PhotoShop Tips for Selective Brightness Masking

Written by Paul Kanevsky, M33 image provided by Rick Yandrick

The following discussion is based on the PhotoShop version 6.0, but similar tools existed in version 5.0 and I believe are still there in version 7.0. 

When  working on an image of a galaxy or another wide-dynamic range object,  you'll often want be able to increase brightness and contrast of an area of the image without altering the background. Or, you may want to apply an Unsharp Mask, but only to the core of the galaxy. Alternatively, maybe you'd like to apply a Gaussian blur to just the dim areas of a nebula. Here is a technique that allows you to select an area of an image in proportion to its brightness. 

1. Load your 16-bit image into PhotoShop



2. Duplicate your image and convert the copy to the 8-bit Grayscale mode:
        - Image/Duplicate... menu followed by
        - Image/Mode/8 Bits/Channel menu

3. Switch to the original image and choose the Select/Load Selection menu option. If this option is grayed out, make sure you converted the copy to the 8-bit mode, as the selection mask must be an 8-bit image.

4. Working with your original image, select the copy image as the source of the selection:

5. If you want to make changes to the dim areas, go ahead and check off the Invert box in this dialog.

6. Click OK and PhotoShop will create a variable transparency mask that is based on the source image. The brighter the pixel in the source image, the more the corresponding pixel will be affected by changes.

Note that the selection band in PhotoShop indicates the area where the image is 50% selected. In the brightest areas, the selection is 100%, and in the dim areas - 0%. The bright areas include most of  the stars, since they are all pretty bright.

7. Now you're ready to apply image adjustments. The adjustment will be much more pronounced in the bright areas (dim, if you selected the Invert check box). Below I applied an unsharp mask and a curves tool to bring out more detail in the galaxy core:

After adjusting the bright areas, you can easily change the selection to focus on the dim areas by selecting the Select/Inverse menu item.

Shortcut

There's a shortcut to load a brightness selection into an 8-bit image.  Just go to the Channels selector and hold down the Ctrl key while clicking the mouse on the Gray channel:

 

Advanced Topics

You have a lot of control as to how much of the image is selected using the brightness mask. Let's say that you really want to select the brightest areas of the image, and reduce the selection of the dimmer areas even more. To accomplish this simply adjust the contrast of the selection image (the 8-bit copy that was created in step 2 above), and then follow the same directions for loading it:

There's no limit as to what you can do with the mask image. Another example would be to eliminate the stars from the selection. To accomplish this, use the Dust and Scratches removal filter (Filter/Noise/Dust&Scratches menu):

Now, when loaded into the main image the selection is focused on just the bright areas of the galaxy, excluding all stars:

There's another neat tool in PhotoShop called the Layered Mask. This allows you to apply curves, contrast adjustments, etc. to a layer without modifying the pixels of the layer. This transformation is temporary and can be removed at any time or applied and made permanent. Furthermore, the layered mask allows a brightness selection mask to be attached to it, so the adjustment is made to a specific area of the image, just like in the above discussion. This is a complex topic and will require another tutorial page yet to be written...

Copyright 2002 Paul Kanevsky