Multi-Pass Rendering
Multi-Pass Rendering
By default, “Final”,
render preset uses an A-buffer renderer. The A-buffer incurs some initial
overhead for edge quality and adaptive computations, but generates images much
faster than its "Multi-Pass" render preset equivalent, and can be efficiently
used with global illumination solutions like radiosity or skylight-rigs.
“Multi-Pass” rendering means that multiple render passes of the complete scene
will be individually computed and averaged with each other. Multi-Pass is used
for selected scenes where the light and camera jittering that happens on each
pass can improve the quality of the shading. The number of passes determine the
level of antialiasing: 4X4, or 16 passes, is the minimum acceptable if a “Blur”
post effect is not applied.
Advantages of A-Buffer Rendering
1) Much faster for the same Multi-Pass equivalent of antialiasing.
2) Practical for global illumination, like radiosity or light rigs.
3) Better bitmap fidelity.
Advantages of Multi-Pass Rendering
1) Light jittering adds some global illumination, including penumbral
shadows. (The A-buffer has “penumbral” klieg lights for a similar effect, and
light rigs also provide this look).
2) Physically correct motion blur, (A-buffer motion blur is adequate in most
cases).
3) Camera jittering blurs crisp geometry. Camera jittering can also be used for
physically correct depth-of-field if enough passes (64) are used, (A-buffer
depth-of-field is adequate in most cases).
Methods to Take Advantage of Multi-Pass Rendering
Since the entire scene is being rendered multiple times (1 time per pass) in Multi-Pass rendering, it is important to take as much advantage of this computationally expensive rendering method as much as possible. Here are a few tricks to giving you a higher quality rendered image at no extra cost.
Motion Blur
1. Motion Blur - Use the motion blur option when rendering animations. With each rendered pass, time will be advanced slightly based on the motion blur "Percent" property. The Motion Blur percent actually represents the camera's shutter and symbolizes how long the shutter is open exposing the image during rendered. The default motion blur is 20%.
4 Pass with Motion Blur 16 Pass with Motion Blur
As seen in the above rendered images, it may take a substantial number of passes to remove the strobe looking motion blur causes by inadequate number of passes.
2. Depth of Field - Focusing the camera on objects of importance and allowing the foreground and background objects to be out of focus can be a very desired filming technique. Multi-Pass rendering depth of field adds no render time expense, but the same applies true with inadequate number of passes using depth of field as does with motion blur. With each pass the camera is slightly moved to render the scene from a different angle causing near and far objects to because blurry.
Depth of Field (DOF) No Depth of Field
Depth of Field Property
To render your scene using depth of field, turn on the Depth of Field render settings on the camera or in the Options Dialog Rendering tab.
Depth of Field manipulators
When the Depth of Field property is set to "ON", the camera will display the Depth of Field manipulators when selected. Use the Focus Distance manipulator to set the focal point, and the Near and Far Focus manipulators to set where the out of focus starts.
3. Light Width - When multi-pass rendering the light width can be used to simulate area lighting, global illumination, and penumbral shadows. For each pass the light will jitter inside the area defined by the light's width property giving a different lighting angle. The benefits from this are endless. Shadows will have crisp edges closer to the object casting them and fall off as the distance increases. Surfaces will be lit from multiple angles giving more accurate specular highlights.
Light Width of 0
Light Width of 80
Light Width of 300
Light Width Property
4. Soft Antialiasing - The multi-pass render can be set up to have a different averaging mechanism used for combining each pass. This averaging technique slightly softens the image but gives the appearance of smoother antialiasing especially at lower passes. To invoke this softening effect use the Blur post effect on the camera with a softness value of 0. Note any softness value larger than 0 will invoke a true blur on the rendered image.
4 Pass 4 Pass with Blur Post Effect
Blur Post Effect
See Post Effects for information on how to you them.
Blur Softness
Use a softness of 0 to only invoke the Multi-Pass softening pass averaging technique.