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Question

Photoshop not presenting alpha channel as expected (like I'm seeing GIMP does)

Forum|Forum|13 years ago
November 8, 2012
5 replies
37875 views

Greetings,

I have a need to edit the channels independently (the values in my image mean things numerically rather than just what it looks like visually - so I need to edit the grayscale values in each of the R - G - B - and A channels seperately).

I've searched through google and other forum posts and others bring up the concept of not understanding alpha versus transparency - but I have yet to see an explanation that fully explains what I'm experiencing.

The closest was http://forums.adobe.com/message/2563436#2563436 But while I saw the exact same symptoms being described, the specific question was never really answered.

Hopefully, a concrete example with pictures will be able to communicate the issue and solicit a useful explanation and/or solution.

While, I'm specifically working with an RGBA image, below is a screenshot of a PNG that demonstrates the exact same behavior or symptom. The screenshot below represents the PNG inside GIMP and PS CS4. BTW, if you'd like to see or play with the image yourself, you can grab the same image at http://www.libpng.org/pub/png/img_png/imgcomp-440x330.png

GIMP, interestingly enough loads the PNG as expected - I see the transparency of the image treated as a true alpha channel.

In PS CS4 however, the transparency seems to be somehow embedded in the Red - Green - and Blue channels; but NO seperate alpha channel.

I can create the alpha channel through Select - Load Selection - Layer 0 Transparency - and pasting that into a newly created alpha channel - but the color channels still have the transparency mixture symptom - rather than showing the JUST the color component of that channel. (Note the channel differences of the GIMP's Red Channel versus PS's Red Channel - Said in another way, the yellow ball in the upper left of the image should have a red value of 255, so the Red Channels grayscale upper left ball should be white like it is in GIMP, rather than Gray as it is in PS).

I never thought I'd experience GIMP being superior to Photoshop - so hopefully somebody can shed light on what is going on - what configuration I may have myself in - or a process to get the channels seperated out in the manner I wish to work with them. Furthermore, maybe somebody can educate the Photoshop community what the difference between transparency and alpha[transparency] is. I haven't found quite the right explanation that makes the light go off in my head yet. As best as I have gleened "Alpha", which can be applied to any kind of channel, typically is in reference to transparency - and is applied as a document-whole transparency (as opposed to transparencies applied at a layer level - such as a layer mask).

This topic has been closed for replies.

JJMack

Community Expert

Photoshop's PGN support is disappointing seeing that Adobe bought Macromedia a couple of links you might find amusing

http://www.adobe.com/support/fireworks/export/fw_export_vs_sav/fw_export_vs_sav02.html

http://www.rotheblog.com/2009/06/design-topics/editing-layered-fireworks-pngs-in-photoshop/

JJMack

Jeff Arola

Community Expert

If you just want to recover the transparency so you can edit it, it's easy to do on a file like that in cs4.

There are several ways including a pixel bender plugin, but the easiest is to Ctrl click on the layer and save

the resulting selection to an alpha channel.

Then Select>Deselect and do Ctrl+J (layer via copy) and Ctrl+E (merge layers).

Repeat the above 8 more times, load the selection from the saved alpha channel and add a layer mask.

After the above steps:

Shift click on the layer mask to see the image without the mask

Semaphoric

Community Expert

Transparency is in fact the Alpha, but is not directly editable as the other channels are. The way I've worked around this is to use the ancient Filter Factory plugin (32-bit only). Load and save the transparency selection, as you've done. Edit that and the color channels. Load the saved selection as an active one, and run the Filter Factory with 255 in the A field. This will replace the current transparency with your edited version. Now, save the file (you may want to delete the saved and edited alpha, since it has been copied into the transparency).

Noel Carboni

Legend

Long story short: It's working properly, as designed. Photoshop embraces the concept of transparency - it has for years. As pointed out above, there are multiple ways to manage it (e.g., layer transparency or masks), though I think the Layer Mask from Transparency feature may have gone in in Photoshop CS5.

Not to put too fine a point on it, but when Gimp and Photoshop differ, it's Gimp that's not performing up to "standard".

-Noel

gener7

Community Expert

Noel Carboni wrote:
Not to put too fine a point on it, but when Gimp and Photoshop differ, it's Gimp that's not performing up to "standard".
-Noel

Noel,

I would be interested in a reference to the differences between Photoshop and Gimp. Is there document/link out there that would explain those differences and where Gimp does not perform up to standard?

Gene

Noel Carboni

Legend

Hi Gene,

I am not a Gimp expert nor do I know of much in the way of documentation, BUT, what I was getting at is that one does not normally expect to see the leading professional graphics software measured to the standard of freeware. The "thing" that most would expect to compare to in common language would be that which is considered the leader.

Ask yourself: When people mean to ask "Has that image been manipulated digitally?", do they say (in slang), "Has that been Photoshopped?" or do they say "Has that been Gimped?"

Whether Gimp actually might work better is an entirely different conversation, and one I'm just not qualified to have.

-Noel

conroy

Participating Frequently

Open the PNG then do "Layer > Layer Mask > From Transparency" to make the image opaque with a layer mask causing transparency. Shift-click the mask thumbnail to disable it and make the image and its channels opaque.

You might find the SuperPNG plug-in useful: http://www.fnordware.com/superpng/

.

S

Steve_PorterAuthor

Participating Frequently

That is what I'd like to see, but unfortunately I don't seem to have the option of "Layer > Layer Mask > From Transparency" in Windows Photoshop CS4 .

Jeff Arola

Community Expert

conroy, I'm am able to load the PNG per your instructions of manually opening up the .png with the SuperPNG format. Now I'm trying to figure if it is better to get my .rgba's through to a png with the method.

R_Kelly's method almost worked - except the color layers still have transparency in them rather than being black in the absence of that color - and more importantly, I can't seem to edit the individual channels. (and I'm a little concerned about the 8 copies that I merge - if they will retain the exact value the original image would have if it would come in like GIMP imports the image).

My goal is to have each of the channels R - G - B - A all be editable grayscale channels - I need to have it come in exactly because the values actually mean information (rather than just visual imagery). ...basically how GIMP treats channels; but obviously I want to use Photoshop.

All the black means is the transparency in the case of your image.

If your were to load the image as conroy has you would need to Crl click on the alpha channel to get your selection and then add a layer mask so you can see the image as it looks in gimp.

In the method i posted, the image should look just like it does in gimp and you should be able to edit the transparency or edit the individual channels.

Gimp just shows the alpha channel (transparency) where as newer versions of photoshop do not, but the alpha channel (transparency channel) is still there in photoshop

as Semaphoric explained in post #3.

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