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DPChallenge Forums >> Tips, Tricks, and Q&A >> Background replacement
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 06/24/2011 04:43:31 PM · #1


I've got a project I'm working on- It involves combining photography and typography and a few other graphic elements. In order to make some of the photos more graphic, I'm replacing the background with non-photographic elements (solid colors, patterns, etc). I've intentionally tried to isolate a single color sky or blown out to make it easier to replace.

does anyone have any suggestions on what the best way to accurately mask the sky out?

I have CS5, and if you suggest any plugins, I'm on OSX.

edit:: the important thing in this photograph are the wires, so methods that kill those (I know they're rather fine) aren't as helpful.

Message edited by author 2011-06-24 16:46:35.
 06/24/2011 05:01:44 PM · #2
Shouldn't be too hard.

- Duplicate the blue channel
- Apply a curves adjustment to the blue channel and drag the top point to the left and the bottom point to the right just enough so that the sky goes completely white while everything else goes black.
- Now you have your sky selection.

You can also use the brush tool set to overlay to dodge or burn areas on the duplicated channel to fine tune it more (example any white spots left on the bark).

Message edited by author 2011-06-24 17:06:16.
 06/24/2011 05:05:19 PM · #3
Also, once you have used the mask to delete the sky you could also apply Layer > Matting > Defringe which can help remove any remaining halos.
 06/24/2011 05:15:17 PM · #4
With a sky like that one that is pretty homogeneous, you could goof with Refine Edge in CS5 after making a selection. I bet it would work pretty well with a sky like that. You can find tutorials on it on YouTube or the like.
 06/24/2011 05:29:14 PM · #5
Originally posted by yanko:
Shouldn't be too hard.

- Duplicate the blue channel
- Apply a curves adjustment to the blue channel and drag the top point to the left and the bottom point to the right just enough so that the sky goes completely white while everything else goes black.
- Now you have your sky selection.

You can also use the brush tool set to overlay to dodge or burn areas on the duplicated channel to fine tune it more (example any white spots left on the bark).


I'm trying it to learn. Having trouble duplicating just the blue channel. Also, the curves selection left some of the dark part of the blue sky. Could you detail just a little bit more what you do in CS5 please.
 06/24/2011 05:40:38 PM · #6
Have you tried "select color range"?

You can use the color sampler and the shift key and draw on the colors you want to select. Make sure "sampled colors" is selected under Select.

That would at least give you a good mask you can start and play with.
 06/24/2011 05:53:17 PM · #7
The channels mixer gave me the best contrast & fine selection, but only marginally better than the selection method.

Even with my 3.2 Quadcore + 8gb of ram, I couldn't fine tune the selection's made with the selection method -- It just bogged down my pc while trying to calculate.

Clouds, want me to upload a fullsize file for you? You know what, I'll do that anyway. I'll edit with a relink to a larger image.

Originally posted by cloudsme:
I'm trying it to learn. Having trouble duplicating just the blue channel. Also, the curves selection left some of the dark part of the blue sky. Could you detail just a little bit more what you do in CS5 please.


anyway, mini-tut, ask a question if you don't get it.

After opening the image, go in the the channels palette. Drag the blue channel to the bottom right of the palette where the "create new color channel" button is. Now you should have a duplicate blue. On that one, open up your curves, and adjust them so that the sky turns white, and the tree turns (almost) completely black. Now select your brush and paint (either solid white for the sky, or solid black for the tree) any areas that didn't fully turn. Command click (or Control click for PC) on the image in the channels to select it, and then switch to the layers palette. Select the image layer (make sure it's not locked), and press the mask button to apply the channel selection as a mask, then fine tune as you see fit.

Message edited by author 2011-06-24 18:04:37.
 06/24/2011 05:58:39 PM · #8
Originally posted by adigitalromance:
Have you tried "select color range"?

You can use the color sampler and the shift key and draw on the colors you want to select. Make sure "sampled colors" is selected under Select.

That would at least give you a good mask you can start and play with.


There are a few ways to do this as others have already mentioned, but I think this is what I would try first. Color Range is a very strong tool. I used this feature to replace the sky in my supermoon shot and it worked perfectly.
 06/24/2011 06:00:01 PM · #9
Okay, so here's a fullsize of the same image,



and the quick and dirty edit to see the results.

06/24/2011 06:15:55 PM · #10
Originally posted by adigitalromance:
Have you tried "select color range"?

You can use the color sampler and the shift key and draw on the colors you want to select. Make sure "sampled colors" is selected under Select.

That would at least give you a good mask you can start and play with.


That should work too but the drawback is you can't zoom in to see what you're doing since it all functions within that small pop up window.
06/25/2011 11:50:53 AM · #11
Originally posted by jamesgoss:
The channels mixer gave me the best contrast & fine selection, but only marginally better than the selection method.

Even with my 3.2 Quadcore + 8gb of ram, I couldn't fine tune the selection's made with the selection method -- It just bogged down my pc while trying to calculate.

Clouds, want me to upload a fullsize file for you? You know what, I'll do that anyway. I'll edit with a relink to a larger image.

Originally posted by cloudsme:
I'm trying it to learn. Having trouble duplicating just the blue channel. Also, the curves selection left some of the dark part of the blue sky. Could you detail just a little bit more what you do in CS5 please.


anyway, mini-tut, ask a question if you don't get it.

After opening the image, go in the the channels palette. Drag the blue channel to the bottom right of the palette where the "create new color channel" button is. Now you should have a duplicate blue. On that one, open up your curves, and adjust them so that the sky turns white, and the tree turns (almost) completely black. Now select your brush and paint (either solid white for the sky, or solid black for the tree) any areas that didn't fully turn. Command click (or Control click for PC) on the image in the channels to select it, and then switch to the layers palette. Select the image layer (make sure it's not locked), and press the mask button to apply the channel selection as a mask, then fine tune as you see fit.


Thank You so much! Don't need a full size image, have many of my own that I have wanted to select blue sky out of trees.

Message edited by author 2011-06-25 11:52:10.
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