libgeometry revamp

1 files changed, 40 insertions, 0 deletions

diff --git a/sys/src/libgeometry/triangle.c b/sys/src/libgeometry/triangle.c
new file mode 100644
index 000000000..1ed6cfcda
--- /dev/null
+++ b/sys/src/libgeometry/triangle.c
@@ -0,0 +1,40 @@
+#include <u.h>
+#include <libc.h>
+#include <geometry.h>
+
+/* 2D */
+
+Point2
+centroid(Triangle2 t)
+{
+	return divpt2(addpt2(t.p0, addpt2(t.p1, t.p2)), 3);
+}
+
+/*
+ * based on the implementation from:
+ *
+ * Dmitry V. Sokolov, “Tiny Renderer: Lesson 2”,
+ * https://github.com/ssloy/tinyrenderer/wiki/Lesson-2:-Triangle-rasterization-and-back-face-culling
+ */
+Point3
+barycoords(Triangle2 t, Point2 p)
+{
+	Point2 p0p1 = subpt2(t.p1, t.p0);
+	Point2 p0p2 = subpt2(t.p2, t.p0);
+	Point2 pp0  = subpt2(t.p0, p);
+
+	Point3 v = crossvec3(Vec3(p0p2.x, p0p1.x, pp0.x), Vec3(p0p2.y, p0p1.y, pp0.y));
+
+	/* handle degenerate triangles—i.e. the ones where every point lies on the same line */
+	if(fabs(v.z) < 1)
+		return Pt3(-1,-1,-1,1);
+	return Pt3(1 - (v.x + v.y)/v.z, v.y/v.z, v.x/v.z, 1);
+}
+
+/* 3D */
+
+Point3
+centroid3(Triangle3 t)
+{
+	return divpt3(addpt3(t.p0, addpt3(t.p1, t.p2)), 3);
+}