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Intelligent Scissors in GIMP

by Alan German


In a previous article (Digital Image Editing with GIMP), I suggested that you could get most of the benefits of a $1000 image editing program at zero cost by using the open-source GNU Image Manipulation Program (GIMP). This program offers a vast array of features, from the most basic editing tools, to some extremely powerful techniques to achieve quite complex tasks.

I have been using GIMP as my main image editor for several years, mainly using simple features such as tools to crop photographs, or change the brightness and contrast. But, as noted in the earlier article (or simply hinted at) GIMP has a number of very powerful tools. For example, suppose that we wanted to cut an object out of a photograph for use as a stand-alone image. How would we go about doing this? In Gimp, one answer is to use the “Intelligent Scissors”.

As the name of the tool suggests, the “scissors” are used to cut out part of an image, but this is accomplished with minimum effort on the part of the user – hence the “intelligent” qualifier.

Suppose, for example, that we want to extract the image of the bull from the photograph shown below (left). Firstly, we open the original image in Gimp. Now, since we are only interested in the portion of the image containing the bull we can crop the image appropriately and then enlarge the selected area. Click on the Rectangle Select Tool, draw a box around the bull, select Image – Crop to Selection, then View - Zoom – Fit Image in Window.


 

Now, it’s time to click on the intelligent scissors’ tool. The cursor changes to crosshairs with an adjacent tiny scissors icon. We position the crosshairs along the bull’s back, immediately ahead of the junction with the tail, and left-click on the mouse. A white dot shows our first selected point. We move along the bull’s back, selecting individual points as we go.

We don’t have to be too precise as the scissors are intelligent so the selected outline pretty well follows the desired contour. However, sometimes it’s necessary to select an inflection point so as to avoid the selected contour from taking a short cut away from the desired outline of the object. Note, for example, in the above screenshot (right) that the selection follows the bull’s contour, even when we select two points that are quite far apart. However, if we are too ambitious, i.e. going from the back of the neck to the tip of a horn, the selected line fails to follow the contour of the neck and jumps across to a point near the base of the horn.

There are “tricks” to make our lives easier in such situations. Edit – Undo Modify Scissors Curve (Ctrl-Z is even easier!) will remove the selected point at the tip of the horns. We can then select a couple of points along the neck to the base of the horns before continuing to define the horns themselves. It is also useful to temporarily enlarge our view in areas where the image has considerable detail (e.g. the horns and tail). Selecting View – Zoom In (or just using the + key), and re-centering the image in the window using the scroll bars, enlarges a section of the image and makes point selection more definite.

Once all the desired points on the outline have been selected, we can click on the very first point in order to complete the selection. Hovering the mouse over this point produces a message on the status bar indicating "Click to close the curve". Having done this, the status bar message indicates "Click or press Enter to convert to a selection". Clicking once more on the image changes the solid line with the selected points into a continuous dashed line that "shimmers" to highlight the selected area. Now, we can copy the selected image to the clipboard using Edit – Copy.

Finally, we can create a blank canvas on which to store our extracted image. Select File – New and choose an appropriate width and height. In my case, the image of the bull would fit into a frame of 2500 x 1400 px. Clicking OK displays the new blank image window onto which we can copy the extracted image of our bull using Edit – Paste. Save the new image as a new JPG file. Job done!



Note that the final product isn't perfect. (For example, do you see the grey area in the lower portion of the end of the tail?) I could have spent more time refining the final image but, for me, the end result is good enough for government work! And, actually, I think it's an amazing result for a few minutes work with a free image editing program!


Bottom Line:

GNU Image Manipulation Program (GIMP) (Open Source)
Version 2.10.0
Spencer Kimball, Peter Mattis and the GIMP Development Team

https://www.gimp.org




Originally published: January 2019


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