Question

I’m programming a nice little game that uses shader generated simplex noise for displaying on the fly computed random terrain.

I’m using Objective-C and Xcode 4 and I have gotten everything to run nicely using a subclass of NSOpenGLView. The subclass first compiles the shader and then renders a quad with the noise texture. The program has no problems running this at an acceptable speed (60Hz).

The subclass of NSOpenGLView uses an NSRunLoop to to fire a selector which in turn calls the drawRect:(NSRect)dirtyRect. This is done every frame.

Now, I want the shader to use a uniform that is updated each frame.
The shader should be able to react to a variable change that might occur every frame thus I’m trying to update the uniform at this frequency. The update of the uniform is done in the drawRect:(NSRect)dirtyRect function.

I am partially successful. The screen updates exactly as I’d like for the first 30 frames, then it stops updating the uniform even though I have a call to glUniform1f() and NSLog right next to each other and the NSLog always fires..!

The strange part is that if a hold space pressed (or any other key for that matter) the uniform is updated as it should be.

Clearly I am missing something here in regards to how OSX or OpenGL or something else handles uniforms.

An explanation of what might be ailing me would be appreciated but a pointer to where I can find information about this will suffice.

Update: After fiddling with glGetError() and glGetUniform*() I’ve noticed that the program works as intended when left alone. However, when I use the trackpad for input the uniform is reset to 0.000 while the rest of the program shows no errors.

Answer 1

First, have you tried calling glGetError() right after glUniform1f() to see what comes out?

I have a very limited knowledge of Mac OS programming (I did some iOS programming two years ago and forgot most of it since then) so the following is a wild guess.

Are you sure drawRect:(NSRect)dirtyRect is called by the same thread as the thread that owns the OpenGL context? As far as I know, OpenGL is not thread safe, so it could be that your glUniform1f() attempts are called from a different thread thus being unable to do anything.