existing app. I followed the example in Apple's LayerBackedOpenGLView example code. The NSOpenGLView is given a backing layer using:

[glView setWantsLayer:YES]

Then the Cocoa NSView with the widgets is added as a subview of the glView. This is basically working and is twice ad fast as my previous approach where I added the NSView containing the widgets to a child window of the window containing the glView (this was the other solution I found on the web).

There were two problems.

• The first is that some textures that I use with blending were no longer getting the blend right. After searching around a bit it looked like I might need to clear the alpha channel of the OpenGLView. This bit of code that I call after drawing a frame seems to have fixed this problem:

Code:

glColorMask(FALSE, FALSE, FALSE, TRUE); //This ensures that only alpha will be effected
glClearColor(0, 0, 0, 1); //alphaValue - Value to which you need to clear
glClear(GL_COLOR_BUFFER_BIT);
glColorMask(TRUE, TRUE, TRUE, TRUE); //Put color mask back to what it was.

Can someone explain why this is needed when using the CALayer, but not without?

• The second problem I don't have a solution for. It seems that when I pan to the part of the scene where problem is #1 was observed, the frame rate drops from something like 110 FPS down to 10 FPS. Again, this only started happening after I added the backing layer. This doesn't always happen. Sometimes the FPS stays high when panning over this part of the scene but that is rare. I assume it must have something with how the textures here are blended, but I have no idea what.

Any thoughts?

回答1:

I did figure out a workaround to the slowdown. The OpenGL view has a HUD (heads up display) view that goes on top of it. I had installed another NSView as a subview if it. Both the HUD and the subview have lots of alpha manipulation and for some reason that tickled a real slowdown in compositing the layers. I could easily instal this subview as a subview of the OpenGL view and when I did this everything sped up again. So although I don't fully understand the slowdown, I do have a good work around for it.

来源：https://stackoverflow.com/questions/6039636/drastic-slowdown-using-layer-backed-nsopenglview