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The PreTeXt Guide

Subsection 4.14.2 Vector Graphics

An image is a vector graphic if the file describes the geometric shapes that constitute the image. So a simple diagram would be a good candidate, but a photograph would not. Popular formats are Portable Document Format (PDF) and Scalable Vector Graphics (SVG). You will get the best results with PDF images in output and SVG images for HTML. The principal advantage of these formats is that they scale (big or small) smoothly, along with fonts. This is critical when you cannot predict the screen size for a reader of an electronic version.
Unless you describe these images with a language (see Subsection 4.14.3), you are responsible for providing the PDF and SVG versions. The pdf2svg utility is very useful if you have PDF images only. To have these different images used for different output formats, you simply follow the instructions above, but do not include a file extension. This alerts the conversion to use the best possible choice for any given output, and to embed it correctly. So presuming you made available the files images/toad-life-cycle.pdf and images/toad-life-cycle.svg, the following example would incorporate the PDF version with output and the SVG version for HTML output.
<image source="images/toad-life-cycle" width="85%">
    <description>The four stages of a toad's life.</description>
Vector graphics images can be created with source code in different languages (Subsection 4.14.3) or with applications, such as Inkscape (Section 5.7). If you are creating non-technical graphics that have lots of geometric shapes and simple text (a look like a movie poster), then using a tool like Inkscape is a great choice since its native file format is an enhanced version of SVG and a faithful PDF is easy to create.