Operating on alpha channels

[taob]

AFAIK, Photoshop CS2 does not provide a way to operate directly on the alpha channel of a layer or image, at least in the same way you can with the RGB or CMYK channels. Photoshop's concept of "alpha channel" really only applies to layer masks and selections. For example, the demo PSD (five layers) looks like this:

http://www.luxography.ca/Images/PST/alpha_channel-1.jpg

The green squiggle is a stroke from a soft brush. The colour of the stroke is 0/255/132 (in RGB), but the alpha channel values vary from 0 (fully transparent) to 255 (fully opaque).

I would like to arbitrarily modify just the alpha channel values of the pixels. How do I do that in CS2? Using the fill/layer opacity sliders is an easy way to make an entire layer less opaque, and layer masks to do selective reduction in opacity... but what if I want to make the "PST" layer more opaque? Or if I want to use Curves to modify the opacity ramp on the green squiggle? Or Levels to heighten the opacity difference between the paint and wet edge of the orange/yellow paint layer?

Is there a way to extract just the alpha channel values, modify them with the standard CS2 tools (like perhaps doing a Gaussian blur on the alpha channel... that could come in handy in some situations), then composite the alpha channel back into the pixel data?


P.S.: Oops, I didn't know there was a 75K limit on attachments here. Download the alpha_channel_demo.zip (http://www.luxography.ca/Images/PST/alpha_channel_demo.zip) (112K) file from my site instead.

[sPECtre]

Ah, you call the transparency mask (http://photoshoptechniques.com/forum/faq.php?faq=photoshop_techniques#faq_pst_transpare ncy_mask) alpha channels ? Do you come from the video world, I presume?

Well, to beef up opacity, you can copy the layer, and lower the opacity of the copy...

Or Ctrl+click the name of the layer with the paint stroke, to select its transparency, then create a new layer, fill with green, and add the selection as a layer mask, that you would be able to tweak...

A problem I see is that full transparency in a transparency mask strips out color info...

To work the way you want, you might for instance paint a whole layer in color, then put your brush strokes on the layer mask, that would be the closest to working like in the video compositing world, I guess.

[sPECtre]

I've been putting features requests to have some advanced masking options, similar to the advanced blending ones, where you would have a slider to blur the transparency mask, and another to change the White, Gamma, and Black values, like you would do using levels, to have (yet another) non-destructive way to affect transparency.

[markzebra]

Layer masks are your answer Taob.

As Spectre said they are modifiable with Levels and Curves. They are Photoshops opacity masks, and once you get used to them they are extremely flexible, and will do everything you need. Follow all Spectres advice.

Heres some extra tips for working with them easily
Press your backslash \ key to view the mask as a red overlay
then press the tilde key ~ to see the mask as a greyscale channel.
Control M opens Curves
Control L opens Levels

[markzebra]

Oh and the phrase "alpha channels" means something slightly different in Photoshop - saved selections basically

[sPECtre]

I'm not sure that it is still present in the latest versions, but in the CD from version 7 WIN, you can find in the folder (CD)\goodies\optional plug-ins\Ffactory\transexpl\ the "Settrans.8bx" filter, that allows you to pump up the transparency of the layer, with explanations. Look also for the other filters in the set.

[Scott20015]

Also, the layer masks are editable by almost all the relevant tools (blurring, dodging, burning, etc.)

I use masks extensively everyday. Like the last post said, once you learn to use them there is no going back!