Today's Topics:  Geometry Shaders!

The pipeline:  https://www.khronos.org/opengl/wiki/Rendering_Pipeline_Overview

Last time, we copied trees
	Cool

Let's do that with a geometry shader!
What geometry shaders do:
	Input:  Primitives, POST vertex shader!
	Output:  Primitives, not to be vertex shaded!
		Remember the order here, since it makes a fair bit of difference

Ok, copying the tree:
	input, and layouts (lines)
		Comes as an array
	output, and layout again
		EmitVertex() sort of takes care of the array thing
		No GL_LINES here, but we can use line_strip and EndPrimitive()
		Copying over of colors:  Can make a difference
		Tutorial library supports GL_GEOMETRY_SHADER, but there's not good coverage for these in the tutorials
			I haven't found any really...GLES example is kinda weird
			Suggested reference:  https://www.khronos.org/opengl/wiki/Geometry_Shader
			Another suggestion:  http://www.lighthouse3d.com/tutorials/glsl-tutorial/geometry-shader/
			Use today's example as a starting point
	How about we lawnmower the trees?
		Culling

Exploiting Parallelism:  Invocations
	We can make the duplicate using another invocation, with gl_InvocationID
	Only on OpenGL 4.0+
	Can't run it in a loop, or recursively, or anything like that
		Can have a loop inside it if you want
		Feedback, which we haven't got to yet, can sort of give you this

Can access uniform variables in the geometry shader
	So we could kinda just not use the vertex shader...
	But we have less invocations to work with
	And we have to have a vertex shader program anyway
	Certainly can, though!

Another use:  Normal Vectors
	The cube demo doesn't calculate per-vertex normal vectors
	We can calculate the per-face normal vector

Note:  How to fix the 3x repeated vertices with index drawing
	Another note:  The .slo file format

Lab today:  Use the geometry shader to make the cube draw with bulging sides
	Start with the cube demo.  May have to adjust opengl version to at least 3.2 (GLSL version 150)
	I suggest just updating to version 450, since we'll have to go to at least 430 for compute shaders anyway
	1.  Calculate face normal vectors
	2.  Determine the middle of the triangle hypotenuse
	3.  Add the normal vector to the middle of the hypotenuse to get the bulge point
	4.  Draw two triangles, each using two original points plus the bulge point
	This is geometry amplification, or tesselation
	To see what you're doing, consider drawing with lines_strip instead of triangles_strip
	Remember the nature of triangles_strip