Using GIMP to Mask Green Screened Photographs

GIMP is a free alternative to Photoshop. If you download it from the GIMP website, you may decide it is too ugly and too difficult to use. However, you can download "skins" and add ons that make it less ugly. Also, if you want to do some digital painting, you may find Krita better suited for your needs. However, today we are going to be working with green screened photographs, and while this is also possible with Krita, I found that it was quicker and more effective in GIMP. So without further delay!

 Step 1 Open up GIMP and click file open and select your photo.

 Step 2 Click on the 'fuzzy select tool'. Don't use it yet, we need to change the settings.
Step 3 Look for the tool options for the fuzzy select tool. It should be in the bottom right hand corner. Click on the bar that says 'Threshold' and change the number by clicking on it and moving your mouse. I find that 15 is a good number.

 Step 4 This step is important. At the bottom of the 'fuzzy select tool' tool options it says "select by". Change this from 'Composite' to 'Hue'. Hue means color, because we want to select by color.

 Step 5 Using the fuzzy select tool, click on the green screen. The fuzzy select tool should select all of the green. There will be some problem areas, and those will be fixed in the next step.

Step 6 At the top of gimp, press for the 'select' menu and then choose 'toggle quick mask'. The selected area will now turn red and we can now use the paint brush to select or deselect more areas. 

Step 7 Select the paint brush and change the color from black to white by clicking on the arrow between the two colors (the colors are under the tools and above the tool options).

Step 8 Remove any unwanted red areas using the paint brush. Don't worry about being perfect right now, we will finish this later. In the tool options, change the size of the brush to a bigger size so that you can work more quickly.

Step 9 Change the color of the brush to black and paint any parts that you want to keep (even though you are using black, they will be painted red). After you are finished you can clean up using the white brush again.

Step 10 Use the zoom tool and change to a smaller brush to clean up the more difficult parts of the picture. You can press the space bar and move your mouse to help you move around the picture while zoomed in.

After a few minutes of cleaning up the picture you should be left with something like this.
 Step 11 Go to the top menu again and select toggle quick mask.
 Step 12 Go to the top menu and select 'layer' -> Mask -> Add Layer Mask
 Step 13 Change the add layer mask options - choose selection and 'invert mask'. If you want you can try it without invert mask to see the difference.
 Step 14 You are now ready to add in a background (use file -> open as layers)! Make sure that you save your image. You could also export your image and finish the rest of the process in Inkscape.

Step 15 (optional) If you need to clean up your image more, click on the mask in the layers panel (on the right) and you can still use the paint brush to take away parts of the picture or add parts back in. This time the mask is not red.