Revision: 52884
http://brlcad.svn.sourceforge.net/brlcad/?rev=52884&view=rev
Author: n_reed
Date: 2012-10-09 14:16:47 +0000 (Tue, 09 Oct 2012)
Log Message:
-----------
a few more epa comments
Modified Paths:
--------------
brlcad/trunk/src/librt/primitives/epa/epa.c
Modified: brlcad/trunk/src/librt/primitives/epa/epa.c
===================================================================
--- brlcad/trunk/src/librt/primitives/epa/epa.c 2012-10-09 04:20:16 UTC (rev
52883)
+++ brlcad/trunk/src/librt/primitives/epa/epa.c 2012-10-09 14:16:47 UTC (rev
52884)
@@ -660,8 +660,7 @@
* plane (x = 0.0). The parabola and line are described by arguments p and m,
* from the respective equations (z = y^2 / 4p) and (z = my + b).
*
- * The line is assumed to intersect the parabola at the origin and one other
- * point (i.e. m > 0, m < infinity).
+ * The line is assumed to intersect the parabola at two points.
*
* The portion of the parabola between these two intersection points takes on a
* single maximum value with respect to the intersecting line when its slope is
@@ -793,6 +792,19 @@
return (r * r) / (4 * mag_h);
}
+/* The contour of an epa in the plane H-R (where R is one of the epa axes A or
+ * B) is a parabola with vertex at H, opening toward -H. We can transform this
+ * parabola to get an equivalent parabola in the Y-Z plane, opening toward
+ * positive Z (-H) with vertex at (0, -|H|).
+ *
+ * The part of this parabola that passes between (0, -|H|) and (r, 0) is
+ * approximated by num_points points (including (0, -|H|) and (r, 0)).
+ *
+ * The constructed point list is returned (NULL returned on error). Because the
+ * above transformation puts the epa vertex at the origin and the parabola
+ * vertex at (0, -|H|), multiplying the z values by -1 gives corresponding
+ * distances along the epa height vector H.
+ */
static struct rt_pt_node *
epa_parabolic_curve(fastf_t mag_h, fastf_t r, int num_points)
{
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