As you all noticed, Maya doesn't provide at all motion blur with shadows.

This tutorial shows how to generate motion blurred shadows in a separate pass(es), to be composited or just remapped into an attribute of a material (mainly the diffuse attr). It's been tested with depthmap shadows, but I don't think ray-traced ones would be a problem.

But let's get into the character...

Considering that a depthmap shadow is just a generated mage from light's point of view, what if we set a camera at the very light source's location, and render the sequence from that cam, with motion blur on.

For my test, I used a directionnal light (sun light source). I have created a camera, point & orient constrained to that dir light.

Now, in order to match with real generated depth shadows (dir light provides parrallel rays, in other words, it's like generated from an orthogonal view), we must set the focal length attribute so that the perspective looks like the dir light point of view. You can visualize that by opening 2 windows of your 'shadow' camera and the real dir light (with Look through selected).

Adjust the 'shadow cam' focal length so that its point of view lies as exactly as possible with the point of view from the dir light.



Be careful to set the same parameters of film aspect ratio for both cameras (the one that renders shadows, and the renderable cam) orelse your projection will not fit exactly with the objects (the shadows will be a bit 'offseted' from the 'supposed to cast shadow objects')

When this is done, you may have to determine what object is in shadow casted by your moving object, so that you'll render it or them in a separate pass.

Create 2 shading map shaders. One black, and the other white (to generate a mask), and apply the white shader to the objects that are NOT shadowed by the motion blurred object, and apply the black shading map to the moving object.

Now you just have to render from the 'shadow cam' the whole sequence with your moving object(s). You'll get a 'mask' sequence of the moving object (now moving shadow).

here are sample frames of the rendered animation from 'shadow light source cam'


This done, remap the motion blurred shadow sequence with a perspective projection (set to the 'light source' cam, with fit to resolution), and plug it into the diffuse of your, say ground material, and do the same for the diffuse attr. of your shadowed objects.



Now you have a perfect motion blurred shadow, with your motion blurred object
(obviously, it's better with than without, huh?!!)


maya default shadow



motion blurred shadow



Ok, that's it. For other light sources, I have obtained very convincing results with spot lights, but I didn't try with a point light which is much more complicated than that (but I'll give you some news about that stuff when I'll get some time to do some more tests)

Limitations: unfortunately, the produced images are not exactly like the real ones provided by maya light sources. Simply because of the projection through a camera...

If you have any questions, critics or comments, let me know at pixho@pixho.com



Emmanuel Campin - june 2000