Time Convolve Node
The Time Convolve node enables you to modify each pixel in an image by averaging the values of corresponding pixels across a range of frames. Time Convolve only affects those pixels that change from frame to frame.
Time Convolve can be used to create various trailing or strobe effects. Depending on the nature of the movement in a shot, this node can also create an effective motion blur because it blurs moving elements without blurring the background. This characteristic means that Time Convolve is excellent for noise reduction on background images (images without moving elements).
You can specify the following options for the Time Convolve operation:
- the width of the filter, in frames (how many frames will contribute to the operation for each frame convolved)
- the shape of the filter (the type of function curve, which specifies how the values will be weighted)
- the direction (from current frame) in which the filter will operate
- an offset from the current frame (so that the frame being modified is no longer the central frame of the filter)
The Time Convolve node panel displays graphically how the current settings affect the filter:
You may want to experiment with different settings to get a feel for the effect they will have on your imagery. For example, using the Box filter, so that each frame in the operation contributes equally to the result, can produce a strobelike or a banding effect.
On the other hand, using the Triangle filter with the Past Only setting in the Direction parameter can create a diminishing trail behind the moving subject.
- Note:
- Due to the nature of the time convolve operation, you may encounter a problem rendering if the Time Convolve is part of a flow that connects to a multilayer-input node (such as Mcomp) downstream. This problem occurs only if the background (top) input to the Mcomp node is a Color node.
The solution is to insert a Sequence node between the background input and the Mcomp to "hold" the single frame until it matches the length of the other inputs.
Convolve Parameter Tab
Filter Width
The Filter Width parameter enables you to specify the number of segments into which the filter is divided, where each segment is a frame of the input image. The default value is 3, which is the minimum size.
You can enter any odd integer value in the range of 3-31. (If you enter any other value, it will be automatically replaced by the nearest allowable value.) The odd value is stipulated because there must be a center frame to reference by other parameters that modify the filter.
- Note:
- The higher the Filter Width value, the longer the node takes to process, since the data from that number of input frames must be accessed and calculated for each frame of the input as it is modified.
Start and End Frames of the Input Sequence
When the first frame of the input image sequence is being processed, no previous input frame exists to use as a segment of the convolution filter. In such cases, the image data from the first frame of the input sequence is used as the value for all filter segments that would otherwise use the values from previous input frames.
The same holds true when the final frame of the input is being processed; the data from the last input frame is used for all filter segments that would otherwise use the values from subsequent input frames.
Filter
The Filter parameter enables you to select one of three function curves, each of which applies a different weight to the distribution of the effect:
- Box filter: The box filter (constant function) averages the values of each frame evenly, so that each contributes in equal measure to the result.
- Triangle filter: The triangle filter (linear function) performs a weighted average of the values in each frame, with the center frame contributing most to the result.
- Gaussian filter: The Gaussian filter uses a bell-shaped function curve to determine the contribution of each frame to the result.
Frame Offset
This parameter allows you to specify an offset from the center of the filter. If you set a Frame Offset value other than 0, the current frame (the frame being modified) is no longer the center segment of the filter.
It may be helpful to visualize the offset as sliding the frames of the sequence back and forth under a stationary window (the filter), as dictated by the offset value entered:
Valid Offset Values
The range of valid Frame Offset values is tied to the Filter Width parameter value, and a value of 0, which specifies no offset, is always the center value of the offset range. At the default Filter Width value of 3, for example, you can only specify a Frame Offset of -1 or 1. At a Filter Width value of 5, the Frame Offset range is -2 to 2, and so on.
Direction
The Direction parameter enables you to specify that only part of the filter be used in the convolve operation. The Direction popup menu enables you to specify Past and Future, Past Only, or Future Only. The terms past and future in this case refer to the direction from the current frame.
You can see the effect of each setting on the filter in the graphic display in the node panel. (Two examples are illustrated in the introductory section of this node description.)
Channel Mask
The Channel Mask parameter enables you to inhibit the modification of any channel by deselecting the Red, Green, Blue, Alpha, or Other channel icons.
Auto Cache Input
When checked, the Auto Cache Input option affects the cache setting of whatever node is serving as the input node to the Time Convolve node. The Auto Cache Input option resets the frame cache of the input node to match the Filter Width setting in Time Convolve.
Auto Caching will greatly improve the speed of rendering; however it requires substantial storage space (in RAM or on disk) to cache the needed input frames.
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