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).
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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/
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Try going to "File > Open" and seeing if SuperPNG is showing up in the Format dropdown menu when a PNG is selected. I find the plug-in to be used by default for PNG when dragging files to Photoshop or opening via right-click in Finder when the plug-in is installed, but check in your "File> Open" dialog, anyway.
When "Always bring up this dialog" is disabled in the plug-in interface shown in previous post, you need to use "File > Open" and hold down Shift when clicking Open button to get access to the plug-in's options.
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Seems like a lot of wheel-spinning to avoid having to upgrade to Photoshop CS6, with the Layer Mask From Transparency option. The combined time people have put into writing here exceeds the $200 upgrade price, almost no doubt. 
-Noel
Noel,
will the "Layer Mask From Transparency option" keep place the color channels in their "opaque" state like I require?
...if so, you are correct about the cost/benefit. Or I could use GIMP for free! (I couldn't help myself Noel!)
...but seriously I really wanted to figure out a way to stay in Photoshop to benefit from the other features where it clearly is a winner. I am just amazed that it seemingly doesn't have a way to treat incoming files with alpha in what I consider the "correct" way - or at the very least have someway of getting it easily into that form - color and alpha channels in grayscale form that I can edit independently.
I learned alot (and I hope other people benefited as well) from the journey! ...Thanks everybody!
I'd love to see if anybody has a brilliant or clever solution - but I agree with Noel - probably enough wheel-spinning on this one.
If your interested in a plugin that mimicks the layer mask from transparency in cs5/cs6 you could install the pixel bender plugin
for photoshop cs4 and then use the removeTransparency.pbk (pixel bender kernal file).
It also works on files other than pngs.
The only steps you have to do is load, save the selection and add the layer mask (if want the layer mask part, which could be recorded in an action).
Pixel bender from adobe labs:
(you have to click where it says pixel bender for cs4 then look on the next page for archived downloads)
http://labs.adobe.com/technologies/pixelbenderplugin/
removeTransparency.pbk
(right click on the page and Save As)
http://neo.cycovery.com/removeTransparency.pbk
Example action and the result:
After shift clicking on the layer mask:
pixel bender plugin:
If you want to install the plugin and need directions in addition to those at adobe labs, we can provide help.
I guess I just don't see a problem manipulating R, G, B, and a layer mask.
The Layer Mask is available in the Channels panel if you want to manipulate it directly, and you can view the RGB with or without the transparency just by shift-clicking on the layer mask to disable/enable it.
Seems like everything that's needed to me. Did I miss something?
-Noel
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