Common Performance Issues
Here is a list of things that can go wrong, roughly in order from most likely to least likely. Each of these has a section to explain it in greater detail.
Too Many Meshes. A well-made typical 3D model contains one mesh. Huge 3D models, such as the model of an entire dungeon or an entire island, might contain multiple meshes. 3D models created by inexperienced modelers can contain dozens of meshes. Most video cards can render about 300 meshes total for the entire scene. Panda3D contains tools to coalesce multiple meshes into one, but they aren’t fully automatic.
Too Many State Changes. The state of an object is the sum of its color, material, light, fog, and other attributes. It can be expensive, for a variety of reasons, to have too many different states in your scene. It is better if many objects share the same state.
Too Many Text Updates. If you have lots of text in your game that gets updated every frame, it will often take a long time for Panda to keep regenerating the text. You need to minimize the amount of text to regenerate per frame.
Performance Issue: Too Many Pixel Shader Instructions. If you are using per-pixel lighting, or hand-written shaders, you need to be conscious of how long your shaders are. Adding one pixel shader instruction can slow the video card a lot. Adding a texture lookup can slow it even more. Professional pixel shaders contain 20-30 assembly-level instructions.
Excessive Fill. The fill rate of the video card is number of pixels it can render per second. Objects that are occluded (behind other objects) still consume fill rate. The total fill-consumption of the scene is the total screen real estate of all objects, including the occluded ones. Particles, in particular, can consume fill-rate like crazy, especially if the camera gets close to the particles.
Memory Full. A floating-point number takes four bytes. Just one vertex contains (X,Y,Z), and a normal, and a texture coordinate. An RGBA color takes four bytes, so a 1024x1024 texture is four megabytes. Do the math, and you’ll see how fast it all adds up.
Python Calculation. Python is a very slow language. Most Panda3D programs only run a few thousand lines of python per frame, since all the real work is done in C++. Sometimes, though, you need to do some complex calculation, and Panda3D just doesn’t contain any C++ code to do it for you. In that case, trying to write the calculation in python can cause problems. You may need a C++ plug-in.
Failure to Garbage Collect. It’s easy to get used to the fact that Python’s garbage collector can automatically clean up Panda3D data structures. Unfortunately, there are a few structures that can’t be cleaned up automatically. You need to know what they are, or you may end up with a leak.
Collision System Misuse. The collision system can detect most types of collisions very rapidly. However, it is possible to construct situations that the collision detection system just can’t handle. Know what it’s good at, and what it’s not.
Motherboard Integrated Video. Motherboard video is very misleading. The chips have names like “Radeon” and “GeForce” that we have come to associate with speed, but these chips are an order of magnitude slower than real video cards. Programming for these chips requires special consideration.
Too Many Polygons. This is at the bottom of the likelihood list, but it can still happen. Usually this happens in combination with something else, e.g. if you have a large vertex shader, performance can be drastically reduced for each vertex you add.
Miscellaneous. There are a lot of small things that have a surprisingly large impact on performance. For instance, printing messages on the console can be very slow in Windows. This section lists a number of miscellaneous things that can bog you down.