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How can you combine multple alpha channels into a single mask?

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  #1  
Old 10-03-2007, 06:04 AM
beestripe beestripe is offline
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How can you combine multple alpha channels into a single mask?

I have wedding photo where the bridal party is walking down a path. I want to Lens Blur the backround by creating a gradient mask leading away behind her up the path, but keep the bridal party sharp in the foreground.

Normally I would do this by:
- Creating a gradient in quick mask and hard brush the subject in the mask I want to keep sharp.
- Then save the mask as an alpha channel and choose it as the Depth Map in the Lens Blur options.

Problem is, this somewhat crude technique can be limiting if you want to change either the gradient or the main subject selection and often means you need to start all over again.

Ideally it would be best if I could combine alpha channels (in this case a foreground mask and a gradient mask) in to one mask for the Lens Blur Depth Map.

It's been fruitless with my google enquiries, any suggestions?

TIA

Last edited by beestripe; 10-03-2007 at 09:26 AM. Reason: grammar
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Old 10-03-2007, 09:15 AM
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mistermonday mistermonday is offline
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Re: How can you combine multple alpha channels into a single mask?

TIA, you can add alpha channels together. Go Select > Load Selection. Choose the alpha channel you want. Then Go Select > Load Selection. When you select the next alpha channel you will get a bunch of options, one of which will be to Add the selection to the active selection.

In general for these types of compositions it a good idea to create a mask of the main subject and save it as an alpha channel. This gives you a lot of flexibility to manipulate it on its own layer.

Regards, Murray
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Old 10-03-2007, 09:26 AM
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DannyRaphael DannyRaphael is offline
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Re: How can you combine multple alpha channels into a single mask?

Hope the following gets you where you want to go...

Suppose you have 2 alpha channels named, say, Alpha 1 and Alpha 2...

In the Channels Palette:
* Click on (highlight) Alpha 2
* From the Cnannels Palette menu choose Duplicate channel... (assign a name of your choice), e.g., Combined
* Now Ctrl + click on Alpha 1 to "load its selection" (marching ants). The same results can be achieved via Select > Load Selection... and specifying Alpha 1
* Next Select > Save Selection...
* From the Channel dropdown menu (default is New) choose Combined.
When you make this selection the options below with become "un-grayed."
* Choose the applicable option -- probably Add -- and click OK. Alpha 1 and Alpha 2 will be merged into Combined


This approach keeps the original alpha channels in tact. If you needed to do this often, an action would streamline the process.

More complex merging of alpha channels can be done via Image > Calculations, but that's probably overkill for what you're trying to do here.
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Old 10-03-2007, 09:27 AM
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DannyRaphael DannyRaphael is offline
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Re: How can you combine multple alpha channels into a single mask?

Quote:
Originally Posted by mistermonday View Post
TIA, you can add alpha channels together. Go Select > Load Selection. Choose the alpha channel you want. Then Go Select > Load Selection. When you select the next alpha channel you will get a bunch of options, one of which will be to Add the selection to the active selection.

In general for these types of compositions it a good idea to create a mask of the main subject and save it as an alpha channel. This gives you a lot of flexibility to manipulate it on its own layer.

Regards, Murray
Nice going... you beat me to the punch while I was playing!
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Old 10-03-2007, 09:57 AM
beestripe beestripe is offline
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Re: How can you combine multple alpha channels into a single mask?

ahha, I knew it would be something logical eluding me, problem solved

In the meantime I figured out a work-around, simply by creating a lens blur with the foreground subject masked on a copied layer, then made a Layer Mask gradient to reveal the original layer's sharper foreground. This way I could redo the gradient if desired and keep the mask of the main subject.

Cheers
Attached Images
File Type: jpg DOFexample.jpg (96.1 KB, 27 views)

Last edited by beestripe; 10-03-2007 at 10:06 AM.
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Old 10-03-2007, 10:07 AM
pixelzombie pixelzombie is offline
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Re: How can you combine multple alpha channels into a single mask?

i was gong to suggest duplicating the main layer, but it looks as if you've figured that out already...
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Old 10-03-2007, 10:30 AM
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Graphics23 Graphics23 is offline
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Re: How can you combine multple alpha channels into a single mask?

Use the Calculations and Apply Image commands.

Regards,

Michael
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