### 9.4.2 How do I increase rendering speed?

This question can be divided into 2 questions:

1) What kind of hardware should I use to increase rendering speed?

(Answer by Ken Tyler)

The truth is the computations needed for rendering images are both complex and time consuming. This is one of the few program types that will actualy put your processors FPU to maximum use.

The things that will most improve speed, roughly in order of appearance, are:

1. CPU speed
2. FPU speed
3. Buss speed and level one and two memory cache - More is better. The faster the buss speed the faster the processor can swap out computations into it's level 2 cache and then read them back in. Buss speed therefore can have a large impact on both FPU and CPU calculation times. The more cache memory you have available the faster the operation becomes because the CPU doesn't have to rely on the much slower system RAM to store information in.
4. Memory amount, type, and speed. Faster and more is undoubtably better. Swapping out to the hard drive for increasing memory should be considered the last possible option for increasing system memory. The speed of the read/write to disk operation is like walking compared to driving a car. Here again is were buss speed is a major player in the fast rendering game.
5. Your OS and number of applications open. Closing open applications, including background items like system monitor, task scheduler, internet connections, windows volume control, and all other applications people have hiding in the background, can greatly increase rendering time by stealing cpu cycles. Open task manager and see what you have open and then close everything but the absolute necessities. Other multi-tasking OS's have other methods of determining open application and should be used accordingly.
6. And lastly your graphics card. This may seem unlikely to you but it's true. If you have a simple 16 bit graphics card your render times, compared to other systems with the same processor and memory but better CG cards, will be equal. No more no less. If you play a lot of games or watch a lot of mpeg movies on your system then by all means own a good CG card. If it's rendering and raytracing you want to do then invest in the best system speed and architecture your money can buy. The graphics cards with hardware acceleration are designed to support fast shading of simple polygons, prevalent in the gaming industry, and offer no support for the intense mathematical number crunching that goes on inside a rendering/raytracing program like Pov-Ray, Studio Max, and Lightwave. If your modeling program uses OpenGl shading methods then a CG card with support for OpenGL will help increase the speed of updating the shading window but when it comes time to render or raytrace the image it's support dissapears.

2) How should I make the POV-Ray scenes so that they will render as fast as possible?

These are some things which may speed up rendering without having to compromise the quality of the scene:

- Bounding boxes: Sometimes POV-Ray's automatic bounding is not perfect and considerable speed may be achieved by bounding objects by hand. These kind of objects are, for example, CSG differences and intersections, blobs and poly objects. See also: CSG speed.
- Number of light sources: Each light source slows down the rendering. If your scene has many light sources, perhaps you should see if you can remove some of them without loosing much quality. Also replace point light sources with spotlights whenever possible. If a light source only lights a little part of the scene, a spotlight is better than a point light, since the point light is tested for each pixel while the spotlight is only tested when the pixel falls into the cone of the light.
- Area lights are very slow to calculate. If you have big media effects, they are extremely slow to calculate. Use as few area lights as possible. Always use adaptive area lights unless you need very high accuracy. Use spot area lights whenever possible.
- When you have many objects with the same texture, union them and apply the texture only once. This will decrease parse time and memory use. (Of course supposing that it doesn't matter if the texture doesn't move with the object anymore...)
- Things to do when doing fast test renderings:
  - Use the quality command line parameter (ie. `+Q`).
  - Comment out (or enclose with `#if`-statements) the majority of the light sources and leave only the necessary ones to see the scene.
  - Replace (with `#if`-statements) slow objects (such as superellipsoids) with faster ones (such as boxes).
  - Replace complex textures with simpler ones (like uniform colors). You can also use the `quick_color` statement to do this (it will work when you render with quality 5 or lower, ie. command line parameter `+Q5`).
  - Reflection and refraction: When an object reflects and refracts light (such as a glass object) it usually slows down the rendering considerably. For test renderings turning off one of them (reflection or refraction) or both should greatly increase rendering speed. For example, while testing glass objects it's usually enough to test the refraction only and add the reflection only to the final rendering. (The problem with both reflecting and refracting objects is that the rays will bounce inside the object until max_trace_level is reached, and this is very slow.)
  - If you have reflection/refraction and a very high `max_trace_level`, try setting the adc_bailout value to something bigger than the default 1/256.