Alpha Channel – Targa Transparency Photoshop CS2

JL
Posted By
Jose_Lira
Oct 23, 2006
Views
1013
Replies
11
Status
Closed
Adobe Photoshop CS2
Windows XP SP2

I admit its been a long time (years) since I had any classes and up till now I have never done anything with channels….

Need to add a transparent alpha channel (8-bit) to existing images and then save as a targa file type to be used in a OpenGL application. We must keep the transparency of the images.

Creating a new channel adds a black channel, which I thought was transparent, but when I switch back to the layers it gets deselected and loses the transparency. If I remember correctly its supposed to be a grayscale (or was it black & white) image of the RGB channels, but I can’t seem to get it to work correctly. What I am doing wrong?

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B
Bernie
Oct 23, 2006
What are you trying to accomplish?
JL
Jose_Lira
Oct 23, 2006
Create a TGA file (Targa) with an alpha channel to retain the image’s transparency.
CC
Chris_Cox
Oct 23, 2006
First: alpha channels and transparency are two different things. (the video and 3D worlds tend to get them confused)

TGA saves an alpha channel, not transparency (we tried it the other way and too many users complained because they used the alpha channel as a general mask, not transparency).

You can load the transparency from your image and copy it to an alpha channel.

Or you can use another file format that has a better distinction between channel types (TIFF, PNG, etc.).
JL
Jose_Lira
Oct 24, 2006
Ok, thanks

That’s what I have been doing. Selecting the transparent areas in the layers and then copying them over to the Alpha Channel, so that when I create the TGA file and its opened in the application the correct transparency is being displayed.

Perhaps in the next version, they can add a dedicated transparency channel. It would help when creating images for game UI and objects.
B
Bernie
Oct 24, 2006
Transparency channels are a function of the file format you save your image in, not of the software

(and you should have seen the fuss raised when PS redefined the Alpha channel as transparency for Targa files)
PN
Pekka_Nurmela
Oct 24, 2006
Hei!
I use original Photoshop 7.0 Targa plug-in with PS CS2 (not 7.0.1. version). You don’t have to save any alpha channels. You just make a 32 bit targa copy of your title and that’s that, easy and fast. Lower thirds and other transparency titles made that way look great in Premiere, Avid or any other video editing program…

Pekka Nurmela
CC
Chris_Cox
Oct 24, 2006
Targa has only 4 channels, Red, Green, Blue and OTHER.
That other channel is currently used for an alpha channel. When we tried to use it for transparency (like the file format says), we got a LOT of negative feedback.
GA
George_Austin
Oct 25, 2006
Chris,

“…alpha channels and transparency are two different things…”

You wouldn’t think there was any difference, reading the alpha channel definitions which abound on the web—like the following two examples:

webpedia.com: In graphics, a portion of each pixel’s data that is reserved for transparency information. 32-bit graphics systems contain four channels — three 8-bit channels for red, green, and blue (RGB) and one 8-bit alpha channel. The alpha channel is really a mask — it specifies how the pixel’s should be merged with another pixel when the two are overlaid, one on top of the other.

ypoart.com: Usually, a color image file is composed of 3 channels: RGB for Red, Green and Blue. A transparency channel may be added to describe the importance of each pixel when composited over another image. This transparency channel is called the Alpha channel.

So, just what is the distinction?
CC
Chris_Cox
Oct 25, 2006
those are bad definitions.

Alpha channels are ANY extra channels, not just transparency. Transparency is a particular subset of alpha channel usage.

TIFF even defines them as different types (associated alpha for transparency, and unassociated alpha for other alpha channels).

For at least 20 years now, other channels have been used for things other than transparency, even when they are called the fourth "alpha" channel in a file format.
GA
George_Austin
Oct 25, 2006
Then, would it be correct to say that a transparency channel is an alpha channel but an alpha channel is not necessarily a transparency channel?
CC
Chris_Cox
Oct 25, 2006
Yes.

Alpha channels are the superset, transparency is the subset.

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