Method and system for improving shadowing in a graphics rendering system

In a graphics rendering system for rendering graphical models to a view space to generate successive frames of animation, a method for computing shadows for the frames of animation comprising:

• for a first frame, transforming characteristic points on a shadowing object and a shadowed object to shadow map space, and saving the transformed characteristic points;
• for the first frame, rendering geometric primitives in a view volume from the perspective of a light source to compute a shadow depth map, rendering the geometric primitives from the perspective of a viewpoint to compute an image illuminated by the light source, computing a shadow image comprising an array of shadow attenuation coefficients indicating an extent to which the shadowed object is shadowed by the shadowing object, and applying the shadow image to the image to compute a shadowed image;
• and
• for a subsequent frame, transforming the characteristic points on the shadowing object and the shadowed object to the shadow map space, computing a difference between the transformed characteristic points for the first frame and the subsequent frame, re-rendering the shadow image when the difference exceeds a predefined amount, and re-using the shadow image to compute shadowed image for the subsequent frame when the difference is less than the predefined amount.

A method for shadowing a scene comprised of geometric primitives in a view volume, the method comprising:

• rendering the geometric primitives from a perspective of a light source to create a shadow depth map;
• performing a shadow filtering pass on the geometric primitives including:
• a) transforming pixel coordinates and a depth value from the perspective of the viewpoint into transformed pixel coordinates and a transformed depth value in light space coordinates;
• b) placing a filter footprint in the shadow depth map such that the transformed pixel coordinates are approximately at the center of the filter footprint;
• c) comparing the transformed depth value with samples in the filter footprint to compute a shadow mask with entries corresponding to the samples in the filter footprint, d) applying a linear or higher order filter to at least some of the entries in the shadow mask to compute a shadow attenuation coefficient, and repeating steps a) through d) for subsequent pixel locations;
• rendering the geometric primitives from the viewpoint to compute an image illuminated by the light source;
• and
• modulating the image illuminated by the light source by the shadow attenuation coefficients.

In a system for rendering geometric primitives in a view volume to compute an image for a view space, a rendering system for shadowing the image comprising:

• texture memory;
• a rasterization buffer with entries for storing first and second depth values for at least a chunk of the image, where the chunk is an n by m region of the image and n and m are integer numbers;
• a rasterizer for converting the geometric primitives into shadow depth values at locations in a shadow depth map during a shadow map rendering pass, for converting each of the geometric primitives into color values and a depth value at pixel locations in the image while rendering the geometric primitives from a perspective of a viewpoint in a second rendering pass, and for transforming pixel coordinates in the image and depth values of the geometric primitives from the perspective of the viewpoint to light space coordinates during the second or a subsequent rendering pass;
• a pixel engine in communication with the rasterizer for receiving the shadow depth values from the rasterizer and in communication with the rasterization buffer for writing first and second closest shadow depth values to a light source into the rasterization buffer at corresponding locations in the shadow map;
• the pixel engine in communication with the rasterizer for receiving the color values and the depth value for each of the geometric primitives during the second rendering pass, for performing a depth compare operation to determine which of the color values and depth values to store in the rasterization buffer during the second rendering pass;
• an anti-aliasing engine in communication with the rasterization buffer for reading the first and second closest depth values during the shadow map rendering pass and computing sample values for locations in the shadow depth map from the first and second closest depth values at each location in the shadow depth map;
• the anti-aliasing engine in communication with the texture memory for transferring the shadow depth map to the texture memory;
• a texture cache;
• a texture block fetch in communication with the texture memory and the texture cache for transferring the shadow map from the texture memory to the texture cache;
• and
• a shadow filter engine in communication with the texture cache for filtering shadow mask entries derived from shadow depth map samples in the texture cache and computing attenuation coefficients for pixel locations in the image;
• wherein the pixel engine is in communication with the shadow filter engine for receiving the attenuation coefficients, and is in communication with the rasterization buffer for applying the attenuation coefficients to the color values stored in the rasterization buffer from the second rendering pass.