What is a ALPHA channel for in phototshop??

BT
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Black tractor
Jan 29, 2004
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How to use , on where, what purpose?? thanks
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alpha channel

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C
CB
Jan 29, 2004
Alpha channel is the area that is not filled in my subject matter. If an image is a close-up of a face, there is no alpha channel. If the image is a text title for a web site the area inside letters e, O,B,P…, has transparency so the background of the web page shows through the lettering the alpha channel is there.

The trick with alpha channel is assigning a color similar to the subject matter so that the fringe around a subject such as "O" would blind into the rest of the page.

I’m sure there’s a technical explanation but from my perspective alpha channels must be assigned a color or it defaults to gray. Online alpha channel is important in transparent .gif (jiff) if you want the image to blind into the background.

"Black tractor" wrote in message
How to use , on where, what purpose?? thanks

G
Glenn
Jan 29, 2004
The alpha channel of an image file defines opacity where the normal other channels contain color information. How this opacity channel be used is the subject of whole chapters or entire books on digital imagery and composting. Normally an alpha channel is used as a mask.

Glenn

Black tractor" wrote in message
How to use , on where, what purpose?? thanks

BV
Branko Vukelic
Jan 29, 2004
Black tractor wrote:

How to use , on where, what purpose?? thanks

This is what the manual would tell you:

An alpha channel has these properties:

* Each image (except 16-bit images) can contain up to 24 channels, including all color and alpha channels.

* All channels are 8-bit grayscale images, capable of displaying 256 levels of gray.

* You can specify a name, color, mask option, and opacity for each channel. (The opacity affects the preview of the channel, not the image.)

* All new channels have the same dimensions and number of pixels as the original image.

* You can edit the mask in an alpha channel using painting tools, editing tools, and filters.

* You can convert alpha channels to spot color channels.

One of the things I use alpha channels for is saving selections. You can do this by going to "Select -> Save Selection…", choosing a new channel in the current document and naming it. The saved alpha channel is a mask you can load later when you need to restore the selection. It can be edited, and so on. After saving the selection, you may turn on the alpha channel to see the mask.

Other than that, I’ve not been using alphas in any way, but there are probably many more uses for them…

It also facilitates alpha transparency (partial transparency) in formats like JPG, but I’m no expert in that field. Mike Russell seems to have mentioned it somewhere, tho.

—
Branko Vukelic ()
MR
Mike Russell
Jan 29, 2004
Black tractor wrote:
How to use , on where, what purpose?? thanks

Alvy Ray Smith invented the Alpha Channel while at Lucasfilm, an published the earliest description of it in 1981. It is a channel representing transparency. Its values are used for smoothly combining (aka compositing) one image on top of another with no jaggy edges such as you would see with a GIF image.

In Photoshop, a channel mask is a common example of an alpha channel, since it limits the effect of superimposing part of an upper layer on the layer below it. So is the transparency mask supported by the PNG file format.

One practical issue that you will run into early , as CB mentions, is the background mask color that is normally combined with the original image data. If the original image is against a light colored background, the final combined image will have a white fringe around it. The conventional solution to this is match the matte color to the intended use of the image, but this is not really necessary, provided the compositing is done in Photoshop using correctly prepared "pre-multipled" alpha pixel values. This is the only way to create animations where an object may be rendered in a variety of settings.

Here’s a summary of the history of the alpha channel, straight from Alvy, for those who are interested:
ftp://ftp.alvyray.com/Acrobat/7_Alpha.pdf

—

Mike Russell
www.curvemeister.com
www.geigy.2y.net
E
edjh
Jan 29, 2004
Black tractor wrote:
How to use , on where, what purpose?? thanks

Alpha channels are incredibly useful in many ways.
Check put Channel Chops. Old but still great. You might find it cheap on the internet or a used book store.

http://graphicssoft.about.com/library/products/aapr_channelc hops.htm

—
Comic book sketches and artwork:
http://www.sover.net/~hannigan/edjh.html

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