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The OpenGL Pipeline Newsletter - Volume 002

Table of Contents
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Next article: Climbing OpenGL Longs Peak – An OpenGL ARB Progress Update

“Clean” your OpenGL usage using gDEBugger

Cleaning up your application's OpenGL usage is the first step to optimize your application and gain better performance. In this article we will demonstrate how gDEBugger helps you verify that your application uses OpenGL correctly and calls the OpenGL API commands you expect it to call.

Remove OpenGL Errors

Removing OpenGL errors before starting to optimize your graphics application's performance is an important task. It is important because in most cases, when an OpenGL error occurs, OpenGL ignores the API call that generated the error. If OpenGL ignores actions that you want it to perform, it may reduce your application's robustness and dramatically affect rendering performance. The OpenGL error mechanism does not tell you the location of the error and therefore it is hard to track GL errors. gDEBugger points you to the exact location of the OpenGL error. It offers two mechanisms for tracking errors:

  1. gDEBuggerBreak on OpenGL Errors: Breaks the application run whenever an OpenGL error occurs.
  2. Break on Detected Errors: Tells gDEBugger's OpenGL implementation to perform additional error tests that OpenGL drivers do not perform. gDEBugger will also break the application run whenever a detected error occurs.

After the application run is suspended, the gDEBugger System Events view will display the error description. The Call Stack and Source Code viewers will show you the exact location of the error.

Remove Redundant OpenGL Calls

Most OpenGL-based applications generate a lot of redundant OpenGL API calls. Some of these redundant calls may have a significant impact on rendering performance. We offer a two-step solution for locating and removing these redundant calls:

  1. Use gDEBugger's OpenGL Function Calls Statistics view to get an overview of the last fully rendered frame. This view displays the number of times each API function call was executed in the previously fully rendered frame. You should look for:
    • functions that have a high percentage of the total API function executions.
    • functions that are known to reduce rendering performance: glGet* / glIs* functions, immediate mode rendering, glFinish, etc.
    • redundant OpenGL state changes: changing the same state over and over.
    • repeatedly turning on and off the same OpenGL mechanisms.
  2. gDEBuggerUse the Breakpoints dialog to break on each redundant API function call. When the application run breaks, use the OpenGL Function Calls History, Call Stack and Source Code views to view the call stack and source code that led to the redundant API call

This process should be repeated until the redundant calls that seem to have impact on rendering performance are removed.

Locate Software Fallbacks

When the application calls an OpenGL function or establishes a combination of OpenGL state that is not accelerated by the graphics hardware (GPU), the driver runs these functions on the CPU in "software mode." This causes a significant decrease in rendering performance. gDEBugger and NVIDIA GLExpert driver integration offers a mechanism that traces software fallbacks. Simply check the "Report Software Fallback Messages" and "Break on GLExpert Reports" check boxes in gDEBugger's NVIDIA GLExpert Settings dialog. In this mode, whenever a "Software Fallback" occurs, gDEBugger will break the debugged application run, letting you view the call stack and source code that led to the software fallback.

We hope this article will help you deliver "cleaner" and faster OpenGL based applications. In the next article we will discuss the use of ATI and NVIDIA Performance Counters together with gDEBugger's Performance Views for finding graphics pipeline performance bottlenecks.

Yaki Tebeka
CTO & Cofounder
Graphic Remedy

Editors Note: You'll remember from our last edition that Graphic Remedy and the ARB have teamed up to make gDEBugger available free to non-commercial users for a limited time.

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