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In this article we will see how we can outline an image with a transparent background, using the Free & Open Source software GIMP 2.10 (GNU Image Manipulation Program). This photo editing program, Gimp 2.10, can be downloaded for free from the gimp.org website, depending on your operating system you have installed. GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software, you can change its source code and distribute your changes. In our case the O.S. is Windows 10. Supported OS: Windows 7 or over. The installer contains both 32-bit and 64-bit versions of GIMP, and will automatically use the appropriate one.
1. After installing and opening the program, select File then Open Image from the main menu, to choose the image you want to outline it. Important in this case is to choose an image in PNG format (Portable Network Graphics), in order to create a transparent background.
To create a transparent background to the image, right-click on mouse, from the menu that opens select Layer > Transparency > Add Alpha Chanel. After this step you have to select Fuzzy Select Tool (Select a continuous region on the basis of color), go back to the image background and with left-click, the all image background will be selected then press the DELETE key to eraise it.
2. From the main menu, select the Image tab > Canvas Size (Adjust the image dimensions) to increase the image dimensions with 200 px, both width and height. You need to make sure that in the Layers section at the bottom, the Fill with: Transparency option is selected. After adjusting the image size, select the Center tab to center the image, then click Resize tab.
3. A new Layer must be created from the bottom right icon which we will call outline (for example), fill with transparency. You will see that the new layer created will be positioned above the Image layer, and in this case you have to drag the Outline layer below the Image layer. Right-click on Image layer and select Alpha to Selection.
4. From the main menu press Select tab > Grow, and in the Grow selection window insert value the outline thickness of the image you want (for example 20px), then click OK. Select from the main menu Edit > Fill with FG Color to fill the outline with solid white color. Click Select tab from main menu, then None to see the final image with Outline. To save the image thus created in the computer, click File > Export as.. and export the image to PNG format. Below you can find a video tutorial on how to create an outline to the image.
Bonus. If you need the image to be mirrored to be printed on heat transfer paper and then printed on an object such as a T-shirt, follow these steps: from main menu, click Image > Transform > Flip Horizontally. The mirror image can be printed directly if you adjust the printer settings accordingly. You can find all the details about these settings in the article Mirror image printing - printer settings.
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