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I understand channel just store color information for an image. But in the real world, what are the actual use cases?

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    Not an answer: Photoshop wouldn't be an industry leading tool without them. Many discreet and powerful Photoshop features are only used by a small (but far from insignificant) market. – Dom Jan 24 '18 at 12:53
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  1. Keying. Channels can be used as a starting point for very fast and very accurate selections.
  2. Storing selections. Storing a selection allows you to do filters and math operations on it.
  3. Storing color channels for spot colors. In other words you want to have that golden color on your print or you want varnish on specified locations or just orange.
  4. Un-premultiplying color.
  5. Viewing raw channel info.
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    Do you have a link to elaborate on #2? – LateralTerminal Jan 23 '18 at 20:55
  • Unfortunately I'm not at my Photoshop now, but if I recall correctly, you can make a selection (marquee, magic wand, quick mask, etc), then right click it, and choose "save selection". This will save the selection as a new channel, which you can then Ctrl+Click on the thumbnail of to recall. – Adam Barnes Jan 24 '18 at 12:25
  • @AdamBarnes your right, but you can also use the Select->Save selection... and Select -> load selection... menu items. That does the same thing. – joojaa Jan 24 '18 at 12:36
  • @LateralTerminal When you have one channel active (others hidden) then all menu items in Image -> adjustments and Filter menu affect that channel only! This only works well if you have a normal layer active and most of the stuff you dont want on that layer hidden though. BUt yes its possible to blur only the blue channel. – joojaa Jan 24 '18 at 12:38
  • @joojaa I'm sorry I specifically meant math operations. I wasn't sure what you were referring to with that. – LateralTerminal Jan 24 '18 at 14:32
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It's extremely useful for the press industry.

Color separation is important.

It's the only way you can clean up crappy non-vector art for the printing press.

For example, prepress has to use Photoshop channels to bring specific colors up to a certain % in order to get enough dot gain for the image to look clean.

enter image description here

To reproduce color and continuous-tone images, printers usually separate artwork into four plates (called process colors)—one plate for each of the cyan, magenta, yellow, and black portions of the image. You can also include custom inks (called spot colors). In this case, a separate plate is created for each spot color. When inked with the appropriate color and printed in register with one another, these colors combine to reproduce the original artwork. https://helpx.adobe.com/illustrator/using/printing-color-separations.html

That quote is for Illustrator but the same concept applies to Photoshop when dealing with the press industry. You won't always have vector art that has easy color manipulation

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    Hey, sorry to bother, but how does that help clean up "crappy non-vector art"? I understand it's the method of separation that the physical printer can act upon, but nothing is mentioned about vector art or cleaning up, there. – Adam Barnes Jan 24 '18 at 12:24
  • @AdamBarnes Almost all press work is done in vector art. Manipulating color is easy in Illustrator which is why the preferred format is always vector. Of course some parts of the design may not be vector. The only way to clean up non-vector art is in Photoshop. Photoshop is raster based image editor. Illustrator is a vector based image editor. – LateralTerminal Jan 24 '18 at 14:26
  • My apologies; I understand the difference between photoshop and illustrator, vector and raster, I just don't understand how splitting a raster image into channels will help vector industries "clean it up". :( – Adam Barnes Jan 24 '18 at 14:27
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    @AdamBarnes For example, prepress has to use Photoshop channels to bring up specific colors up to a certain % in order to get enough dot gain for the image to look clean. If colors are below a certain % it gets nasty. – LateralTerminal Jan 24 '18 at 14:30
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    @AdamBarnes Alright. Another reason is you may want to remove a specific color from printing in certain areas of your print. – LateralTerminal Jan 24 '18 at 14:35
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Channels don't only store color.

They can store transparency (alpha) as well. In fact every single layer mask in Photoshop creates an alpha channel.

While alpha channels may not be what one immediately thinks about when thinking of "channels", rest assured there would be no such thing as transparency in Photoshop without channels.

I start almost all extractions with a selection based upon a color channel. Convert it to an alpha channel and refine.

Example: How to remove white background in Photoshop

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When taking pictures with an infrared filter in front of the sensor, the results often appear pink and unpleasing.

Here's an example of what a picture could look like straight-out-of-camera: Original picture

After white balance adjustments, clouds are white again but the sky still looks weird: enter image description here

By switching the red and blue channels, the sky becomes blue again: St Lawrence infrared St Lawrence, Stratford-sub-Castle, in infrared..

Note that the first picture isn't the original one. I didn't have access to the RAW file, so I applied the described process in reverse to show the steps.

Here's a youtube video describing this process : "Color Infrared Channel Swap in Photoshop". The swap is done with Channel Mixer but could also be done by copy-pasting the Red and Blue channels.

David Keochkerian is a photographer who took many great IR pictures (example).

His workflow is described here and here. Red/Blue channel swap is mentioned in both.

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  • Interesting practical effects! I wasn't expecting that. – LateralTerminal Jan 24 '18 at 14:38
  • I'm not totally convinced. Would it not work quite the same way with an originally grayscale image? Is the infrared a one-channel or a two-channel image? – usr2564301 Jan 24 '18 at 23:19
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    @usr2564301: Well spotted. I didn't have access to a RAW image taken with the infrared-blocking filter removed, so I just modified this picture to show what it could look like. With a completely grayscale image, applying this process would render everything blue, including the trees. When removing the IR-filter in front of the sensor, the 3 channels are very different to one another, and the red one gets the biggest part of IR. With a cheap IR passing filter, only the red channel gets any light at all. – Eric Duminil Jan 25 '18 at 8:09
  • @usr2564301: Updated with examples. – Eric Duminil Jan 25 '18 at 8:23

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