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xform: add a reference for the equations of persp and ortho projections

Song Ho Ahn (안성호) does a magnificient job at deriving the two major
projection matrices, and it served as a fundamental resource since our
very first implementation, as well as with the latest change to get the
orthographic matrix to map correctly to our standard right-handed frames.

--- a/xform.c

+++ b/xform.c

@@ -138,6 +138,9 @@

 	return rframexform3(p, *e);

 }

+/*

+ * adapted from the equations in https://www.songho.ca/opengl/gl_projectionmatrix.html#perspective

+ */

 void

 perspective(Matrix3 m, double fovy, double a, double n, double f)

 {

@@ -152,6 +155,9 @@

 	m[3][2] = -1;

 }

+/*

+ * adapted from the equations in https://www.songho.ca/opengl/gl_projectionmatrix.html#ortho

+ */

 void

 orthographic(Matrix3 m, double l, double r, double b, double t, double n, double f)

 {

-- 

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