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The Unsharp Mask Filter

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The Unsharp Mask Filter is a powerful technique to reveal small details, especially in planetary images. The idea is to create a low-pass filtered image (the unsharp mask) which suppresses the details in an image. This mask is combined algebraically with the original image to get a superior result.

To see how this works, consider multiplying the original image by 3 and the unsharp mask by 2. When the 2x unsharp mask is subtracted from the 3x original, the small detail (contained in the high frequency parts of the original) is emphasized by a factor of 3. If we think of the original image consisting of (High + Low) and the unsharp mask consisting of only Low, we have, mathematically (3*High + 3*Low) - 2*Low = 3*High + Low.

NGC 2261 - Hubble's Variable Nebula
Original (A)Unsharp Mask (B)Merged (3*A - 2*B)
Image of Hubble's Variable Nebula (NGC2261) taken 11 February 1997 Unsharp Mask (3x3) of Hubble's Variable Nebula (NGC2261) Algebraic merge of Left Image and Right Image (3*Left - 2*Right)

The Unsharp Mask Filter creates the unsharp mask. It works on the image in the A buffer and leaves the result in the B buffer. You are given choices as to how big to make the unsharp mask in both the row and column directions. The size of the filter is given by (2*rows + 1) x (2*cols + 1). The example above used a 3x3 filter.