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View Full Version : Channels in Gimp (and photoshop) - what are they useful for?



torstan
08-30-2007, 12:16 PM
I've been playing around with Gimp for a few months now and think I've got most of the functions under control. However I can't figure out the use of channels. It seems that I can use them to view the different colour information of each layer, and edit them individually. However I can edit the different colours directly using tools like the levels or curves dialogue. I'm just curious to know if I'm missing out on some tricks here.

RobA
08-30-2007, 02:22 PM
Well you understand the primary function. Each of the channels lets you independently edit the colours of that channel. If you have a RGB image then the channels you have are Red, Green and Blue. If, however the image is HSV or CMYK, then you will have the appropriate channels. One thing to keep in mind is that all channels in the GIMP are 128 bit greyscale.

The next major function of channels in GIMP is for transparency (namely the Alpha channel). If you want an image with transparency (like png) you can add an alpha channel that masks the image, allowing the pixels to be opaque, transparent, or somewhere in between.

The last function of channels is storing selections. You can take any selection (including feathering) and save it as a channel. You can then edit this channel as if it were a greyscale image, including painting, erasing, and all the associated effects (blur, scrip-fu, etc). This lets you "save" different selections that you can "load" as selections later.

Hope that helps. If you have any other questions (or are looking for use examples), just ask!

-Rob A>

HFP
08-30-2007, 05:49 PM
I've been playing around with Gimp for a few months now and think I've got most of the functions under control. However I can't figure out the use of channels. It seems that I can use them to view the different colour information of each layer, and edit them individually. However I can edit the different colours directly using tools like the levels or curves dialogue. I'm just curious to know if I'm missing out on some tricks here.

The use I put them to most is when doing digital art, specifically my webcomic. No, I'm not the artist, but I am the one who does the coloring as necessary. I use channels to create the "inks" that I lay on top of the colored areas. If you'd like, I'll go and post the link to the tutorial I use for that.


Lisa

----
Lisa Hartjes
Hart-Felt Productions: http://www.hartfeltproductions.com
Personal Website: http://www.lisahartjes.com
World of Lisacraft World Building Podcast: http://wol.lisahartjes.com
The Domino Girls Webcomic: http://www.dominogirlscomic.com

RPMiller
08-31-2007, 03:18 AM
Lisa! Welcome to the community. It is great to see another person from the CC list make it over here.

Please do post the link. We love tutorials here and would be glad to have another.

torstan
08-31-2007, 04:04 AM
Ah ha. If I can use it for saving selections that makes my life ten times easier. Any links to tutorials are much appreciated. This should reduce the clutter in my layers significantly...

ravells
09-03-2007, 06:36 AM
Channels are also useful in conjuntion with magic wand colour selections (which is what I mainly use them for). Quite often if you split a picture into its individual channels, one of them will yield much better magic wand selection results than the original. It also helps if you play with the brightness and contrast settings too.

Not sure how useful that would be in cartography terms though!

Ravs

torstan
09-04-2007, 05:45 AM
Thanks for the tips. I've been playing with channels after these suggestions and found them to be really useful. I can see that splitting the colours is useful for photography. I'm primarily using them for keeping track of selections - one area for walls, one for floor, another for water and so on. It has made life a lot easier.

palehorse
09-11-2007, 02:29 AM
What's really interesting (well, it's interesting to me anyway) is that, as far as Photoshop is concerned (and Gimp too, I suspect), Channels, selections, layer masks, and the Quick Mask are all the same thing under the hood. They're a way for the program to say, "OK, if the pixel on the channel/in the selection/on the mask/etc is white, apply X effect at 100%. If the pixel is black, apply the effect at 0%. If it's somewhere in between, apply the effect at an equal percentage." If you follow Photoshop through it's various iterations, you can see how Channels evolved into all these tools which have different applications but are all still essentially the same. Pretty cool!


Quite often if you split a picture into its individual channels, one of them will yield much better magic wand selection results than the original. It also helps if you play with the brightness and contrast settings too.

You'll find that the red channel contains most of the contrast, the green channel contains most of the detail, and the blue channel contains most of the noise. So if you've got a really noisy image, you can apply a filter like Despeckle or Dust and Scratches to just the noisy Blue channel instead of the whole image to clean it up, which'll keep you from losing detail, or sharpen your image by applying an Unsharp Mask to just the details of the green channel.

Channels are also really good for building complex selections instead of just saving them. If you make a new channel in the Channels palette, then click on the eye for any of the other channels there, it'll make all the other channels visible and turn your new channel into a rubylith overlay, just like the one you see when you use the QuickMask. So you get the convenience of creating a complex selection with the drawing tools just as you would with the QuickMask, but it'll be saved automatically when you're done because you're doing it all in a channel.

Sorry, it's late and I'm starting to blather!

RobA
09-11-2007, 10:45 AM
And don't forget you can split am image as HSV in addition to RGB. That will sometimes give better definition for selections (i.e light from dark, regardless of hue) with the magic want tool.

-Rob A>

RPMiller
09-11-2007, 03:54 PM
You'll find that the red channel contains most of the contrast, the green channel contains most of the detail, and the blue channel contains most of the noise. So if you've got a really noisy image, you can apply a filter like Despeckle or Dust and Scratches to just the noisy Blue channel instead of the whole image to clean it up, which'll keep you from losing detail, or sharpen your image by applying an Unsharp Mask to just the details of the green channel.

Channels are also really good for building complex selections instead of just saving them.


And don't forget you can split am image as HSV in addition to RGB.
These are awesome tips!!!!!! As soon as I can I'll rep you both. Oh how I wish I knew this stuff years ago...

RobA
09-12-2007, 11:01 AM
magic want tool.

hehe! I like my own typos - I think we all have a magic want tool :)

-Rob A>

torstan
09-12-2007, 11:17 AM
I agree. Those are the kind of tips that I just don't seem to find in tutorials. Thanks for those.