Chrome can draw vectorized images using Skia. Vector images have the advantages of looking better at different scale factors or sizes, can be easily colorized at runtime, and reduce the chrome binary size.
Chrome uses .icon files to describe vector icons. This is a bespoke file format which is actually a C++ array definition. At build time, the .icon files are composed into a single .cc file which is compiled into the binary.
The vector icons shared across different platforms live in src/ui/gfx/vector_icons/ and the icons which are specific to Ash live in src/ash/resources/vector_icons. Some of the icons have .1x.icon variants which are used when the device scale factor is 100%. For any other scale factor, the .icon variant will be used. The 1x variants are generally only necessary for very small icons which may look fuzzy if shrunk from a larger icon.
Converting an SVG to .icon format
This tool generates .icon file output from SVGs. (If you want to contribute improvements, here's the project.)
It handles only a small subset of SVG (paths, circles, etc.) and it's finicky about what it expects as the format, but with a minor amount of manual intervention beforehand, it mostly spits out usable .icon output. It will often work better if you run the SVG through SVGO first, which is a separate project (an SVG minifier). Jake Archibald's SVGOMG is a web interface to SVGO. If any manual adjustments need to be made to the output, the SVG Path spec is a helpful reference.
Some SVGs are already pretty minimal, like the ones at https://design.google.com/icons/ so they don't require much if any adjustment, but some SVG editing tools like Sketch leave a lot of random cruft so SVGOMG helps a lot. Take the output and insert into a .icon file.
Using .icon files
Once you have created an .icon file, place it in src/ui/gfx/vector_icons or src/ash/resources/vector_icons (the latter is used for Ash-specific icons) and add the filename to the corresponding BUILD.gn.
The icon with filename foo_bar.icon found in src/ui/gfx/vector_icons is mapped to the value FOO_BAR in the VectorIconId enum. Typically the call site would look like:
gfx::CreateVectorIcon(gfx::VectorIconId::FOO_BAR, 32, color_utils::DeriveDefaultIconColor(text_color));
If foo_bar.icon is located in src/ash/resources/vector_icons, it would instead be mapped to the constant name of kFooBarIcon ('k' + camel-cased filename + 'Icon'); work is in progress to make this the standard naming convention for all icons. A call site for an Ash-specific icon would look like:
gfx::CreateVectorIcon(kFooBarIcon, 32, color_utils::DeriveDefaultIconColor(text_color));
If the size argument is unspecified, the size will be taken from the .icon file (or the .1x.icon if more than one exists). The icon's name should match its identifier on the MD icons site if that's where it came from. For example, ic_accessibility would become accessibility.icon.
Where can I use vector icons?
Chrome's native UI on desktop platforms. Currently the vector icons are in extensive use on views platforms, where skia is the normal drawing tool. Mac uses them sometimes, but optimizing performance is still a TODO so many places stick with raster assets. The files in chrome/app/theme/default_*_percent/legacy are ones that have been switched to vector icons for Views but not yet for OS X. If you need to add raster assets (PNG) for mobile or OS X, please make sure to limit their inclusion to those platforms.
How can I preview vector icons?
Build and run the views_examples_exe (or views_examples_with_content_exe) target and select "Vector Icons" from the dropdown menu. This loads a simple interface which allows you view any vector icon file at a specified size and color. Contributions to improve this interface are welcome (bug). Note that currently clicking through the interface will only show the icons in src/ui/gfx/vector_icons (not the Ash-specific ones).
Can my vector icon have more than one color?
Yes. You can hard-code colors for specific path elements by adding a PATH_COLOR_ARGB command to the appropriate place within the .icon file. Any path elements which are not given a hard-coded color in this manner will use the color provided to CreateVectorIcon() at runtime.
When introducing a new icon, should I use a PNG or a vector icon?
Use a vector icon, unless the icon is extremely complex (e.g., a product logo).
Why is the outside of my icon being filled instead of the inside?
This is a bug (contributions welcome). See the bug description for a manual workaround.
Why am I seeing ???, ??? as the first two arguments to CUBIC_TO?
Another bug. The workaround for the time being is to manually delete ???, ??? and replace CUBIC_TO with CUBIC_TO_SHORTHAND.