Rob Jenkins (robj++at++quid)
Mon, 2 Jun 1997 08:27:25 -0700
ogldebug ( or gldebug for Iris GL ) is very useful, attached is a Pipeline
article with suggestions and references for use.
Cheers
Rob
-- ________________________________________________________________ Rob Jenkins mailto:robj++at++csd.sgi.com Silicon Graphics, Mtn View, California, USA
---------------------------------------------------------------------------- PIPELINE ARTICLE 19970207 Ogldebug: An Overview
Published: March/April 1997 in Pipeline Volume 8, Number 2 Revision: 1.1 ----------------------------------------------------------------------------
Ogldebug(3G) is a tool that allows a programmer to capture and examine the underlying OpenGL calls of selected sections of an application.
This article presents a brief overview of ogldebug, some of its practical uses, and some of the features added to ogldebug recently. This article is intended for programmers who are already familiar with OpenGL.
System Requirements
Ogldebug will run on all SGI systems with graphics that support OpenGL. Ogldebug is part of the subsystem gl_dev.sw.ogldebug on the IRIS Development Option (IDO) CD-ROM. This subsystem is not installed by default.
The IRIS GL equivalent of ogldebug is gldebug(1) which is part of the subsystem gl_dev.sw.gldebug on the IRIS Development Option (IDO) CD-ROM. This subsystem is not installed by default.
Purpose
Because only OpenGL calls are captured by ogldebug, it is extremely useful for identifying and solving OpenGL programming errors. It is also very good at uncovering OpenGL programming style issues that may slow an application. Note that the OpenGL debugger isn't a general-purpose debugger. Use dbx(1) and related tools such as cvd(1) (CASEVision/Workshop Debugger) to find problems in the non-OpenGL portions of a program. Ogldebug has a user interface control application (ogldebug) and a special library (libogldebug.so) that intercepts OpenGL or GLX calls from the application to the OpenGL library (libGL.so).
Using Ogldebug
The manual OpenGL on Silicon Graphics Systems is available on-line with InSight(1) if the subsystem gl_dev.books is installed. Chapter 10 of this manual provides information on using ogldebug to debug OpenGL programs. Rather than duplicate that information here, this article assumes familiarity with that manual and suggests some practical uses for ogldebug. There are some changes to the latest version of ogldebug which make it slightly different from the version described in the on-line manual. Although the changes represent improvements in performance, user interaction and functionality, the core of the tool and principles of use remain close to those documented. Changes to ogldebug are outlined below, and should be used in conjunction with the on-line manual.
New Features
The most recent version of ogldebug is version V14.0 which is available on the MIPSPro 7.1 IRIX Development Option CD-ROM. The majority of the changes in this version of ogldebug are in the user interface. These changes include:
o smaller main window o call information is now separated o only one call is displayed in the main window o a new call history panel is provided where the programmer can adjust how much of the call history to retain (10 to 1000 calls) o only one main window appears for all application GLX contexts (previously, ogldebug opened a new main window for each GLX context)
In addition, a significant improvement has been made in call history. In previous versions of ogldebug, call history updating could be suspended to allow an application to run faster. In the latest version, call history is always produced, and if the call history panel is not open the application still runs reasonably fast.
A number of minor bug fixes have also been made to ogldebug. Refer to the gl_dev release notes for more information.
When to Use Ogldebug
This section describes a few of the many possible uses for ogldebug.
To Improve Application Performance
The manual OpenGL on Silicon Graphics Systems contains two chapters on tuning: Chapter 11, "Tuning Graphics Applications: Fundamentals" and Chapter 12, "Tuning the Pipeline". Ogldebug can be used as an aid to performing some of the techniques suggested in those chapters. For example, ogldebug can be used to look for frequent OpenGL mode changes, or unnecessary vertex bindings. This could include things like unneeded per-vertex colors, or normals being defined for a flat-shaded object. In extreme cases it may be found that operations are repeated, or large parts of the scene are drawn more than once by mistake.
To Isolate Programming Errors
OpenGL never signals errors but simply records them. Within an application, glGetError(3G) can be used to look for errors, or ogldebug can be used to automatically call glGetError after every OpenGL call. Ogldebug also attempts to highlight programming style issues that although correct, may be inefficient. For example, if an application loads an identity matrix with glLoadMatrix(3G), ogldebug will suggest using glLoadIdentity(3G) instead. Although the two operations are semantically equivalent, glLoadIdentity can be more efficient in some cases.
To Isolate Parts of an Application
Isolating parts of a large application can help in narrowing down potential bugs and in analyzing the performance of discrete stages in an application. Tracing the very first frame of an application is most likely to show a majority of the graphics state initialization and selection. This would be useful for examining and tuning operations that happen only once at program start time. Tracing a later frame would be useful for examining the application during normal drawing.
The following steps can be used to examine one frame of a double buffered application in an ogldebug session:
1. Run ogldebug with the application.
2. Turn breakpoints and output off from the control panel.
3. Set a breakpoint at glXSwapBuffers(3G).
4. Click on the "Continue" button and manipulate the application to the area of interest.
5. Turn on breakpoints.
6. When the execution stops at glXSwapBuffers, turn on tracing.
7. Click on the "Continue" button
8. When execution stops at the next glXSwapBuffers, one full scene frame of OpenGL calls will have been written to a trace file.
The ogldebug trace file is legal C code and may be compiled and executed. To compile the trace file, change the file name to <someName>.c and the use the command:
cc <someName>.c -lGL -lX11
It might be useful to experiment with customizing the trace output to reduce the size of the trace file by omitting commands that are not of interest or that are repeated often. For example, a scene with geometry generates a lot (potentially thousands), of glVertex(3G) commands each frame, and it may not be useful to trace them all.
The process described above can be applied to any application that calls OpenGL directly or that makes OpenGL calls via a library, including SGI libraries like IRIS Performer, Open Inventor or Imagevision.
To Generate Benchmarks
It is possible to create test cases to represent the graphics workload for different stages of an application by tracing parts of the application.
When Opening a Support Call
The techniques discussed in this article can be used to create a small test case. Providing a test case is extremely helpful when a support call on OpenGL or any of the SGI libraries that use OpenGL is opened with your support provider. Depending on the problem, it may also reduce the time needed to resolve the issue.
When Developing New Applications
Ogldebug can be used when developing new applications or adding graphics code to an existing application. Use an iterative coding process: add some graphics-related code and then build and test to ensure expected results.
As a Learning Aid
Using ogldebug to examine an application can help to visualize what the application is doing and how OpenGL is being used.
Conclusion
The intent of this article is to encourage graphics developers to use ogldebug as part of their development environment. Ogldebug can contribute to the design and development of new applications, the improvement of existing applications, and in learning more about OpenGL or any library that uses OpenGL.
For More Information
More information on OpenGL is available on the SGI Web site (http://www.sgi.com). Use the find or search tool with the keywords "OpenGL WWW Center", or use the direct URL http://www.sgi.com/Technology/OpenGL/index.html.
Customers interested in OpenGL may subscribe to the public newsgroup comp.graphics.api.opengl.
The document "Frequently Asked Questions About Graphics and user environment customization" is regularly updated and posted to the comp.sys.sgi.misc, news.answers and comp.answers newsgroups twice per month. FAQs are also available on the SGI Web site http://www.sgi.com under "Sales & Support".
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