Thursday, November 29, 2007

FG Point Density Optimization

Just a quick note to help speed up hires renders that use FinalGather. The new method is easier to setup in maya 8 and 2008. Accuracy can now often be left at low levels like the default of 100 and still provide smooth results, but some things to keep in mind regarding the new method...

a point density of 1 relates to the default layout of FG points. These points are normally set in a hexagonal sort of layout (really just offset rows) every 10 pixels or so. The "Point Density" value adjusts this. The default value of 1 is usually a very fine starting place, and many may never have to change the Point Density value. However, as resolutions get larger, and much larger for print images, this can be detrimental. The number of FG points that are created is resolution dependent. The larger your image, the more FG points will be added (still every 10 pixels or so). This is normally a wise thing, as the larger you render, the more FG detail you should need.

In practice, however, you probably don't need all the FG point detail (Density) you did for your lowres tests. Something that is nice to know is the following relationship:

If you double your image size (from 640 to 1280 for example) you are squaring the number of FG points. 2x2... if you leave your density at 1. If you quarter the density to .25, you will exactly match the FG points used in your 640 render. Since the render is now larger, your FG solution will probably not be quite detailed enough. A nice compromise might be to set Point Density to .5 thereby doubling (but not squaring) your FG points. This could provide much faster FG performance for higher res renders.

1280 is not a very hires render, so this example may not provide very high quality... but it illustrates an optimization that can make a huge difference for print-res renders (if you're into that sort of thing). If I'm rendering a print res at 10x640 (or 6400 pixels wide), that's a LOT of FG points. I would plug .01 into density to match the same FG points used in my 640 test. This will obviously look poor, but the render would be very fast. Finding the right relationship may be a bit of trial and error, but there is certainly a medium number of points that will retain enough FG detail, and still allow for fast rendertimes. I might start a 6400 render at a density of .1 for a nice speed optimization.

Good luck.

1 comment:

Anonymous said...

I'm not sure I follow. Density increases with resolution automatically. It is resolution dependant but mental ray computes internally (FG points are also contrast sensitive, so just like sampling, details receive more.) From a 320x240 render to a 640x480 render my density remained at 1.0 but the points generated went up almost 4 fold. Interpolation also averaged a few more points. So for a big print the rule would be more detail for everything (samples, FG, etc) but mr does increase the density on its own with resolution increases. May not be adequate, but then "automatic" rarely is. ;-)