Filters (Spatial): Unsharp Mask

When applied to an image, this algorithm produces a sharpened version of the image or the image volume defined by a VOI of the image.

The sharpening process works by utilizing a low-pass filtered (blurred) version of the original image. This is then subtracted away from the original to detect the presence of edges, creating the unsharp mask (effectively a high-pass filter). Contrast is then selectively increased along these edges using this mask — leaving behind a sharper final image.

The effect of a high-pass filter, like the unsharp mask technique, is to enhance sharp intensity transitions (i.e., edges) but at the cost of enhancing noise.

Image types

You can apply this algorithm to all image data types, except complex and RGB images, and to 2D, 2.5D, 3D, and 4D images.

Applying the algorithm

To run this algorithm, complete the following steps:

1. Select Algorithms > Filter (spatial) > Unsharp mask. The Unsharp Mask dialog box (Figure 1) opens.
2. Complete the information in the dialog box.
3. Click OK.

The algorithm begins to run, and a pop-up window appears with the status. The following message appears

"Unsharp Masking Image."

When the algorithm finishes running, the pop-up window closes, and the results either appear in a new window or replace the image to which the algorithm was applied.

Figure 1. Unsharp Mask dialog box

X Dimension	Specifies the standard deviation (SD) of Gaussian in the X direction.
Y Dimension	Specifies the SD of Gaussian in the Y direction.
Z Dimension	Specifies the SD of Gaussian in the Z direction.
Use image resolutions to normalize Z scale	Normalizes the Gaussian to compensate for the difference if the voxel resolution is less between slides than the voxel resolution in-plaine. This option is selected by default.
Process each slice independently (2.5D)	Filters each slice of the dataset independently of adjacent slices.
Image - (k<1) * blurred image	Determines the amount of filtering, or sharpening, applied to the edges of the image or a VOI of the image. Low values sharpen image edges but increase noise. The default value is 0.75.
New image	Shows the results of the algorithm in a new image window.
Replace image	Replaces the current active image with the results of the algorithm.
Whole image	Applies the algorithm to the whole image.
VOI region(s)	Applies the algorithm to the volumes (regions) delineated by the VOIs.
OK	Applies the algorithm according to the specifications in this dialog box.
Cancel	Disregards any changes that you made in this dialog box and closes the dialog box.
Help	Displays online help for this dialog box.

See also:

• • Filters (Spatial): Adaptive Noise Reduction
  • Filters (Frequency)
  • Filters (Spatial): Adaptive Path Smooth
  • Filters (Spatial) Anisotropic Diffusion
  • Filters (Spatial): Coherence-Enhancing Diffusion
  • Filters (Spatial): Gaussian Blur
  • Filters (Spatial): Gradient Magnitude
  • Filters (Spatial): Haralick Texture
  • Filters (Spatial) Laplacian
  • Filters (Spatial): Local Normalization
  • Filters (Spatial): Mean
  • Filters (Spatial): Median
  • Filters (Spatial): Mode
  • Filters (Spatial): Nonmaximum Suppression
  • Nonlinear Noise Reduction
  • Filters (Spatial): Regularized Isotropic (Nonlinear) Diffusion
  • Filters (Spatial): Slice Averaging