In addition to the portability issue, CoreGraphics does not compete directly with Skia or Direct2D for that matter (it’s mostly CPU based). If you combine it with CoreAnimation you get something of a quasi superset. Direct2D is +/-, but it also doesn’t have some higher level features like PDF and SVG backends, and it doesn’t have the software rendering capabilities of skia by design.
Of course there are 2D drawing libraries on the major platforms (and CoreGraphics and Direct2D are very different, there’s also GDI+), but it was a weird question to start: what’s the use of a portable library. But furthermore, skia is not just a portability wrapper.
Well, the whole point is the native 2D library on the OSes that Skia depends on, APIs that take direct advantage of OS 2D and 3D OS APIs, and not some third party.
Are Vulkan and OpenGL OS “native” or 3rd party? I think this is sort of a semantics hairsplitting argument that isn’t terribly useful.
Skia is useful because it provides a powerful, performance oriented common target 2D drawing library, and it is more than a lowest common denominator wrapper.
Of course there are 2D drawing libraries on the major platforms (and CoreGraphics and Direct2D are very different, there’s also GDI+), but it was a weird question to start: what’s the use of a portable library. But furthermore, skia is not just a portability wrapper.