Creating guides inside GIMP is a bit of a limited task as of the time of this tutorial. However, there are some very simple and effective workarounds to achieve custom guides at any angle and any position to help you accurately place objects or paint at angles. And yes, you will be able to snap objects to these “custom guides” we create. You can watch the video version of this tutorial below, or scroll past the video for the full help article available in 30+ languages.
For starters, you can always place center guides in your composition by going to Image>Guides>New Guide by Percent.
Set the dropdown value (red arrow in the image above) to either vertical or horizontal, then type “50” in the numerical field (green arrow) as the value. Click OK. If you do this for both “Vertical” and “Horizontal” options, you will now have center guides on your image.
If you want guides placed at an angle on your image, you’ll have to draw paths and set those paths at the angles you want.
To do this, grab the paths tool from your toolbox (red arrow in the image above – shortcut key “b”).
In this first example, I’ll be creating custom guides that rotate from the horizontal center of the image. You can also perform this technique from the vertical center of the image.
So, I’ll place my path along the horizontal center of my image by hovering my mouse over the horizontal center guide. To ensure I am precisely placing the first point of my path, I am looking at the measurement in the lower left corner of the image window (green arrow). Because my image is 1080 pixels tall, I know that the exact horizontal center of my image is at 540 pixels. So, once the position of my mouse reads 540, I’ll click to place the first point of my guide (blue arrow).
Notice that I’ve placed the first point off the canvas – creating a little bit of distance from the left edge of the document. This is to ensure that when I rotate the guide later it will be long enough to span the entire length of the document.
Now that I have my first point of my path placed, I’ll hover my mouse over the other end of the document at the horizontal center guide. Once again, I’ll make sure the measurement reads “540” in the lower left corner (blue arrow in the image above) of the image window. I’ll click to place the second point of my path (red arrow).
Note: if you are having trouble precisely getting your mouse positioned at the halfway measurement (540 in this case), simply zoom in a bit more on your composition (you can do this using ctrl+mouse wheel or cmd+mouse wheel on a MAC).
Once my path is placed across the center of my composition, I can now use the Rotate tool to create custom guides at any angle. I’ll start by grabbing my rotate tool from the Toolbox (red arrow in the image above – shortcut key shift+r) and setting the “Transform” mode to “Path” (blue arrow) in the Tool Options for this tool.
Next, I’ll come over to the “Paths” tab (red arrow in the above image). Here, you’ll see the path we just drew – titled “Unnamed” by default). I’ll click the “Show/Hide” icon (blue arrow) so I can see this path on the document/image.
I’ll now duplicate this path so that we can create multiple custom guides at various angles. To do this, I’ll click the “duplicate” icon at the bottom of the Paths dialogue (red arrow). A new path will be created titled “Unnamed copy” (blue arrow).
If you want to rename your paths, simply double click on the current path name and type a new one (I renamed my paths “Original” for the first path we created and “30 degrees” for the duplicate path – outlined in green).
Click on the path you wish to rotate, making it the active path (I will click on the “30 degrees” path to make it active – red arrow in the image above). Now, with the “Rotate” tool still selected, click on your composition. This should bring up the Rotate dialogue box (green arrow).
Make sure the “Center X” (red arrow in the image above) is set to whatever the mid-point is of your image’s width (this value will be 960 in my case since my image’s width is 1920 pixels – 1920 divided by 2 is 960), and the “Center Y” (green arrow) is whatever the mid-point is of your image’s height (in my case it’s 540). This “Center” is going to be the center of axis for our rotation (blue arrow).
Once your Center X and Center Y are set to the proper values, you can now set the angle for your path’s rotation. For example, I can rotate the path by 30 degrees by typing “30” in the angle field (red arrow) and hitting the enter key.
You can also manually click and drag the slider below the angle field to adjust the value of the angle, though this method tends to be less precise.
Once I have rotated the path at the angle I want, I’ll click “Rotate” (blue arrow). This will give us an angled path, which we can use as a custom guide.
To make this path effective as a guide, we’ll need to turn on the snapping of objects to the path. I can do this by going to View>Snap to Active Path (red arrow). Objects will now snap to whatever path we have active over in our Paths tab.
So far we have used the horizontal center of the image as the rotational axis, but we can use any position on our image. For example, I can create an angled path from the bottom of the image. To do this, I’ll start by duplicating the “Original” path over in the paths tab (blue arrow). This will create a new path named “Original copy.” I’ll rename this path “Bottom 60 Degrees.”
I’ll then use the “Alignment” tool from the toolbox (red arrow in the image above – shortcut key “q”) to align this path to the bottom of the image. Once you have the alignment tool selected, click on the path in the center of the image. You’ll know you’ve selected the path because the corners will show small squares (blue arrow).
Change the “Relative to” option to “Image” (red arrow), then click “Align bottom of target” (blue arrow). Your path should now be sitting at the bottom of the image (green arrow).
Now, I’ll once again grab the Rotate tool from the toolbox (shift+r shortcut key). I’ll click on the bottom guide with this tool if it doesn’t automatically pop up with the Rotate dialogue box.
Next, I’ll make sure the “Center X” and “Center Y” values are set to 0 (blue arrow) and 1080, respectively. It’s important that the Center X value be set to “0” in this case because I want the path to rotate from the bottom left corner of the image (green arrow). If you want it to rotate from a different point, i.e. the right corner of the image, make sure the value you set here matches that value (for the right corner it would be 1920 in my case, since that is the full width of the image and therefore the location of the right corner of the image).
Finally, set the value of your rotation in the “Angle” field inside the Rotation dialogue box (red arrow). You’ll have to place a “minus” sign at the beginning of the value if you are rotating from the bottom left corner. This will ensure that path rotates upwards and not downwards (in my case, I put -60 to rotate upwards 60 degrees).
Click “Rotate” to rotate the path.
As an example of how you can snap objects to these paths, effectively using them as angled guides, I’ll draw an ellipse shape on my composition using the Ellipse Select Tool (red arrow in the image above – shortcut key “E”) and will drag the shape over to my active path. The center crosshairs of this shape (blue arrow) will snap to the path like a guide.
Voila! Note that you can perform this technique on any path angle or shape (since you can convert shape selection areas to paths in GIMP). So, in other words, you could create a path based on any drawable shape, and then use that path as a guide with this method. Very powerful stuff!