What is the backend of Rust game engine Piston?

From Piston overview, Backends:

These integrates Piston with variable external APIs.The window and graphics backs are duplicated.The reason is greater flexibility when shipping a product, easier to compare API design and performance, and works both for cross and native platforms.

Window (uses the piston core):

• pistoncore-glfw_window
• pistoncore-glutin_window
• pistoncore-sdl2_window

2D graphics (usespiston 2d-graphics:

• piston2d-opengl_graphics
• piston2d-gfx_graphics
• piston2d-glium_graphics