Episode: 3160
Title: HPR3160: GIMP: Transform Tools
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3160/hpr3160.mp3
Transcribed: 2025-10-24 18:00:20

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This is Hacker Public Radio Episode 3,160 for Friday 11 September 2020. Today's show is entitled,
Gimp, Transform Tools,
and is part of the series Gimp. It is hosted by Ahuka
and is about 17 minutes long, and carries a clean flag. The summer is
with Transform Tools, you can stretch, move, and crop the image.
This episode of HPR is brought to you by AnanasThost.com.
Get 15% discount on all shared hosting with the offer code
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the shortcut is Q, the move tool shortcut is M, crop, shift plus C, rotate, shift plus
R, scale, shift plus T, shear, shift plus S, perspective, shift plus P, flip, shift plus
F, engage, transform, shift plus G. So those are the tools. Now let's get to each one
of them. The align tool moves one or more layers to align with a target. Well, what's the
target? Could be any of several things. The target could be the first item. So if you
had selected multiple layers using the shift key when clicking on the layers, the first
one you clicked on is the first item. So what you can do is you can click that one and
then click a few others and align those other layers to line up with that first one. You
can align with an image. You can align with a selection. You can align with the active
layer or the active channel or the active path. And you make this choice in the tool options
panel on the bottom left. Now 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. Now if you need to add an
offset, you do that with the distribute buttons, which are right under the align buttons and look
exactly the same. The only difference is you add an offset in the box at the bottom. And that's
how many pixels, I'm going to say pixels because I'm assuming we're going to be working with
computer screen images at this point. Remember we talked before about the differences between
pixels and points and print versus computer screens and all of that. So for the moment,
I'm just going to assume it's pixels and so you add how many pixels. So if you added a 10 pixel
offset, which is actually a very small distance by the way, and then align to the left, what would happen
would be you wouldn't align all the way to the left side, you'd align to 10 pixels away from the left
side. So that's how the distribute buttons work. Now move is pretty straightforward. You can move
a layer, a selection or a path. And again, you choose the mode in the tool option section on the
lower left. You can also make a choice between picking layer or guide or only being able to move
the active layer. Now personally, I don't see the point in restricting your options. So I leave mine
to pick the layer or guide. The crop tool, Shift plus C, the crop tool is pretty easy to use.
When you select the tool, your cursor will become a cross like target site 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 little rectangles that show in the corners
of the selection. And 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 as an example. Rotate Shift plus R.
This lets you rotate a layer, a selection or a path. And like most tools, the mode selection
is in the tool options on the lower left. When you select this tool and then click on the image,
you'll get a window that pops up and lets you set an angle, which you can specify numerically
or use a slider right under it to create the angle. You can adjust the center of the rotation.
By default, it will be the mathematical center of the image, but you can change that.
It is set using units of pixels by default, so you can set everything there and then just click the
rotate button on the lower right of this window and it will do that. Now you can also free-hand
the rotation by just clicking on the image and dragging it around. If you want to constrain
this a little, you can use the trick we saw before with the paint tools and hold down the control key
while you're clicking and rotating, and that will 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 the canvas. When you do, you can, if you like, go to image menu and select fit canvas to
layers, and then the entire rotated image will fit inside a new larger canvas. But note the
canvas itself won't be rotated. Canvases are always rectangles that are aligned in perfect
vertical and horizontal directions. Scale tool, Shift plus T. This tool changes the size of the image.
When you select this tool and click on the image, again, you get a window that appears. Let's
you select the width and the height of the final image. Units again are going to be by default in
pixels, and then when you have it where you want it, you can just click the scale button and the
lower right of this pop-up window. Now, as with rotate, this pop-up window gives you a little finer
control by setting the precise definitions. But there will also be a grid overlaying the image
and eight handles, four of them at the corners, four of them on the sides, that you can use to
adjust this as well. Now, one thing to keep in mind in changing the scale is you can introduce
distortion if one dimension changes more than the other so that elements are stretched in either
the vertical or horizontal dimension. Now, if you want to prevent this, use the check box 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 the image menu, select fit
canvas to layers to correct this. Shared tool, Shift plus S. The shared tool distorts the image by
a kind of a twisting. In the horizontal direction, it can drag the top of the image one way and then
the bottom will automatically go 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. Again, when you select the tool and click on the image or selection, you will get a pop-up
window and it will let you select the sheer magnitude for either the x-axis, the horizontal direction,
or the y-axis, which is the vertical direction. Now, using this gives you finer control, but note 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.
Now, you can also freehand it using the mouse with a grid that appears, but again, only one direction
of the time, and as before, you can resize the canvas if needed. As perspective tool, Shift plus P.
This is really a tool for distorting the image. It is not a true perspective tool. It can, however,
be used to create the appearance of perspective. You can see what I mean. If you create a blank canvas
with a white background and you create a black rectangle and move that around, you will get a good
idea of what I am talking about. When you do this, you select the perspective tool. It will pop 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 it with Kapedia, and I will put the link in the show notes
for those of you who really want to subject yourself to that, but I say it is highly technical.
But what the tool does, basically, is let you stretch the image by pulling any corner,
and it will stretch that corner independently. None of the other tools do that thing exactly.
You can then pick a different corner to add more distortion, and it can 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. Now, flip is the simplest of the transformation tools. It creates mirror images,
and it can either flip the image vertically or horizontally, and you choose which it will be
in the tool options area on the lower left. Easy peasy. And last of them is the cage transform,
and that is Shift plus G. And what this does is let you define an area on your image
by clicking to select anchor points or control points, and we've seen things like that before
with the selection tools. And what you need to do is make a free form kind of box. Every time you
click to set another control point or another anchor point, a line will connect those two points,
and you keep clicking around until you get back to the beginning and click on your initial point,
and then you've got the whole thing selected. And then when you do that, click on any one of those
anchor points and drag it to distort the image. So this is one of those ones that at first glance
looks really weird, but it gets some very interesting effects with it. Now, in summary,
as with the other tools we've covered, the best way to learn these is playing around with them.
Now, for most of these, I suggest using a photo to see what each of them does.
After all, if you had a black rectangle and you used the flip tool, you're not going to see anything.
If you want to dig deeper, also I always put a link in the show notes to the GIMP documentation site,
and there's a lot more information there about all of these if you really want to plumb the
depths. And bear in mind that these tutorials are not plumbing the depths. They're an introductory
look at this to get you familiar. So I would suggest playing around with these a little bit,
get to know them, and we've still got one more group of tools to look at. And after that,
I think we can probably get on to something else. Maybe the layers would be next. Let's see what
happens. So this is a hookah for Hacker Public Radio, signing off, and as always, asking you to support
free software. Bye-bye.
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