Document Properties: Render Details
Render acceleration grids
Controls the size/number of the grid cells used for eye ray and shadow ray speed. Building the grid takes some time, and storing the grid takes memory, lots of memory if the scene is big, so sometimes it may be better to turn off the grid, or change the grid parameters. The eye ray is the primary ray shot from the camera into the scene. To increase calculation speed of the intersections between the eye ray and geometry, the render plug-in divides the render viewport into smaller rectangular regions, builds a list of polygons in each of these regions (grid cell), and sorts the polygons by the distance to the camera. To render the scene, the ray only needs to be intersected with the polygons within the cell the ray is in, starting from the closest one. When an intersection is found, some of the polygons within the cell can be skipped based on their depth.
Screen grid, cell size: ___ pixels
The width and height of each grid cell in pixels.
The smaller the grid cell size, the more memory it takes and the more memory it takes to build, but the faster the final render. The default is 16 pixels.
Spotlight shadow grid, size
The shadow ray is the ray shot from the scene towards each light when the intersection between the eye ray and the scene is found. Usually the lions share of the render time is spent tracing the shadow rays.
To increase calculation speed for the spotlight shadow rays, the render plug-in divides the spotlight cone into rectangular regions, and again builds a sorted list of the objects within each region. This speed increase only applies to spotlights, because they are very similar to the viewports, the light location is like the camera location, and the light cone defines the viewport.
The spotlight grid is defined in number of grid cells instead of pixels, because there is no pixel size associated with lights. The default is 256 (the cone divided into 256 x 256 cells).
Prevents self-shadowing artifacts. When the intersection between the eye ray and the scene is found, the intersection point is moved (offset) towards each light before calculating the shadow ray. The reason is there’s always some numerical fuzz in calculating the intersections, and if the point is not offset, the shadow ray might hit the very same polygon again, putting shadows in wrong places. You can see the self shadowing artifacts if you set the setting to zero, and then render the scene. The default is 0.001.
Object & polygon BSP tree
The BSP tree is another way to increase rendering speed. Instead of testing each polygon one-by-one, the objects and polygons are divided into a tree-like hierarchy based on the location in space. The render plug-in builds multiple trees, one that contains the whole object bounding boxes, and one per object that contains the polygons for that object. The BSP tree again takes some time to build and takes some memory. The bigger the depth and the smaller the node size, the more memory it takes and the longer it takes to build.
Max tree depth
Controls how many times the scene can be subdivided when building the tree.
Target node size ___ objects/polygons
Defines the optimal size for each node that contains the objects or polygons.
The transparency bounces setting controls how many times rays that hit transparent objects are traced: 12 means a stack of 12 transparent sheets will render properly, but the 13th sheet will render opaque. The limit is there to keep the render times reasonable.
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