implementation of curveTangent() from davbol (bug #715)

core/src/processing/core/PGraphics.java

@@ -1796,10 +1796,23 @@ public abstract class PGraphics extends PImage implements PConstants {
   }
 
 
+  /**
+   * Calculate the tangent at a t value (0..1) on a Catmull-Rom curve.
+   * Code thanks to Dave Bollinger (Bug #715)
+   */
   public float curveTangent(float a, float b, float c, float d,
                             float t) {
-    System.err.println("curveTangent not yet implemented");
-    return 0;
+    if (!curve_inited) curve_init();
+
+    float tt3 = t * t * 3;
+    float t2 = t * 2;
+    float m[][] = curve_basis;
+
+    // not optimized (and probably need not be)
+    return (a * (tt3*m[0][0] + t2*m[1][0] + m[2][0]) +
+            b * (tt3*m[0][1] + t2*m[1][1] + m[2][1]) +
+            c * (tt3*m[0][2] + t2*m[1][2] + m[2][2]) +
+            d * (tt3*m[0][3] + t2*m[1][3] + m[2][3]) );
   }
 
 

core/todo.txt

@@ -13,8 +13,13 @@ X     perhaps something gets corrected?
 X   had a fix, but decided not to re-implement the loadTIFF stuff too
 X fix for bezierTangent() problem from dave bollinger
 X   http://dev.processing.org/bugs/show_bug.cgi?id=710
+X implement curveTangent (thanks to davbol)
+X   http://dev.processing.org/bugs/show_bug.cgi?id=715
 
-test
+_ loadStrings(".") should not list directory contents
+_   http://dev.processing.org/bugs/show_bug.cgi?id=716
+
+_ remove saveFile() methods?
 _ ENABLE_DEPTH_SORT causing trouble with MovieMaker
 _   need to make the flush() api accessible
 _   http://dev.processing.org/bugs/show_bug.cgi?id=692
@@ -435,9 +440,8 @@ _   rewrite line and stroke code, it's a buggy mess
 _     lines become 2 pixels thick after a 3D transform 
 _     better handling of single-pixel special case
 _       flat_line_retribution is a hack, can go away
-_ implement curveTangent
-_   fix-up the curve_init() and the rest to use matrices
-_     and not have ugly names (i.e. just g.curveDetail is good)
+_ fix-up the curve_init() and the rest to use matrices
+_   and not have ugly names (i.e. just g.curveDetail is good)
 _ ellipse scaling method isn't great
 _   http://dev.processing.org/bugs/show_bug.cgi?id=695
 _   weird ellipse bug with an alpha line in same image