Inverse Mask Noise Reduction

This method is designed to concentrate your preferred method of noise reduction on areas in your image that need it most (i.e. the background). It works by creating a mask that's black where there are stars or other bright objects. masks control what parts of the image will be visible--the lighter the greater the visibility.

  1. Open your image
  2. Copy it to the clipboard (Ctrl-A and Ctrl-C)
  3. Paste the copy to a new layer (Ctrl-V)
  4. Create a mask for the duplicate layer (click on the mask mini-icon)
  5. Alt-click the mask and copy the image to it (Ctrl-V)
  6. Invert the mask (Ctrl-I)
  7. Click the duplicate layer's thumbnail and apply whatever noise reduction you would like. This can be Photoshop's Noise Reduction or a simple Gaussian blur. Focus on the appearance of the background to judge your noise reduction parameters.
  8. Alt-click the mask and apply the same noise reduction. Some descriptions of inverse masking omit this step, but I think it helps feather the noise reduction.
  9. Use Levels (Ctrl-L) to ajust the mask's black and white points inward. The amount you move them will depend on your image.
  10. Click the duplicate layer's thumbnail, and then toggle the visibility of the layer visibilty to see what it has done. If you're satisfied with the result, merge the layers and save the image.
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