The following tutorial is a short excerpt from Mesmer«s XSI 102: Materials, Textures, Lighting, and Rendering web-based distance learning class. Click on the following links to learn more about the XSI 102 class in particular, and Mesmer distance learning in general. Enjoy.
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Motion blur makes fast-moving objects look more natural and realistic. The faster an object moves, the more blur it will receive.
Load up the scene called motion_blur_spheres.scn (located in the Scenes folder of the XSImaterials project, available for download by browsing to http://www.mesmer.com/docs/distlearn/olxsi2.html and following the appropriate download links and the bottom, or by browsing to the FTP site at ftp://ftp.mesmer.com/books/XSImaterials).
In this scene I«ve animated three spheres. Play back the animation by clicking the Repeat then the Play button at the bottom of the screen.
As you watch the animation, you«ll notice that the three spheres move at different speeds. The top is the slowest, the bottom one is the fastest, and the one in the center travels at a speed somewhere in between. Regardless, they each travel directly in front of the camera at frame fifty.
Stop the animation by clicking the play key again and move the time slider to frame fifty by typing 50 in the current frame number box.
Turning on motion blur is very easy. Turn viewport D into a property editor and open up the render region options by clicking Render]Region]Options. The motion blur settings in here work the same as they do in the Render]Render]Options, but since we want feedback in the viewport, we«ll use these options instead.
Draw a render region in viewport B by pressing the Q key and dragging from left to right.
Motion blur can be pretty processor-intensive so turn render region auto refresh off by clicking Render]Region]Auto Refresh. Also, turn on the background by clicking Render]Region]Show RGB
Even though the spheres are moving very fast, there is no motion blur. Click the Motion Blur Tab in the region options property page. Turn on the blur by checking the Motion Blur Enable check box.
***See figure xm.motionblur_none.jpg
Click the Refresh button to see the results.
***See figure xm.motionblur_on.jpg
Now the spheres are blurred. The top one has the least amount of blur because is moves the slowest. The bottom one has the most because it moves the fastest.
As soon as you turned on the motion blur, many parameters that were once grayed-out light up and are now available for use.
The Shutter Speed controls how much motion blur your objects get. This works just like the shutter on a real camera. If you left the shutter open longer it would record more of the action in that frame, thus giving you more blur. In XSI the higher you set this number the more your objects will blur. Set this number to zero and your motion blur will disappear. Although the slider only goes to 1 it is possible to set it higher by typing in values manually. Unless you need it, I wouldn«t go above 1. Too high a motion blur setting can cause unwanted artifacts. Sometime you are better off speeding up your animation to get more motion blur. Set this slider to 0.8
The Sampling Threshold controls the quality of the motion blur. It works in much the same way that the antialiasing threshold does. The threshold defines a minimum difference that can occur between the pixel that is currently being blurred and the ones surrounding it. If the difference is greater than the minimum number set in the threshold, mental ray continues on to another sampling level until it dips below the defined sampling threshold. High settings produce poor motion blur, but it renders very fast.
Try setting the motion blur threshold to 1 by dragging the sliders to the right while holding down the CTRL key. Click the Refesh button. The motion blur looks horrible but it renders much faster.
Low settings produce excellent motion blur but take much longer to render. Try setting it somewhere around 0.1. Now it looks much better, but it«s taking quite a lot longer to render.
While the antialiasing threshold is separate from the motion blur threshold, itÕs wise to keep the motion blur threshold a little higher than the antialising threshold. This speeds up rendering but can lower the image quality a bit. If that happens then you«ll need to lower the motion blur threshold and plan for longer rendertimes.
Estimated motion blur is a simpler version of full motion blur. It is really good for test rendering. With estimated motion blur on, your scene will render faster. While the differences between real and estimated motion blur are subtle, they can make or break a scene. This setting should be turned off when you are doing final renders.
Another way to speed up motion blur rendering is by turning off the raytracing rendering mode. When only scanline is being used then motion blur will render faster.
If you are using shadow mapped shadows in your scene then you«ll need to turn Motion Blurred Shadow maps on. Otherwise, shadows from motion blurred objects will look like they came from static ones instead.
***See figure xm.motionblur_no_shadowmaps.jpg
If you«ve got objects in your scene that deform, squash, or stretch and they need proper motion blur, then turn Deformation motion blur on. It works great for characters that have arms and legs that bend. This is because deformation motion blur will compute the blur for vertices of your object, unlike normal motion blur which just uses the object transformations to compute the blur. This setting is off by default because it uses a lot of memory and slows your render down. If you«ve got a large scene, then you«ll want to use local deformation motion blur.
