Decrease Star Size
Peter Shah's Method

This is an interesting method that works by edge finding and clever masking.

  1. Open your image
  2. Create a duplicate using the duplicate mini-icon
  3. With the duplicate layer active find edges using Filter > Stylize > Find Edges
  4. Apply a very weak Gaussian blur, Filter > Blur > Gaussian Blur... with radius = 0.1
  5. Adjust brightness and contrast with Image > Adjustments > Brightness/Contrast... using Brightness = 15 and Contrast = 15
  6. Do some color range selecting with Select > Color Range... Select = Highlights, Invert checked
  7. Modify the selection, Select > Modify > Expand... Pixels = 1, then Select > Modify > Feather... with pixels = 1
  8. Now something tricky. Alt-click on the background layer, which makes it the active layer without showing it. You'll still be looking at the duplicate layer.
  9. Apply a minimum filter: Filter > Other > Minimum... with Radius = 1 pixel
  10. Fad the minimum filter: Edit > Fade... with Opacity = 40% and Mode = Normal
  11. Deselect the selection: Select > Deselect
  12. Alt-click on the duplicate layer
  13. Delete duplicate layer

How does this work? Let's look at it step by step:

3: Find Edges. Changes the image so that the intensity at each pixel determined by the magnitude of the local intensity grandient. The important effect is in star halos, where we go from saturation to black in a short distance. This steep fall-off in brightness will be replaced with darkness, while the star's center (an area with fairly uniform brightness) will remain light. The end result is that each star becomes a dark ring.

4. My guess is that this is performed to eliminate any noise generated by the edge-finding.

5. This sharpens the dark edge rings of brighter stars and reduces the effect of edge finding on fainter stars

6. Edge finding leaves the background white and so in the range of highlights, so this step selects the background and white spots within the stars' edge rings. Appling Invert means that in the end it will be only the dark rings that are selected.

7. Expanding and feathering extends the dark ring both outward into what is the darker area of the halo and inward into the star's bright core.

8. I believe this applies the copy layer's selection as a clipping mask to the background layer. Now anything done to the background layer will only take effect in its selected areas .

9. The minimum filter here is working on those little dark rings areas in the background layer, eroding them by letting darker values replace lighter values. On the ring's outer edge, some of the darkness that was included during the Expland/Feather erodes inward, shrinking the star. On the inner edge there's nothing but the light of the star's center, so no change takes place.

10. Fading weakens the change and blends back in some of the star's original halo for a more pleasing effect.

Anyway, Mr. Shah was kind enough to make this into an action that you can read about here and download. He made this relatively non-aggressive so that it can be run a number of times without generating significant artifacts.

Return to main Processing page