Cameras are the center of the CGI universe.
In live action film, motion blur is the result of objects moving during the exposure of the film. The faster the exposure, the less things will blur. Conversely, a long exposure results in more motion blur. Audiences are quite familiar with the look of motion blur and creating this effect in 3D fast and efficiently is critical for convincing visual effects.
Pixar's RenderMan can mimic the behavior of real cameras, with extraordinarily fast 3D motion blur. By default, RenderMan renders scenes with an instantaneous shutter speed, an exposure which is impossible in the real world. However, without motion blur the effect would look unnatural, as moving objects would strobe across the screen, causing temporal aliasing.
Thankfully RenderMan is extremely efficient at rendering motion blur and has a number of options to help tune the final look.
Control the Blur
There are many knobs for controlling the quality and character of motion blur. Here are some of the most important.
Shutter Opening — An innovative creative control for artistically sculpting the falloff of blur
Motion Factor — An innovative solution for rendering highly blurred objects faster, because they are so blurry they don't necessarily need to be shaded as much as other objects in the same scene
Shutter Angle — The main knob to set the degree of the effect
Depth of Field
In the real world, when a camera focuses on a close object, the background can go blurry. This effect is known as the depth of field, and Pixar's RenderMan offers many controls for creating this effect.
With Pixar's RenderMan, we can decide, depending on requirements, whether we want finite or infinite depth of field, an instantaneous shutter speed or a long one to create motion blur. RenderMan provides us with several options to control how our virtual camera behaves, which is important for creating believable 3D imagery.
Depth of Field in RenderMan is dependent on three main factors: the focal length of the lens, the diameter of the aperture, and the focal distance of the camera. With these controls RenderMan can convincingly emulate real film cameras.
RenderMan also offers aperture controls, to simulate the artifacts of physical cameras. This allow custom control over the virtual camera to create special effects, such as Bokeh, as it is known in the world of professional photography. Bokeh controls the look of out of focus points, and can be seen more clearly in the highlights of an image. With the aperture controls the shape of the out-of-focus regions of our image can be brighter in the inside or outside of blurry regions.
Sides — The number of diaphragm blades controls the shape of your blur circle
Roundness — Controls the curve of the edges of the aperture blades
Angle — For rotating the out-of-focus points
Density — You are able to mimic the good (or bad) light falloff in your lens by using this control
Motion Blur in Presto
Case Study: Creative Control
In Pixar's Animated short, Presto, one of the challenges was rendering motion blur on the quickly moving characters. RenderMan's innovative blur control, Shutter Timing, was used to help create convincing motion blur.
This subtle effect was critical for creating a specific look, and is an example of the amount of creative control Pixar's RenderMan provides, so any type of creative vision can be realized.
This is the default motion blur from an instantaneous shutter. Look at how even the blur effect is. Sometimes that is the desired look, but in Presto the director wanted blur with more of a stylized falloff, and RenderMan has controls for just that type of effect.
In this second image, the image was rendered with a slow opening shutter. You'll notice that the blur is shaped ... with the blur looking like it's streaking off the magician. This type of stylized blur was the kind of look the director wanted in this particular case.