Monday, April 25, 2011

Mental Ray Sampling cont. -- Fast Rasterizer, Shades per Pixel

In this post we'll look at some renders using the Fast Rasterizer having determined that 64 Samples per Pixel will provide a good quality render, as good as the  Minimum Maximum render sampling option set to 4 & 64.
Rendering with 1 Shade per Pixel seems to look good enough but further evaluation of some different parts of our renders will show that we need to increase the 'Shades Per Pixel" to insure the best quality.

Example 1 shows a benchmark using the Minimum Maximum method reviewed a few posts back. We've set min = 4 and max=64 as demonstrated in that post for best quality. Let's focus on the shadow created by the sign highlighted in red. Render time = 16 minutes for the full frame (864 x 486) of which this is only a small portion:

Next is an Example using Fast Rasterizer with Samples set to 64 and Shades Per Pixel set to 1. Clearly some artifacts in the shadows of the signs. These will flicker in an animation also. However, note that the render time = 2.5 min!:

Example 3 shows the result of increasing Shades Per Pixel to 4. Clearly an improvement, but careful examination will show that the shadows are still not quite as cleanly anti-aliased as example 1:

Finally, let's increase the Shades Per Pixel to 6. This will yield a render as good as the Min Max option in example 1, but the render time?.....4.3 minutes!

In summary, the Fast Rasterizer is a much faster rendering algorithm than the default Minimum Maximum option and with some tweaks can provide superb quality renderings in 1/2 to 1/4 the render time. I noted an exception to this in an earlier post. This occurs when you are rendering a frame that contains mostly a blank background or background image, for example a render intended for compositing. The Minimum Maximum sampling option discriminates about the amount of sampling to do for each pixel based on the Spatial Contrast settings. Some buckets will render very fast while others will take a long time. With an absence of 3D elements in the bucket, the renderer will blow through these buckets very quickly. The Fast Rasterizer is not so discriminating and will render all buckets with a more uniform render time, regardless of what is being rendered and hence these types of frames can take longer to render with fast rasterizer.

Next I'll show some quick examples of the parking lot stripes rendered in an animation to show how the results affect anti-aliasing "flicker".

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