Spatial Filters Command


The Spatial Filter command applies a filter to an Image Set or a single image. This dialog organizes a large number of filters into a tree format based on their category. For example, all of the smoothing filters are grouped together under one heading and all the Sharpening filters are organized under a different heading. Selecting any filter loads all of its controls.

Filters are selected from the tree control on the left side of the dialog. The tree organizes filters into categories based on what they do. For example, the Smoothing node lists the group of filters whose primary effect is to smooth the image.

Applying a Filter

  1. Click on the node containing the type of filter you want. Then click the desired filter to load its preferences.

  2. Change the filter parameters if necessary.

  3. To process the image using the selected filter, click [Process]. This action also updates the parameters you have set.

  4. If you do not like the result, use Ctrl+Z or Edit > Undo to recover the original images.

Description of Individual Filters

The table below describes each of the filters available in the Spatial Filters dialog.

Filter Descriptions

Unsharp Mask

Applies an unsharp mask sharpening filter. The parameters specify the blurring profile which is subtracted from the image.

Highpass

Applies a "high pass" sharpening filter. The Strength parameter controls the effect. A setting of 0 performs almost no sharpening.

Rectangle

Applies a rectangular smoothing kernel to the image.

Binomial

Applies a filter kernel having binomial weights. This filter approaches a Gaussian filter shape for sizes beyond 5--7 pixels, and works very fast.

Elliptical

Applies an elliptical smoothing kernel to the image.

Gaussian

Applies a Gaussian smoothing kernel to the image.

Block Average

Combines adjacent pixels by averaging their values in blocks of a specified size. Each block of pixels becomes 1 pixel in the output image.

Minimum

Replaces pixels with the minimum value of all pixels inside the filter kernel at each point.

Maximum

Replaces pixels with the maximum value of all pixels inside the filter kernel at each point.

Percentile

Replaces pixels with the percentile value of all pixels inside the filter kernel. The minimum, maximum, and median filters are special cases of this filter having ranks of 0, 100, and 50, respectively.

Median

Applies a Median filter to the image. This rejects outlying pixel values, no matter how extreme, and tends to preserve edges relatively well, but images can become "blocky" from using too large a kernel. This filter is accessible both from the Smoothing group and the Rank Filters group.

De-speckle

Removes isolated speckles, or outlying pixel values. This is usually applied to RGB images to remove color channel noise.

Cosmic Ray

Applies a contrast-based point source rejection filter. This works well with bright, isolated bad pixels like those resulting from cosmic ray hits and radioactive decay tracks. The filter rejects deviant pixels that exceed an adjustable threshold compared to the brightness of their neighborhood. Also see the Apply Cosmic Ray Filter comamnd and the Clean Image Set command.

Clip Low

Replaces all pixels below a threshold value with a new value.

Clip High

Replaces all pixels above a threshold value with a new value.

Compass Gradient

Applies a directional gradient filter. This type of filter enhances details lying along the pointing ("compass") direction of the filter.

Rotational Gradient

Applies a filter that mainly enhances structure that winds outward from points like the spokes of a bicycle wheel.

Laplacian

Applies a Laplacian operator to enhance changes in slope or gradient. The actual gradient is discarded by the filter, leaving only changes that occur at edges and discontinuities. Use this type of filter to locate edges.

DDP

Applies "Digital Development Processing", which is primarily an exponential stretch above a threshold value.

Related Topics

Spatial Filter Overview, 3x3 Custom Filter, Custom Filter, Apply Cosmic Ray Filter, Clean Image Set