How to cut an irregular pattern with proper transparency?

Question

This may be hard to describe and I'm not proficient enough to draw it yet so please bear with me.

I'm cutting parts of some photos of me and my girlfriend to put on a board game I'm making for her. Now simply cutting the photos creates a sharp edge which doesn't look beautiful, besides the fact that the cut is not very regular either.

My idea is cut a bigger part of the image, select what's left and shrink the selection. This way, I have a stripe around the actual image that I want to use. What I intend is to have a gradient in the transparency channel only where the gradient at the inner edge of the stripe is 100% and on the outer edge is 0%.

Is this at all possible? If so, how?

Another way I thought of doing this was to cut the part that I want and add a glow to it in the alpha channel only ( I didn't find a glow effect but I have a recollection of seeing it in the past). Then somehow try to replace that stripe's alpha channel with the glow I just created.

I didn't have any luck with either of these, but I believe this shouldn't be such a rare use-case. Does anyone have any suggestions how I can do this?

Here's an ASCII art of it!

                            ____
           GRADIENT        /    \
         TRANSPARENT      /  __  \
            STRIPE  -----/> /  \  \
                        /  /    \  \
                       /  /      \  \
                       |  \ CLEAR \  |
     COMPLETE         /  _/ IMAGE |  |
      TRANSPARENCY   /  /         /  |
                     |  \__       \  |
                      \_   \_  _  /  /
                        \_   \/ \/  /
                          \_   _   /
                            \_/ \_/

Answer 1

There are a few ways to do that.

With the selection tool:

• First select your subject as usual (around the "clear image" part)
• Enlarge your selection by half the width of your "fuzzy border" with Select->Grow (so if you want, let's say 50 pixels of fuzzy, grow the selection of 25 pixels)
• Smooth the selection with Select->Feather for the same amount of pixels
• Copy the selection; You should have in the clipboard what you're looking for

With a layer mask:

• First select your subject as usual (around the "clear image" part)
• Right click on your layer and select "Add layer mask"
• Initialize layer mask with "Selection"

A layer mask is basically a grayscale layer where color will be copied into the alpha layer of the target layer (white=fully visible (100%), black=invisible(0%)). So you may now use any filter you like to add a black to white gradient border to your white object of interest; when you like the result, right click on the layer again and select "Apply layer mask" (In your case, a blur filter should be a good start)

Alternatively, you may "paint" your selection using regular painting tools directly on the layer mask (Use white to extend the selection, black to reduce it and any other gray for alpha effects.

There are probably other ways to do that, like using the "Toggle quick mask" button, but they are mostly variants of those two solutions.

Bottom-line: The layer mask approach is ways more flexible, and worth spending some time learning how to use it if you are considering using Gimp seriously, but in your case, the first solution is probably good enough, and will be a lot faster if you never used layer masks before.

Answer 2

This is easily accomplished using the built in Feather tool. Make your selection as normal, then click the Select menu and Feather. Choose the size of the feather and click OK, then copy and paste the selection into a blank document to see the effect.

Incidentally, you should almost always feather your selections when applying any effects to avoid an artificially hard edge.

Answer 3

You don't need to do anything fancy with the selection, just use layer masks with a blended edge. I'm not familiar enough with GIMP to walk you through it myself, but the basic idea is to use a mask behind your image to create a blending along the edges. There are numerous tutorials already available on the subject. I searched for "gimp blend layer edges" to find a wide selection of them.

Update: Given the less powerful layer blending options available in GIMP and the presence of a selection feather option, I'd suggest that LeFauve's suggestion is the better way to go about it unless you need some additional flexibility in controlling the exact way that it blends. I'll leave my answer since using a blending mask does still give greater control (at the cost of complexity) though.