Computer Graphics with Photoshop - How do I edit only the A channel of an RGBA image?

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

Gammaburst

2006-03-17, 6:14 am

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.

Clyde

2006-03-17, 6:14 pm

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

Johan W. Elzenga

2006-03-17, 6:14 pm

Gammaburst <eye@hate.spam> 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

iehsmith

2006-03-17, 6:14 pm

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

> Gammaburst <eye@hate.spam> wrote:
>
>
> 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.

Mike Russell

2006-03-17, 6:14 pm

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

Gammaburst

2006-03-18, 6:14 pm

On Fri, 17 Mar 2006 20:21:00 GMT, "Mike Russell"
<RE-MOVEmike@Curvemeister.comRE-MOVE> 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.