Debugging my render code

OpenGL Development JOGL Development
#1

I’m currently going crazy trying to work out why an object I think I should be rendering can’t be seen. Off the top of my head it might be too small, on the wrong side of one of 6 clip planes, or behind something nearer and hence killed by the z-buffer.

I could extract the various matrices etc. from the GL object and compute the projected positions of the vertices, but that sounds like a lot of work with potential for its own subtle bugs, so I thought it would be worth asking whether anyone knows of a library or GL call which would allow me to call a method to reset counters, then later ask how many pixels have been drawn to, how many faces have been clipped entirely away, etc.

Any other suggestions also welcomed.

#2

I’ve had a similar problem before. In my case I think I messed up setting the clipping planes(put zNear where zFar should have been).
A good idea if your IDE doesn’t have a good debugger is to rig your spacebar or enterkey to use a series of System.out.println statements such as:

Camera| X: # Y:# Z:#
Object| X: # Y:# Z:#
Object Scale| X:# Y:# Z:#

I’m sure I recapped things you’ve already checked for but I figured no one else responded so I might as well.
but as for GL commands that return that kind of information, to be honest I’m not sure.

#3

If that’s the debugging approach you want, try looking into the feedback and selection render modes for opengl. However, I don’t recommend this because they seem complicated to use and interpret.

An approach that works for me, is to use a TraceGL to print out all of the gl commands I execute per-frame. Then I copy these into a simple GLEventListener (and fix up the syntax) and start tweaking from there to get the test case working.

Some other options as to why it’s not appearing include: you have lighting enabled but individual lights are not enabled (so it’d be black on a black background), or you it might be culled in appropriately on the CPU if you’re doing frustum culling yourself. I find it useful to use a gray background so that an object rendered as black (because of bad textures/shaders/ or gl state mishaps, etc.) is still visible.

#4

I did in fact manage to get that particular problem fixed by finding some code elsewhere in the project which I could refactor to get the transform I wanted. It’s cargo cult programming at its finest, but.

The feedback selection mode does look interesting. If I run into similar problems in the future I will try using it, although I suspect that it might not be helpful if the problem is due to the object being outside the clip frustrum.