May 31, 2012, 06:32:32 pm
Shadow Cache off:
The shadows are updated every frame (Slow!)
The shadow from the light uses a static 128x128 pixel texture (Fast, very low memory usage)
The shadow from the light uses a static 256x256 pixel texture (Fast, low memory usage)
The shadow from the light uses a static 512x512 pixel texture (Fast, moderate memory usage)
Every time you add a light with "Shadow Cache off", the entire scene has to be rendered once more. Add 10 lights and the scene has to be rendered 10 times more every frame.
So if you make some of the lights use static shadows (e.g. the 512x512 pixels option) the graphics card has to render the scene fewer times per frame at the expense of slightly higher memory usage. This will almost always result in shorter render times and better performance.
The downside is that the shadow texture is a lower resolution than those of dynamic shadows (2048x2048 per light).
To reduce render times, we recommend that you turn distant lights (e.g. street lights) in your scene into lights with static shadows (so that the shadows are not updated every frame). If your graphics card has enough RAM, the best option is "512".
You would only ever use "Shadow Cache off" for your lights if something that casts shadows is moving near the light (a car driving past a street light for example). Or if the light itself moves/rotates (stage lighting for concerts for example). Or if you need high-resolution shadows in certain areas (typically close-ups of interiors).