atan2(y, x) in AGAL using atan(x) = Pi / 2 * x / (1 + x)

atan2.as

context3D.setProgramConstantsFromVector(Context3DProgramType.FRAGMENT, 0, Vector.<Number>([
	1, 
	0, 
	Math.PI, 
	2 * Math.PI,
	
	1e-10, 
	Math.PI / 2, 
	0, // not used
	0, // not used
]));


// ft4.x = atan2(ft2.y, ft2.x)  Uses: ft3, ft4, ft5
"add ft2.x, ft2.x, fc1.x            \n"+ // fudge to prevent div zero

"div ft3.x, ft2.y, ft2.x            \n"+ // ft2.x = ydiff / xdiff
"neg ft3.y, ft3.x                   \n"+ // ft2.y = -ydiff / xdiff

"mul ft4.y, fc1.y, ft3.x            \n"+ // ft4.x = atan(ft2.x)
"add ft4.z, fc0.x, ft3.x            \n"+ // atan(x) = Pi / 2 * x / (1 + x)
"div ft5.x, ft4.y, ft4.z            \n"+

"mul ft4.y, fc1.7, ft3.y            \n"+ // ft4.y = atan(ft2.y)
"add ft4.z, fc0.x, ft3.y            \n"+ // atan(x) = Pi / 2 * x / (1 + x)
"div ft5.y, ft4.y, ft4.z            \n"+

"slt ft4.x, ft2.x, fc0.y            \n"+ // x < 0  ft4.x
"slt ft4.y, ft2.y, fc0.y            \n"+ // y < 0  ft4.y
"sub ft4.z, fc0.x, ft4.x            \n"+ // x >= 0  ft4.z
"sub ft4.w, fc0.x, ft4.y            \n"+ // y >= 0  ft4.w

"mul ft3.x, ft4.z, ft4.w            \n"+ // x > 0 && y > 0  ft3.x
"mul ft3.y, ft4.x, ft4.w            \n"+ // x < 0 && y > 0  ft3.y
"mul ft3.z, ft4.x, ft4.y            \n"+ // x < 0 && y < 0  ft3.z
"mul ft3.w, ft4.z, ft4.y            \n"+ // x > 0 && y < 0  ft3.w

"sub ft4.x, ft5.x, fc0.z            \n"+ // a - Pi  ft4.x
"neg ft4.y, ft5.y                   \n"+ // -a      ft4.y
"mov ft4.z, ft5.x                   \n"+ // a       ft4.z
"sub ft4.w, fc0.z, ft5.y            \n"+ // Pi - a  ft4.w

"mul ft4, ft4, ft3                  \n"+ // multiply grid of possibilities

"add ft4.xy, ft4.xz, ft4.yw         \n"+ // add possibilities
"add ft4.x, ft4.x, ft4.y            \n"+