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Glass_Knife

OpenGL
Debugging Graphics

23 posts in this topic

I am curious what you have in your bag of tricks for debugging 3d graphics.  I know it is a broad question, but I'm hoping there are some really good ideas out there floating around in the ether that I haven't stumbled upon yet.

 

I see this with beginners a lot.  Most of the problems in a beginning OpenGL class were students putting everything into the scene, and then nothing shows up.  To me, it is a "black screen of death".  Where is my model?  There aren't any errors, and no way to really figure out what's wrong.  But that feels like guessing, and debugging by guessing never works out.

 

For example, when I was writing a software renderer from scratch, I was trying to clip in homogeneous coordinates.  But I didn't really understand the math, so when I tried out the clip code I got an array index out of bound exception (this was in Java).  Well great.  I had no idea how to fix that.  I ended up setting up four views: top, left, front, and perspective, and then drawing the view frustum, hand clipping all the lines so nothing would crash.  Then I could see what was happening.  It ended up being a sign error in the near clipping plane.  It was easy to fix, but it took two weeks of side work to find it.

 

What do you do when you load up a scene and nothing is there?

Edited by Glass_Knife
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Now, thats kind of a bare-bones debugging experience. There are much better tools today -- like PIX, Visual Studio Graphics Diagnostics, and Valve's similar upcoming OpenGL tools -- that let you inspect the goings-on of the graphics pipeline in great detail by capturing all your draw commands together with all the data. For example, this topic is the same black-screen-of-death style problem, and the tools make it pretty straight forward to investigate and discover the problem.

 

I imagined that having some kind of tool would be necessary to really get in there and figure out what is going on.  In my example I wrote my own graphical debugging framework so I could play with the problem and figure out what was happening.  Maybe I just haven't put enough time into my code to add  debugging capabilities.  I can think of lots of things that could be done to inspect the pipeline and find problems.

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Obviously tools are great, if they work. Fixing the "where's my triangle?" problem can be hard no matter what, and I think that's a reflection on the complexity of the hardware and the API's.

 

Having a quick loop for recompiling shaders can be really nice for debugging, especially if you can hot-swap them while your program is running. It can make it very easy to quickly isolate problems without having to step through a debugger line by line.

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Don't forget the glValidateProgram function, call this right before you do the drawcalls and write away the output somewhere.
Useful for if your shader linked correctly, but doesn't produce expected output (eg. a black screen of doom).

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when i started development with cuda i discovered nvidia nsight, i've never used it for opengl debugging but it looks very promising :)

 

https://www.youtube.com/watch?v=HAm5ziXE6pA

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AMD have CodeXL aswell, it's a visual studio addin, lets you debug your OpenGL states as though you are using the VS debugger(almost). Really helpful. Aside from that, I have a checkError function that if I create  a debug build (no optimisations) it will call glGetError and log the results with a timestamp and a little information about where it was called, what the state of the program at that point was. I sprinkle it liberally around problem errors, and run my app for a little while, and parse my logs after. I normally write it to a stringstream and on calling the destructor for the singleton I dump the stringstream to a file, formatted.

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For geometry problems I absolutely love OpenGL's immediate mode. It's so easy to render a couple of lines or points to show, where the object, coordinate system, light source, particle effect, ... should be. You can also encode states or properties as colors if you are trying to find out, why a problem only occures for a small subset of entities. Being able to render text at a 3D location can also be extremely beneficial.

If it's shader related, implement hot-swapable shaders like MJP suggested, and output "debug colors" to circle in on the problem. Having had shader debuggers at work and hot swapping in my private engine I would take hot swapping over a shader debugger almost every time.

Also, seeing the API calls not as code but as a list like in apitrace or PIX can give a second perspective and move things out of your blind spot.

What Buckeye said (reverting the last change) obviously plays well with lots of small version control commits.
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If it's shader related, implement hot-swapable shaders like MJP suggested, and output "debug colors" to circle in on the problem. Having had shader debuggers at work and hot swapping in my private engine I would take hot swapping over a shader debugger almost every time.

 

This is a new idea to me.  Can you give me an example of how debug colors work.  It sounds fascinating, but I don't see the benefit.

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To address what issues a beginner/student might make, here is a list of things to do to find silly mistakes:

 

Voted up, mostly because in my career (working on a wide range of platforms), I think I've failed at every one of these. biggrin.png

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If it's shader related, implement hot-swapable shaders like MJP suggested, and output "debug colors" to circle in on the problem. Having had shader debuggers at work and hot swapping in my private engine I would take hot swapping over a shader debugger almost every time.

 

This is a new idea to me.  Can you give me an example of how debug colors work.  It sounds fascinating, but I don't see the benefit.

 

 

Many times when optimizing I draw difference of naive version and optimized version. This way I can spot if I have make silly mistakes with math. This also work well when replacing math with aproximations.

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If it's shader related, implement hot-swapable shaders like MJP suggested, and output "debug colors" to circle in on the problem. Having had shader debuggers at work and hot swapping in my private engine I would take hot swapping over a shader debugger almost every time.

 

This is a new idea to me.  Can you give me an example of how debug colors work.  It sounds fascinating, but I don't see the benefit.

 

 

Let's say that you suspect a certain value is ending up being less than 0, even though that shouldn't be the case. You can check the value in the shader, and set the color to bright if it's less than 0. You can also very easily disable large sections of complicated code by hard-coding a pixel shader output color, which can be very helpful for narrowing down the location of your bug.

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If it's shader related, implement hot-swapable shaders like MJP suggested, and output "debug colors" to circle in on the problem. Having had shader debuggers at work and hot swapping in my private engine I would take hot swapping over a shader debugger almost every time.

 

This is a new idea to me.  Can you give me an example of how debug colors work.  It sounds fascinating, but I don't see the benefit.

 

 

Let's say that you suspect a certain value is ending up being less than 0, even though that shouldn't be the case. You can check the value in the shader, and set the color to bright if it's less than 0. You can also very easily disable large sections of complicated code by hard-coding a pixel shader output color, which can be very helpful for narrowing down the location of your bug.

 

 

Ah, like a visual debug statement.  I hadn't thought of that.  Got it.

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* Wrap glGetError in a function that can print the error enum and a custom string and call it before and after tricky code, e.g.: CheckGLErrors( "DrawShadows begin" ) and CheckGLErrors( "DrawShadows end" )

* Reverse triangle winding order

* Output debug colors in a shader

* Use a frame debugger (eg. Crytek's RenderDoc, PIX or Xcode Instruments)

* Give your state objects debug labels

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For some kind of errors, using some kind of intercepting tool can be pretty useful. In the past I have used both glIntercept (https://code.google.com/p/glintercept/) and gDEBugger (http://developer.amd.com/tools-and-sdks/archive/amd-gdebugger/).

 

I also have some builds of my applications which, at every frame, check the date of the shader files and, if they are newer than the ones that were loaded (I store a timestamp in my Shader class) they are loaded again.

 

This way I can have the application in one screen, the shaders in Notepad++ in the other, and everytime I save in Notepad++ I see the results in the application. For tuning shaders this is awesome. It makes the application somewhat more unstable and of course it is not efficient, so I would never leave this in my production release...

 

EDIT:

Oh, I also have a macro for checking glError. It is a macro called __GL which is blank in Release mode and in Debug it basically calls a function that calls glError, and asserts that the error is GL_NONE. When I am desperate enough I just fill the code with calls to that macro in the sections I suspect.

 

And finally, the new OpenGL 4.1 (I believe) comes with the Debug context, which is a God-sent. More info in: http://www.altdevblogaday.com/2011/06/23/improving-opengl-error-messages/

Edited by Javier Meseguer de Paz
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