Done using GIMP 2.8 on Kubuntu Linux 18.04 LTS
These tools are most commonly done to affect an entire image, though they can operate on areas or paths. There are some general properties shared by most of these tools, which are set in the bottom Properties area. They are:
- Transform – This lets you select if the tool will work on the entire layer, a selected contour, or a path.
- Direction – Either forward or backwards. As an example, if you had rotated an image and overdid it a little, you would like to take it back a bit.
- Interpolation – Some transformations (like stretching) mean that additional pixels need to slotted in to the image. This controls how those pixels will colored. Interpolation in general means looking at the surrounding pixels and more or less “averaging” them to get the new pixels. The None selection will generally be the coarsest and unsatisfactory, while Sinc is the highest quality, though Cubic may worth a try if you don’t like what Sinc did.
- Clipping – If you rotate a rectangular image, for instance, the corners of the image will go outside of the area of the canvas. If you leave this set Adjust, you can then go to Image–>Fit Canvas to layers, and the image will expand to contain the rotated form. Otherwise, you can choose to drop the image as a result of your rotation.
So, these are just a few of the general options you have with the Transform tools. One thing you need to be careful about is knowing what object you are transforming. Some of these tools only work on layers, others can work on layers or selections. Knowing which you are working on is important.
Now let’s take a look at each one in turn. Again, there are keyboard shortcuts. For me, I tend to use the keyboard shortcuts a lot when working with text because my fingers are already on the keyboard, but when working with graphics my hand is on the mouse so I use that more often. Your mileage may vary.
- Align (Q)
- Move (M)
- Crop (Shift+C)
- Rotate (Shift+R)
- Scale (Shift+T)
- Shear (Shift+S)
- Perspective (Shift+P)
- Flip (Shift+F)
- Cage Transform (Shift+G)
This tool moves one or more layers to align with a target, which can be any of:
- First item – If you had selected multiple layers using the Shift key when clicking, the first one you clicked on is the first item.
- Active Layer
- Active Channel
- Active Path
You make this choice in the Tool Options panel on the bottom left.
Once you have your target selected, you can align the layers you chose using the alignment buttons below the target selection. You can align to the Left side, horizontal middle, right side, top, vertical middle, and bottom. If you need to add an offset, you do that with the Distribute buttons right under the Align buttons. Just add the offset, and choose the correct button.
This is pretty straightforward. You can move a layer, a selection, or a path, and make this mode choice in the Tool Options section on the lower left. You also can make a choice between picking the layer or guide, or only being able to move the Active Layer. Personally, I don’t see the point of restricting your options, so I leave mine to pick the layer or guide.
The crop tool is pretty easy to use. When you select the tool, your cursor will become a cross, like a target sight, with a knife icon under it. Center the cross on one corner of the area you want, and click-and drag to create a rectangle. You can adjust the selection using the “handles” (i.e. the rectangles in the corners of the selection). When you have it where you want it, either click in the middle of the selection, or hit the Enter key, and what you have selected will be the only thing left. This tool is one I use frequently on old photos to remove unwanted borders.
This lets you rotate a layer, a selection, or a path, and like most tools the mode selection is in the Tool Options. When you select this tool and then click on the image, you will get this window:
You can fill in the numbers to get the best control, but you can also “free hand” the rotation by clicking in the image and dragging it around. If you want to constrain it a little, you can use the trick we saw before with the Paint tools and hold down the Control key to restrict the rotation to multiples of 15 degrees. This is useful if you want to turn a square into a diamond shape, for instance: three steps gets it to a perfect 45 degree rotation. Note that you can rotate something so that part of it is off of the canvas. When you do, you can, if you like, go to Image–>Fit Canvas to Layers, and then the entire rotated image will fit inside a new larger canvas, but the canvas itself won’t be rotated. Canvases are always rectangles that are aligned in perfect vertical and horizontal directions.
This tool changes the size of the image. When you select this tool, and then click on the image, this window appears:
As with Rotate, this gives you a little finer control on the scaling by letting you specify a precise set of dimensions for the final picture. There will also be grid overlaying the image, and 8 “handles” (4 at corners, 4 on sides) to adjust as well. One thing to keep in mind in changing the scale is that you can introduce distortion if one dimension changes more than the other, so that elements are stretched in either the vertical or horizontal dimension. To prevent this, use the checkbox on the bottom of the Tool Options that says Keep Aspect, or hold down the control key while you click and drag. Finally, if you increase the size beyond the canvas, go to Image–>Fit Canvas to Layers to correct this.
The Shear tool distorts the image by a kind of twisting. In the horizontal direction it can drag the the top of the image one way, and the bottom the other way. Or in the vertical direction it will drag one side up and the other side down. This is not rotation, it is a stretching of the image that definitely distorts it. When you you select the tool and click on the selection, you get this window:
Using this window gives you finer control, but note that you can only do either X or Y at any one time. To do both, you have to use the tool twice in a row. You can also “free hand” is using the mouse with a grid that appears, but again only one direction at a time. And as before, you can resize the canvas if needed.
This is a tool for distorting the image, not a true perspective tool. But it can be used to create the appearance of perspective. It pops up a window with various coordinate values based on the idea of a Transformation Matrix, which is a very technical thing you can read about at Wikipedia. But what it does is lets you stretch the image by pulling any corner and it will stretch that independently. You can then pick a different corner to add more distortion. It could definitely be used to create perspective, but my advice is that unless you are highly technical about this kind of thing, forget about the Transformation Matrix and just use the grid to click and drag until it looks right to you.
This is the simplest of the Transformation Tools. It creates mirror images, basically by flipping the image either vertically or horizontally. You choose which it will be in the Tool Options area on the lower left.
Cage Transform (Shift+G)
Basically, what this does is let you define an area on your image by clicking to select anchor points (or control points) that make a free form “box” around your image. When you have completed this by clicking back around to your initial point, you can then click on one of the anchor points and drag it to distort the image.
As with the other tools we have covered, the best way to learn these is playing around with them. For these tools, I suggest using a photo to see what each of them does. After all, using the Flip tool with a black rectangle won’t show you much of anything. And if you want to dig deeper, please know that there is a lot more information on all of these tools in the GIMP Documentation site https://docs.gimp.org/2.8/en/. Bear in mind that these tutorials are introductory, and are not intended to plumb the depths.