Vector Blur Node

The Vector Blur node enables you to specify precise values to control the blurring of an image. The Vector Blur node is most often used to create motion blurs.

The Vector Blur node blurs each pixel in an image based on parameter values that you set to control the number and location of neighboring pixels to be used in the blur computation, as well as the weight of the contribution of each neighboring pixel value to the blur effect.

The Vector Blur node computation is essentially a convolution whose kernel, or matrix of cells, consists of a single row of cells with the pixel to be convolved in the central cell of the row and neighboring pixels in the cells on either side. For a general explanation of the convolution process, see the Convolve Node description in chapter 18.

To control the blur effect, you specify the vector used to define the kernel. The vector can be thought of as a line drawn on the image, with the current pixel to be blurred in the center of the line.

How Border Pixels Are Treated:
Pixels around the border of the image, which do not have adjacent pixels on all sides, are mirrored; that is, they are treated as if there were adjacent pixels on all sides by mirroring the values of the inner adjacent pixels.

You can control the following, using the indicated parameter on the Vector Blur node panel to do so:

  • the length of the vector, which affects the overall magnitude of the blur ("Magnitude" parameter)
  • the angle of the vector, ("Direction" parameter)
  • the type of distribution curve used to weight the contribution of each cell in the kernel ("Filter" parameter)
  • the portion of the curve used in the computation ("Extents" parameter)

Using a Control Image with Vector Blur

The Vector Blur node accepts one or two inputs. The second, optional input is used as a control image for the blur. The value of each pixel in the control image determines how the corresponding pixel (the pixel with the same x,y coordinates) in the primary input image will be processed by the Vector Blur node.

Note:
The control input must be a one-channel image if the "on/off" control method is specified or a two-channel floating point image if the "direction" control method is specified.

On / Off

The On/Off setting turns the blur operation on and off at each pixel in the primary image. The control image used for On/Off must be a one-channel image of the same resolution as the primary image input.

If a pixel in the control image has a value of 0 (i.e., is black), the corresponding pixel in the primary image will not be processed, and the original value of the pixel in the primary image will be passed through unchanged. However, if a pixel in the control image has a value other than 0, the corresponding pixel will be processed.

Direction

The Direction setting uses the control image to determine the direction and magnitude of the blur operation at each pixel in the primary image. The control image used for Direction must be a two-channel floating point input.

Tip:
Use the Bump or Surface Normal nodes to create such a control image.

The two channel values in the control image represent the x and y coordinates of a vector. Chalice uses this vector to calculate the direction and magnitude that will be used to blur the corresponding pixel in the primary image.

Vector Blur Parameter Tab

Magnitude

The Magnitude parameter enables you to specify the length of the vector used to define the blur. You set this parameter to specify the general level of blur, with higher values resulting in a greater blur effect. The greater the magnitude, the longer the blur will take to process.

For example, if the Magnitude is 6 and you are working in pixels, the total length of the vector would be 7 pixels (the pixel to be blurred occupies the center position in the row). Working in percentages (see the "Measured In" parameter description below), a Magnitude value of 6 would represent a vector measuring 6 percent of the total width of the image. There is no benefit to using negative values in this parameter; that is, a value of -6 is equivalent to a value of 6.

Measured In

The Work In parameter is used to specify whether the Magnitude value will represent number of pixels or a percentage of the total width of the input image.

Direction

The Direction parameter enables you to set the direction of the vector used to define the blur. This parameter, along with the Extents parameter, is used to specify the apparent direction of motion in a motion blur.

You can set the Direction parameter by clicking the directional line on the compass icon and dragging it counterclockwise to rotate it to the desired angle. There is no benefit to rotating the compass icon in excess of 360

For example, a setting of 380

will generate the same result as a setting of 20 (all other things being equal).

You can also set the Direction parameter manually by keying in a numerical value in the range of 0-360 degrees. The default value is 0, which equates to a horizontal left-to-right direction.

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 blur effect:

  • Box filter: The box filter averages the values of each pixel in the kernel, so that each pixel contributes in equal measure to the blur effect.
  • Triangle filter: The triangle filter performs a weighted average of the pixel values, with the central pixels contributing more to the result.
  • Gaussian filter: The Gaussian filter uses a bell-shaped curve to determine the contribution of each pixel in the kernel to the blur.

Extents

The Extents parameter enables you to "clip" the function curve used to control the blur. In effect, this eliminates the contribution of any pixels in the kernel that fall outside the boundaries you set using the Extents parameter. To use the Extents parameter, you key a pair of values between -1 and 1 into the text entry fields:

  • A value of -1 represents one end of the established vector.
  • A value of 0 represents the center pixel of the vector.
  • A value of 1 represents the other end of the established vector.

The Extents parameter can be used to help create the effect of motion in a motion blur by enabling you to specify that only a portion of the pixels in the kernel be used in the computation. For example, to create the effect of an object moving left to right you might set the Extents parameters to 0 and 1.

Using a horizontal vector as an example, keying a value of 0 into the left text entry field and 1 into the right text entry field would eliminate the contribution of all pixels in the kernel that fall to the left side of the central pixel, as in the following illustration:

Extents Values in Relation to Direction of Vector

As the selected vector deviates from 0 degrees (the default), you can think of it as rotating counterclockwise to determine which edge of the vector is represented by the -1 value, and which by the 1 value. For example, for a 90-degree (vertical) vector, a value of 1 represents the top edge of the vector and -1 represents the bottom edge.

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.

Control Image

This menu enables you to specify whether the On/Off or Direction method will be used for the control image, as explained in the earlier section on Using a Control Image with Vector Blur .




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