How do I edit only the A channel of an RGBA image?

Gammaburst wrote:

When I open an RGBA image in Photoshop 7, the Channels palette shows me only the RGB channels. How do I edit only the A (alpha) channel of the image without disturbing the RGB channels?

For example, I have several RGBA images (TIFF and PNG format) that were rendered in Lightwave. These images are full-color RGB images, plus an A channel that masks small areas of the image. Photoshop shows me only the masked areas, the rest of the image is just transparency checkerboard. I want to delete the A channel (or fill it with a solid value) to reveal the RGB information "hiding" behind the transparency. But I can’t find any way to do that in Photoshop.

Why and how would anything hide behind transparency? I don’t understand what you are trying to do. To my knowledge, nothing is behind transparency.

An Alpha channel in Photoshop is pretty much just a saved Selection. It doesn’t get used until you load it as a selection. Otherwise it just sits there as an extra channel taking up space. If you haven’t loaded the alpha channel as a selection, it doesn’t come into play. If it doesn’t come into play, it can’t be hiding anything.

You can edit an alpha channel. Click on the Channels tab and click on the Alpha channel. It can be renamed too. It is just a grayscale bitmap channel. You can do anything you can do to a grayscale image. White and black completely reveal and hide while grays give partial selections.

I’ve never seen it, but I’m guessing that transparency on an alpha channel works like white.

Keep in mind that editing the alpha channel doesn’t change anything on the image. You have to edit the alpha channel, load as a selection, and then edit the image.

If you have an alpha channel that was created in another program, you can’t undo what was done in that other program. The other program used the alpha channel as part of its processing. Once it saved the file, the changes it made using the alpha channel are applied to the image. You can’t undo that in Photoshop. Ideally, that other program would have deleted the alpha channel, but they don’t always.

[In my panoramic creation process I use Enblend. It always leaves a completely white alpha channel in the resulting TIFF. I just trash it.]

I think the confusion might be in the term "mask". Alpha channels are often referred to as masks. They aren’t active though until you load them as selections.

I hope I hit on what was useful.

Clyde

Gammaburst wrote:

When I open an RGBA image in Photoshop 7, the Channels palette shows me only the RGB channels. How do I edit only the A (alpha) channel of the image without disturbing the RGB channels?

For example, I have several RGBA images (TIFF and PNG format) that were rendered in Lightwave. These images are full-color RGB images, plus an A channel that masks small areas of the image. Photoshop shows me only the masked areas, the rest of the image is just transparency checkerboard. I want to delete the A channel (or fill it with a solid value) to reveal the RGB information "hiding" behind the transparency. But I can’t find any way to do that in Photoshop.

Just go to the channels palette and click on that alpha channel.

—
Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl

On 3/17/06 11:19 AM, Johan W. Elzenga commented:

Gammaburst wrote:

When I open an RGBA image in Photoshop 7, the Channels palette shows me only the RGB channels. How do I edit only the A (alpha) channel of the image without disturbing the RGB channels?

For example, I have several RGBA images (TIFF and PNG format) that were rendered in Lightwave. These images are full-color RGB images, plus an A channel that masks small areas of the image. Photoshop shows me only the masked areas, the rest of the image is just transparency checkerboard. I want to delete the A channel (or fill it with a solid value) to reveal the RGB information "hiding" behind the transparency. But I can’t find any way to do that in Photoshop.

Just go to the channels palette and click on that alpha channel.

The OP said there is no Alpha channel in the Channels palette. Basically they’re just images with transparent backgrounds. Chances are there is nothing in the transparent areas.

There are no layer masks or clipping paths showing in the layers palette, right? I don’t know anything about Lightwave, but maybe they wre exported to TIFF/PNG without Alphas.

I think I know what the OP is describing, although I have no solution.

Photoshop supports a per layer transparency that is sometimes called "visible transparency". Most of us are familiar with it, and it works so well that few of us think of it as anything except the image data existing or not at a given location.

Visible transparency is not directly accessible as a mask or channel in Photoshop, though it may be used as a source in Image>Calculations, copied as a selection. It may be reduced by the eraser or the layer’s opacity slider. AFAIK there is no such "anti-eraser" that allows the transparency to be decreased.

When visible transparency is zero, any RGB values at that location are displayed as zeroes, even if the rendering program deposited RGB values at that location. An example would be a z-clipping based compositing operation that is no longer wanted.

Perhaps someone has already provided a plugin that provides access to transparency.
—
Mike Russell
www.curvemeister.com

On Fri, 17 Mar 2006 20:21:00 GMT, "Mike Russell" wrote:

I think I know what the OP is describing, although I have no solution.
Photoshop supports a per layer transparency that is sometimes called "visible transparency". Most of us are familiar with it, and it works so well that few of us think of it as anything except the image data existing or not at a given location.

Visible transparency is not directly accessible as a mask or channel in Photoshop, though it may be used as a source in Image>Calculations, copied as a selection. It may be reduced by the eraser or the layer’s opacity slider. AFAIK there is no such "anti-eraser" that allows the transparency to be decreased.

When visible transparency is zero, any RGB values at that location are displayed as zeroes, even if the rendering program deposited RGB values at that location. An example would be a z-clipping based compositing operation that is no longer wanted.

Perhaps someone has already provided a plugin that provides access to transparency.

Yes, that’s correct. These RGBA images have full-color information in every pixel of the the RGB channels. If I open them in ACDSee (or any other simple picture viewer app that ignores the A channel), I see the full RGB image, so I know it’s there. But Photoshop isn’t letting me access it.