Killzone 2 decision tree rendering

gistfile1.txt

I hope I'm going to be able to describe what we do there - it's really no magic 
involved - we just wanted something simple.

The decision tree is quite simple structure - leaf nodes are individual 
drawparts (drawpart is a structure that contains vertex array, index array, 
shader, matrices, constants... - the minimum data to render something without 
any higher logic required) and non-leaf nodes pretty much contain 
rules/conditions determining what child node to pick - since we don't want 
to have any virtual calls there, we only have predefined set of conditions 
such as LOD node (based on distance from camera), "switch" node 
(explicitly pick certain child) and couple more (just imagine a giant 
switch statement with all possible node types). The whole tree is one 
linear block of memory so we can easily DMA it onto SPU, start in the 
root and go through the decision nodes and output list of the relevant 
leaf drawparts. We then merge all drawparts for the frame and just render 
the list.

The reason why the tree contains only couple of different node types 
is because all higher level game logic is done at different point - 
therefore the representation layer of the game can stay simple.