Help with GIMP channel problem needed.

[Deadshade]

Greetings cartographers.

I have learned the basics of GIMP and wanted to follow a tutorial now ( http://www.cartographersguild.com/tu...mp-wilbur.html )
I am at the stage Bump Map, Creating additionnal layer masks at p.6 and am stuck. I spent1 hour on GIMP tutorial first but still don't understand what to do.

In GIMP there are files, images, layers, masks and channels all referring to similar things so that the instructions are confusing for me.
For me :
- a file and an image are the same thing, this is what I load with "Open file".
- a layer and a mask are the same thing too. I can load a file as layer and then it appears in the layer window. So I know how to create a layer with any file I have. If layer and mask are same things why different words are used ?
- I don't understand what a channel is and how it is different from a layer/mask. I see the channels window but it is empty (excepted for the RGB,alpha channels)

Now the instruction in the tutorial is : "Now we are going to create 2 additional layer masks. The first layer mask is simply a new version of our "Land Mask". In your channels dialogue click on the Land mask channel and then click on the Create Duplicate Channel Icon"

As I opened a file Land mask that I created some time ago, I have a file and an image called Land Mask. I have also a layer that is called Land Mask.
But I have NO Land Mask channel so I can click on nothing there. What should I do to have a "Land Mask channel" containing the Land Mask file ?
Also what does it mean to "create a layer mask" . Does it mean the same thing as "create a layer" ? But as I know how to "create layers", why suddenly jumping to "channel dialogues" ?

Thanks in advance for any explanation helping to unstuck me

[Freodin]

First thing you could do is go to youtube and try to look up some vids about GIMP layer masks.

I admit that I found the few I try to watch rather too confusing to learn the basics, but at least they will show you exactly what to do and where to click when you try to follow my incompetent explanation.


So, let's start...

Layer and mask are two different things.

A layer is just what it says: one layer in a stack of pictures, layered on top of each other, which can be combined in different ways.

A mask is an addition to a layer. It is a black&white image that controls the transparency of this layer. You add such a mask by right-clicking on the layer in the layer-tab, and choose "add layer mask". You get several options of how to initialize this mask - one of these options is "Channel" (more on that later. Just choose "white" to start with and for a little experimentation.)

You will see an additional little representation of this mask in your layer-tab, right behind the layer. Now everywhere this layer mask is white, the layer itself with be fully visible. Everywhere it is black, it will be completely transparent. In-between-grey means partial transparency.

You can see what the layer mask looks like by right-clicking on the layer in the layer-tab and choosing "show layer mask" from the options. You can choose whether to paint on the layer or the mask by left-clicking on the layer or the mask in the layer-tab. The active part will be highlighed by a little white border, just like an active layer.



A channel can be used as a storage space for selections, like in this tutorial.

The tutorial states: "On your “Map Outline” layer, grab the Select by Color tool (Threshold 0) and click on the land area." This adds a selection of all the areas you have painted in as "land".

Now you save this selection for later, in a channel. Go to the "select" menu and choose "save to channel". Now if you switch to the channels-tab, you will see it has added a new channel named "Selection Mask copy". You can change that name, in the same way as with layers.

I hope that helped.

[Deadshade]

Thanks !

This is much better than incompetent ! I think I understand now what the difference between mask and layer is. The channel is still rather obscure but I am at least partly unstuck now.
So yes, it helped.
Btw the few Youtube tutorials seemed to me extremely confused or too superficial when the matter concerned the layers and sélections.

[Freodin]

Hah, I knew that we had a great tutorial on GIMP layer masks right here on this forum. Took me a while to find it... I hope they will fix the "search" function soon.

Take a look a this tutorial. I might be helpful, and it explains a lot of things that you can do with layer masks.


There is a lot more you can do with channels, too. Basically, they are a sort of layer-independend masks. A channel can have a overall transparency and even have a colour.

But for the most part, it is just the easiest way to save a selection for later re-use.

[Deadshade]

Definitely thanks. This tutorial helped.
I still don't really understand why I should "store" masks in some other place like channels when I can duplicate them or have them in the layers anyway. Even if I can understand that it could "store" something, I can't see what it does (if anything) beside storing .

But I guess some more understanding will come with practice. As I started some 2 weeks ago with 0 prior knowledge, I am Learning in parallel Wilbur, FT3 and Gimp and this overloads a bit my biological RAM

[Gidde]

Two reasons for making the channels. First, it's easier to add a mask that = a channel than it is to grab it from a different layer. Second, once the mask is on a layer it's easy to (on purpose or on accident) change it, and the channel stays a "clean" copy. Hope that helps!

[Deadshade]

Thanks much.
OK so basically the channel is something that saves a bit of time. If I understand well it is not really necessary and any work in Gimp could be done without using channels at all, right ?
Could somebody look at my question here : http://www.cartographersguild.com/tu...wilbur-16.html ?

It was this problem with following the tutorial that actually triggered these questions about masks and channels.
Apologies for the newbiness but as I said I only started with map/image software 2 weeks ago.

[RobA]

Hah, I knew that we had a great tutorial on GIMP layer masks right here on this forum. Took me a while to find it... I hope they will fix the "search" function soon.

Hi all-

Search has been (hopefully fixed for good). There was a known error in the init.d scripts that failed when starting the search server on a reboot, and would take admin action (ssh login, kill some processes, manually start it up again).

I've edited the init.d script so it should now restart the search properly when the server is restarted. I've kicked it manually over a half dozen times in the last half hour and it always seems to come back OK now.

Still looking at the vBForum_Item_SocialGroupMessage error at the top of the advanced search page (and the godaweful way that page looks) but at least search is working.

Sorry for the delay, real life, and all that...

-Rob A>

[Midgardsormr]

Sounds like you have a grasp of how masks work now, but you're still hazy on channels. Maybe I can help with that.

First, you expressed some confusion about the difference between "image" and "file." The image is what you actually see displayed on your screen. It's simply the grid of pixels. The file, on the other hand, is sort of a container that the image is stored in. Or perhaps the instructions for constructing the image, to look at it another way.

You can store an image in a Gimp file, or a Photoshop Document, or a PNG, or a Targa. The pixels on the screen, the image, will be the same, but the file is different. If you're interested in the qualities of different file types, I wrote a tutorial on that some time ago here: http://www.cartographersguild.com/so...t-formats.html

Okay, so the image is a grid of pixels (Picture Elements is what that word actually means). Each pixel is typically described with six numbers: its x and y coordinates, and four channel values. X and Y are simply where the pixel is in that grid. The other four numbers are its red, green, blue and transparency (alpha) levels. A screen like the one you're looking at has three phosphors for every pixel, and the color it shows you depends on the brightness of those three phosphors, which are so close together that they appear to be a single dot. When all three are off, the pixel is black (or close to it—backlit screens never get completely black). When all three are turned all the way up, the pixel is white. If you set the red light to 100% and leave the other two off, the pixel will be, unsurprisingly, red. If you turn green and blue all the way up, but leave red off, the pixel will be cyan.

Therefore, the grayscale picture in the red channel is what the image would look like if all of the green and blue phosphors in your monitor were broken.

You can also have channels other than red, green and blue. I mentioned transparency before, and you've already had some exposure to that: the layer mask is actually a fourth channel that describes how transparent a given pixel is. Black for invisible, white for opaque, and gray for translucent. Another possibility you might see here is a channel that stores elevation—the value of the pixel represents the height of the land at that location.

Or the channel might, as alluded to earlier, hold a saved selection, which is essentially the same as a mask. One more reason you might want to store a selection here rather than just leaving it as a mask on your layer is because you might, for instance, need to perform an action that rasterizes your layer, which merges the mask with the color pixels. I almost always at least save my land/sea matte as a channel early in my process because I'll probably want to use it for something later on, and there's no telling what I might have done to any layer masks in the meantime.

It took me quite a while before I really understood what channels were for and how to use them. The good news is that understanding them isn't really necessary for most things, and as you do more image manipulation, you'll pick up how they work along the way. If you're interested in more applications for the knowledge, here's another tutorial that deals heavily with channels: http://www.cartographersguild.com/tu...ackground.html

[Deadshade]

Thanks, this helped a lot !