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B&W image to alpha channel

Mar 21, 2012, 23:16
Post: #1
B&W image to alpha channel
Hi,

I have a B&W images. It's actually in RGB but all shades are gray. I would like to create an alpha channel whose values equal the brightness of each pixel.

How do I do that?

Thanks!

Sam
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Mar 21, 2012, 23:48
Post: #2
RE: B&W image to alpha channel
(Mar 21, 2012 23:16)samtakoy Wrote:  Hi,

I have a B&W images. It's actually in RGB but all shades are gray. I would like to create an alpha channel whose values equal the brightness of each pixel.

How do I do that?

Thanks!

Sam
- Make this layer the only visible layer.
- In the Channels list, drag any of the color channels further down the list: this will create a copy
- Select layer for layer mask, "Layer/Mask/add layer mask" and initialize to "Channel", and pick the color channel copy.

An equivalent method, without layer mask:
- Make this layer the only visible layer.
- In the Channels list, drag any of the color channels further down the list: this will create a copy
- Invert channel
- Channel to selection (or Channel to selection & Invert selection)
- Select layer
- [delete]

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Mar 22, 2012, 00:09
Post: #3
RE: B&W image to alpha channel
Thanks.

I tried the equivalent second approach, but I am not sure it worked. After I copy a channel by dragging (now I have "Blue Channel Copy") what makes it into the alpha channel?

Thanks again,

Sam
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Mar 22, 2012, 00:46 (This post was last modified: Mar 22, 2012 00:46 by ofnuts.)
Post: #4
RE: B&W image to alpha channel
(Mar 22, 2012 00:09)samtakoy Wrote:  Thanks.

I tried the equivalent second approach, but I am not sure it worked. After I copy a channel by dragging (now I have "Blue Channel Copy") what makes it into the alpha channel?

Thanks again,

Sam
2nd way also works (I tried both and they give the very same final image) because, if deleting a 100% selected pixel produces a completely transparent pixel, deleting a pixel with a 20% selection (ie, not very much selected) is making it 80% opaque.. (ie, deletes it only a bit), while a very selected pixel (80% selection) is pretty much deleted (20% opacity).

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