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RE: [APML]Gradient Mask Question



Hi Gary,
 
One easy way to make a light gradient mask in Photoshop is to use the original image to create the mask.  Here are the "quick and dirty" steps:
 
1. Duplicate your background layer - this duplicate will be your mask layer.
 
2. Select > All (Ctrl+A) and Select > Invert (Ctrl+I) your mask layer to create a negative image.
 
3. Run a wide radius gaussian blur on the mask layer (70-100 pixels or so on your image).  If there are bright stars in the image, or subject matter (galaxy, nebula, etc.) you may want to remove those first with the Rubber Stamp tool (a/k/a Clone tool).
 
4. Select "Screen" mode for the mask layer and adjust the opacity of the layer until the light gradients are gone (the image will look overly bright since you are raising the luminosity levels of the dark areas, rather than lowering the luminosity of the brighter areas in order to balance them out).
 
5. Use a Levels and/or Curves adjustment to restore the black point and add contrast back into the image.
 
If you would like me to email you the Photoshop file (with the layers intact) with the above steps applied to your image, please just contact me off list.
 
Good luck with it,

Scott

R. Scott Ireland
ireland@gate.net
www.rsiphotos.com

-----Original Message-----
From: astro-photo-bounces@seds.org [mailto:astro-photo-bounces@seds.org]On Behalf Of StarManGMH@aol.com
Sent: Wednesday, January 14, 2004 5:07 AM
To: astro-photo@seds.org
Subject: [APML]Gradient Mask Question

I'm trying to learn how to do a gradient mask to correct for skyglow near the horizon.  I'm practicing on an image I took of Sagittarius with trees in the foreground and a lot of skyglow.  The picture is way overexposed but I'm trying to learn this technique.  Is it possible to use a gradient mask (linear or radial) to correct it without destroying the trees?  I haven't quite gotten the radial procedure down yet but when I tried the linear it really messed the picture up.  Here's a link to the raw scanned image.  Is it possible to fix something like this or do I need a better picture with less skyglow?  Any advice or tips would be appreciated.
 
Gary
 
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