Starguide - A Practical Guide to Astro-Photography

• Erosion: Erosion is one of the two fundamental morphological operations. Erosion is defined as the set of all points z such that the mask, translated by z, is contained in the image. In other words, erosion outputs a zero if any of the input pixels under the “1" pixels in the mask (structuring element) are zero. In terms of image processing, erosion reduces an object’s geometric area. When all elements in the mask are equal, erosion acts as a simple minimum filter.
• Dilation: Dilation is defined as the set of all points where the intersection of the structuring element and the image are non-empty. For each source pixel, if any of the pixels in the mask are “1” and line up with a source pixel which is also “1”, the output pixel is “1”. When all elements in the mask are equal, erosion acts as a common maximum filter.
• Opening: The opening function is the dilation of the erosion of the image. It tends to smooth outward bumps, break narrow sections and eliminate thin protrusions. The opening filter tends to darken bright details.
• Closing: The closing operation is the erosion of the dilation of the image. It tends to eliminate small holes and remove inward bumps. The closing filter tends to brighten dark details.
• Morphological median: A morphological median computes the median of the corresponding pixels in the target image according to the structuring element.
• Morphological selection: Morphological selection acts as a blend between the erosion and dilation methods. When this option is selected, the Selection parameter becomes available. See the Selection parameter below.
• Midpoint: The midpoint function will execute the transformation by averaging the minimum and maximum.

Credits: PixInsight v1.5 - Unofficial Reference Guide: Morphology (deepskycolors.com)

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