LibreOffice » canvas
UNO-based graphics backend, lesser impedance to modern graphics APIs than vcl.
The canvas framework is the successor of the system GUI and graphics backend VCL. Basic functionality is available, supplying just as much features as necessary to provide a VCL-equivalent feature set (except proper BiDi/CTL support).
The canvas framework consists of the following two modules, canvas and cppcanvas. Additionally, a new generic graphics tooling is used (but not exclusively by the canvas, Armin's drawinglayer module also make use of it), which resides in basegfx.
The UNO API used by the canvas is primarily under css::rendering, with css::rendering::XCanvas being the central interface.
The slideshow engine has replaced the former Impress-embedded presentation framework with a fully independent UNO component, and it is based on the canvas. Some features used there are only available from canvas, like double-buffering, and hardware-accelerated alpha-blending (currently not on all platforms).
cairo canvas is one of backends of canvas component. canvas is mostly used for slideshow rendering and also for emf+ rendering. we hoped it will even be used by drawing layer, but it didn't happen (yet?) for API look at offapi/com/sun/star/rendering/, the implementation is in canvas and cppcanvas modules.
cairo canvas backend uses cairo library for rendering. main advantage is support of alpha transparency and in some cases accelerated rendering.
the backend itself is quite old and stable, not many changes in that area lately, mostly changes for emf+ rendering, communication with vcl and bugfixes
FUTURE work: look at cairo canvas and situation when it is used (mostly slideshow). TODO there still might be more cases when we can save some roundtrips when exchanging data with vcl.