Each of the draw_modes is a way of interpreting the cells of an incoming matrix and making geometry from it. For example, triangles will make a triangle for each 3 cells across the horizontal dimension of the matrix and quads will make a quad from every 4. For tri_grid and quad_grid, jit.gl.mesh creates a grid of connected geometry based on the topology of the matrix. So neighboring cells will be connected with triangles or quads. A good way to see this clearly is to turn on @poly_mode 1 1 for jit.gl.mesh. The patches in this article might show the geometry a little more effectively than what I can describe:
http://cycling74.com/2010/03/08/demystifying-expressions-in-jitter/

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