Setting a performance goal helps you use your time wisely. Typically, you should decide on a desired frame rate, such as running at 20 frames per second with a particular scene. A reasonable performance goal for interactive programs is a frame rate of at least 10 frames per second. Most users find that frame rate acceptable for most tasks (more is always better, of course).

When setting a goal, keep in mind the capabilities of your hardware. If the absolute top speed for drawing polygons on your system is 60,000 unlit, single-color triangles per second, don't try to get 10 frames per second while drawing 6,000 lit, color-per-vertex triangles. Write short OpenGL benchmark programs, or feed test scene graphs to ivview -p to help set your expectations.

It is important to have an objective way of measuring your application's performance. You're likely to waste time on insignificant optimizations if you just watch your application run and try to see if it seems faster.

Adding code to your application that measures the number of polygons in your scene and how fast they are being rendered is fairly simple; see, for example, the source code for ivview in /usr/share/src/Inventor/tools/ivview.

The osview utility can also be useful. The “swapbuf” number in the Graphics section tells you how many frames per second your application is getting (assuming that your application is double-buffered, and that it is the only double-buffered application running).

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Note: Don't confuse osview with its graphical counterpart, gr_osview, which doesn't have this feature.

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Be sure to keep good records of your application's performance before you start optimization. Comparing “before” and “after” performance numbers ensures that you are not making things worse.

Most applications spend most of their time executing a small part of the code. Optimizing a procedure that is taking up only 5% of the total time is probably not worthwhile: even if you manage to double the performance of the procedure, the application will speed up by only 2.5%. In fact, on some systems graphical operations can occur in parallel. For example, filling in polygons and transforming polygon vertices might occur at the same time. If the bottleneck is in the vertex transformation stage, increasing the pixel fill time may not increase performance at all! Find the bottlenecks first, and then work on improving them.

Finding bottlenecks is an experimental science. You should first come up with a theory on where the bottleneck might be, then devise an experiment that proves or disproves that theory. Create experiments that isolate one small part of your application's performance, and make sure you understand what you are measuring every time you run an experiment.

“Optimizing Rendering” and “Optimizing Everything Else” describe frequently encountered bottlenecks, show how to determine the amount of time your application is spending in each of them, and give suggestions on improving them. The following topics are discussed:

Feel free to start with bottlenecks you suspect are responsible for the most noticeable slowdown. You can look at the sections in any order; just make sure you always know what you are measuring and keep good records of your experiments.

When you apply a performance optimization, make sure that the modification is really an improvement: don't assume that all the suggestions made in this (or any other) document automatically apply to your application. For example, render culling usually increases performance. However, if you have an application in which all objects are always visible, render culling actually hurts performance.

Again, keep good records. Record what you do and how much it improves performance. Try to minimize the number of things you change at any one time; for example, if you make two “optimizations” and performance goes up by 10%, the speedup might be caused by a 5% improvement for each optimization, or might be caused by a 100% speedup caused by one optimization and a 90% slowdown caused by the other! It is tempting to read a document like this, make lots of changes, then see if the application gets faster. This not only wastes time, it can also be counter-productive.

One of the most frustrating things about optimizing an application's performance is that it can be difficult to determine when you are done. Once you have successfully eliminated one bottleneck, something else becomes the factor limiting performance. Before spending more time on optimization, you should ask yourself:

To find out whether rendering is the problem, modify your application so that it does everything it normally does except render, and then measure its performance. An easy way of getting your application to do everything but rendering is to insert an SoSwitch node with its whichChild field set to SO_SWITCH_NONE (the default) above your scene. So, for example, modify your application's code from:

myViewer->setSceneGraph(root);

To:

SoSwitch *renderOff = new SoSwitch;
renderOff->ref();
renderOff->addChild(root);
myViewer->setSceneGraph(renderOff);

This experiment gives an upper limit on how much you can improve your application's performance by increasing rendering performance. If your application doesn't run much faster after this change, then rendering is not your bottleneck. See “Optimizing Everything Else” for information on optimizing the rest of your application.

If you have determined that your application is spending a significant amount of time rendering the scene, the next step is to isolate rendering from the rest of the things your application does. This makes it easier to find out where the bottleneck in rendering occurs. The easiest way to isolate rendering is to write your scene to a file and then use the ivperf program to perform a series of rendering experiments.

The code for writing your scene may look like the following:

SoOutput out;
if (!out.openFile(“myScene.iv”)) { ... error ... };
SoWriteAction wa(&out);
wa.apply(root);

The ivperf utility reads in a scene graph and analyzes its rendering performance. It estimates the time spent in each stage of the rendering process while rendering the scene graph.

The process of rendering a single frame can be decomposed into five main stages:

The sum of the times spent in these stages does not, in general, equal the total time it takes to render the scene. Depending on the underlying hardware platform and graphics pipeline, some or all of the above can overlap with each other. Thus, completely eliminating one of the stages does not necessarily speed up the application by the time taken by that stage. ivperf takes this into account; it answers questions of the type “if I could completely eliminate xxx from my scene, how much faster would rendering be?” For example, if ivperf indicates that 50% of your time is spent changing the material graphics state, then making your entire scene a single material would make it render twice as fast. Knowing that materials are taking up a significant part of your rendering time, you can then concentrate on minimizing the number of material changes made by your scene.

If you have created your own node classes, either create DSO's for them (see “DSO Directories and Versions”) or call their initClass() methods just after the call to SoInteraction:init() in the ivperf source and link their .o files into ivperf.

The camera control used by ivperf is simplistic: it calls viewAll() for the scene and just spins the scene around in front of the camera when benchmarking. If you have a sophisticated walk-through or fly-through application that uses level of detail and/or render culling, modify ivperf so that its camera motion is more appropriate for your application. For example, have ivperf use the following little scene instead of just SoPerspectiveCamera:

TransformSeparator {
    Rotor { rotation 0 1 0 .1 speed .1 }
    Translation { translation 100 0 0 }
    PerspectiveCamera { nearDistance .1 farDistance 600 }
}

ivperf correctly reports the performance of changing scenes, as long as you give it enough information. It automatically deals with scenes containing engines and animation nodes, but if you are using an SoSensor to modify the scene, you should mark nodes that your application frequently changes by giving them the special name “NoCache.” For example, if your application is frequently changing a transformation in the scene, the transformation should appear in the file given to ivperf as:

DEF NoCache Transform { }

The first step in the rendering process is clearing the window. It is easy to forget this step, but depending on the size of your application's window and the type of system you are running on, clearing the window can take a surprisingly long time. If your application's main window is typically 1000 by 1000 pixels, run ivperf like this:

ivperf -w 1000,1000 myScene.iv

ivperf performs many different rendering experiments, and eventually prints information on each rendering stage.

For example, on an Indigo2^™ Extreme™ running IRIX 5.3, ivperf reports that for rendering a simple cube in a 1000 by 1000 pixel window 46% of the time is spent clearing the window.

Unfortunately, if clearing the window takes too much time, there is not a lot you can do. One possibility is to make the window's default size smaller (while still allowing users to resize the window if necessary).

After running ivperf, you know how much time your application spends clearing the color and depth buffers. The next experiment is designed to find out how much time Inventor spends traversing your scene. Traversal is the process of walking through the scene graph, deciding which render method needs to be called at each node. Inventor automatically caches the parts of your scene that aren't changing and that are expensive to traverse, building an OpenGL display list and eliminating the traversal overhead.

If most of your scene is changing, or if your scene is not organized for efficient caching, Inventor may not be able to build render caches, and traversal might be the bottleneck in your application. ivperf measures the difference between rendering your scene with nothing changing, and rendering with the camera, engines, and nodes named “NoCache” changing.

If traversing the scene is a bottleneck in your program, there are several ways of reducing the traversal overhead:

You may be able to organize your scene so that Inventor can build and use render caches even if part of the scene is changing. Note that the following things inhibit caching:

For more information on Inventor's render caching, see Chapter 9 of The Inventor Mentor.

If ivperf reports that material changes are the rendering bottleneck, try the following:

For transformation, ivperf reports two numbers: the overhead of changing the OpenGL transformation matrix between rendering shapes and the time it takes to transform the vertices in your scene through that matrix. This section helps with the former, giving suggestions on how to make Inventor execute fewer OpenGL matrix operations. See “Optimizing Vertex Transformations” for hints on optimizing the transformation of vertices.

For best performance when creating SoFaceSet and SoIndexedFaceSet shapes, arrange all the triangles first, then quads, and then other faces.

If your scene contains textures, ivperf reports two numbers: the time you would save if you could turn off textures completely, and the time you would save if you could make your scene use only one texture. On systems with texturing hardware, the number of textures used can dramatically affect performance; see “Optimizing Texture Management” for hints on optimizing texture management. On systems without texture mapping hardware, the bottleneck is probably filling in the textured polygons.

Inventor 2.1 automatically does two things to speed up rendering on systems without texture mapping hardware:

If ivperf reports a lot of time is spent in texture management, then you are running out of hardware texture memory. Try the following:

If the scene given to ivperf contains light sources, ivperf informs you how expensive they are compared to rendering your scene with just a single directional light. If ivperf reports that lights are a significant performance bottleneck, try to use fewer light sources, and use simpler lights (a DirectionalLight is simpler than a PointLight, which is simpler than a SpotLight). If possible, put lights inside separators so that they affect only part of the scene, increasing performance for the rest of the scene.

If ivperf reports that vertex transformations (which include per-vertex lighting calculations) take up a significant portion of the time it takes to render a frame, you can do the following to optimize per-vertex operations:

A common bottleneck on low-end systems is drawing the pixels in filled polygons. This is especially common for applications that have just a few large polygons, as opposed to applications that have lots of little polygons.

If ivperf reports that a large percentage of each frame is spent filling in pixels, try to optimize your scene as follows:

There are several performance problems that ivperf doesn't catch. The following sections describe them, and give hints on how to improve them.

Making Inventor Produce Efficient OpenGL

If your application is rendering only 10 frames per second with 1,000 triangles per frame, and you know that your graphics hardware is capable of rendering 100,000 triangles per second (10,000 triangles per frame at 10 frames/second), and ivperf reports that your bottleneck is vertex transformations, then your problem might be that Inventor is not making efficient OpenGL calls.

Inventor is much more efficient at rendering multiple triangles if they are all part of one node. For example, you can create a multifaceted polygonal shape using a number of different coordinate and face set nodes, as shown in the lower half of Figure 6-1. A much better technique is to put all the coordinates for the polygonal shape into one SoCoordinate or SoVertexProperty node, and the description of all the face sets into a second SoFaceSet node, as shown in the upper half of Figure 6-1.

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Tip: The ivfix utility program collapses multiple shapes into single triangle strip sets.

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Figure 6-1. Condensing Face Sets Into Fewer Nodes

Using fewer nodes to get the same picture reduces traversal overhead for scenes that cannot be cached. Note also that Inventor optimizes on a node by node basis and generally can't optimize across nodes.

An SoFaceSet or SoIndexedFaceSet has special code for drawing 3 and 4-vertex polygons. To take advantage of that, you must arrange the polygons so that the 3-vertex polygons (if any) are first in the coordIndex array, followed by the 4-vertex polygons, followed by the polygons with more than 4 vertices.

For some applications, consider implementing your own nodes that implement the functionality of a subgraph of your scene. For example, a molecular modeling application might implement a BallAndStick node with fields specifying the atoms and bonds in a molecule, instead of using the more general SoSphere, SoCylinder, SoMaterial, SoTransform, and SoGroup nodes. If the molecular modeling application changes the molecule frequently so Inventor cannot cache the scene, using a specialized node could make traversal orders of magnitude faster (for example, a simple water molecule scene graph with three atoms and two bonds might consist of 20 nodes; replacing this with a single BallAndStick node would make traversal 20 times faster). The BallAndStick node could also perform application-specific optimizations not done by Inventor, such as not drawing bonds between spheres whose radii were large enough that they intersected, sorting the spheres and cylinders by color, and so on. See The Inventor Toolmaker for complete information on implementing your own nodes.

The standard performance analysis tools (prof, pixie, or the CaseVision/WorkShop Performance Analyzer) make performance analysis of the non-graphics part of your application easy. See the reference pages for prof, pixie, or cvspeed for information on using these tools.

First, make sure your application isn't running out of physical memory by running gr_osview with the -a flag and looking for “swap” in the “CPU Wait” usage bar. If your application is swapping, try to reduce its memory usage as follows:

If memory is not the problem, start by looking at “inclusive” CPU times for your procedures (inclusive times include time spent in that procedure and all procedures it calls; exclusive times are just the time spent in that procedure). Ignore the very highest level routines like main() or SoXt::mainLoop(); look for Inventor beginTraversal() methods or for application routines that are taking a significant percentage of time. If a lot of time is spent in SoGLRenderAction::beginTraversal(), see “Optimizing Rendering” for information on improving rendering performance.

If your application is spending a lot of time in code written by you, you are on your own! The rest of this section describes Inventor routines that often show up on profile traces, describes what these routines do, and suggests ways of using them more efficiently.

Inventor actions perform a lot of work the first time they are applied to a scene (subsequent traversals are very fast). Therefore, if your performance traces show a lot of time being spent inside an action's constructor or the SoAction::setUpState() method, try to create an action once and reapply it instead of constructing a new action. For example, if you often compute the bounding boxes of some objects in the scene, keep an instance of an SoBoundingBoxAction around that is reused:

static SoGetBoundingBoxAction *bbAction = NULL;
if (bbAction == NULL) bbAction = new SoGetBoundingBoxAction;
bbAction->apply(myScene);

instead of the much less efficient:

SoGetBoundingBoxAction bbAction; 
                             // inefficient if called a lot!
bbAction.apply(myScene);

Every time you change a field in the scene, Inventor performs a process called notification. A notification message travels up the scene graph to the node's parents, scheduling sensors, causing caches to be destroyed, and marking any connections to engines or other fields as needing evaluation.

If your performance traces show a lot of time is being spent in a startNotify() method, try the following to decrease notification overhead:

If your application profiles show a lot of time is spent inside the methods SoHandleEventAction::beginTraversal() or SoPickAction::beginTraversal(), try the following to improve picking and/or event handling performance: