DDP Sharpening for PhotoShop

The following tips are based on PhotoShop CS (8.0), but similar tools exist in PS versions 5-7.

What's it good for?
DDP stands for Digital Development Process and is a non-linear image manipulation used to bring out dim areas in  high dynamic range images, without saturating or overexposing the brighter areas. DDP was first introduced and documented by Kunihiko Okano. In addition to histogram manipulation, DDP applies a level of sharpening to the resulting image. The method presented here was developed to accomplish both, sharpening and bringing out dim areas using standard PhotoShop functions.

NOTE: the process described here is not a real DDP as described by Dr. Okano, but a PhotoShop workflow meant to produce an effect similar to DDP. 

In my testing, this process works best on a large nebulae, but is worthwhile to try on galaxies and even globular clusters. How well does it work? Here are some example images processed this way.

The Method
 In summary, the idea is to use the Overlay layering mode in  PS to simulate division of the original image by a blurred copy of itself. PS overlay mode actually works as both, Screen and Multiply modes, depending on the brightness of the background. This produces the additional histogram stretching effect that brings out the dim areas, while controlling the brightness of  the light ones.

I've developed a PhotoShop Action that performs most of the PS DDP processing for you automatically. At the end of the action, you can decide how much blurring to apply, and then work on the brightness/contrast of the resulting image. Download the PS DDP action here. The action performs steps 3-9 automatically.

Here is the step-by-step workflow:

1. Load the original grayscale image. If you're using version 8 of PhotoShop, keep all the processing in 16 bits, otherwise convert to 8 bits grayscale. 

2. Adjust levels and curves so the image is at the brightness level to you'll ultimately want, without saturating or overexposing bright areas.

3. Copy image into clipboard (Ctrl-A followed by Ctrl-C)

4. Invert image (Ctrl-I)

5. Paste original image as a new layer, Layer 1 (Ctrl-V). Set blending mode to Overlay.:

6. Optional, but recommended:
Add a layer mask to Layer1 and paste the clipboard contents into the mask:

7. Add an an Invert adjustment layer above Layer 1 from Create new Fill or Adjustment Layer icon, circled below:

8. Re-activate Layer 1 by clicking on it in the Layers palette.

9. Apply Gaussian blur to Layer 1 (Filter/Blur/Gaussian Blur... menu). Start with 1-5 pixel radius, but try larger radius also to see what works best.

10. If necessary, adjust the Layer 1 blending percentage to less than 100% to reduce any undesirable sharpening effects.

Optional: 

11. Flatten image using Layer/Flatten Image menu.

PhotoShop Action

This action was created in PhotoShop CS, I'm not sure whether it'll work in any other version of PhotoShop.

C:\Program Files\Adobe\Photoshop CS\Presets\Photoshop Actions 

Note: Although this PhotoShop action will not work on color images, the process described above can be successfully applied to RGB images, simply by following the directions. The action has some dependencies on PhotoShop generated names, and these are different for color images than they are for grayscale, so you'll get some errors if you try the action on a non-grayscale image.

Examples

M13 Globular: Original  and  PS DDP Processed

Pelican Nebula: Original  and  PS DDP Processed

Nebulosity around LDN889 in Cygnus: Original   and  PS DDP Processed.

I'd love to hear from you if this was helpful and if you have other tips or tricks that improve on this method.

Copyright 2004 Paul Kanevsky