Line data    Source code

       1             : /*
       2             :  * Copyright 2015 Google Inc.
       3             :  *
       4             :  * Use of this source code is governed by a BSD-style license that can be
       5             :  * found in the LICENSE file.
       6             :  */
       7             : 
       8             : /*
       9             : http://stackoverflow.com/questions/2009160/how-do-i-convert-the-2-control-points-of-a-cubic-curve-to-the-single-control-poi
      10             : */
      11             : 
      12             : /*
      13             : Let's call the control points of the cubic Q0..Q3 and the control points of the quadratic P0..P2.
      14             : Then for degree elevation, the equations are:
      15             : 
      16             : Q0 = P0
      17             : Q1 = 1/3 P0 + 2/3 P1
      18             : Q2 = 2/3 P1 + 1/3 P2
      19             : Q3 = P2
      20             : In your case you have Q0..Q3 and you're solving for P0..P2. There are two ways to compute P1 from
      21             :  the equations above:
      22             : 
      23             : P1 = 3/2 Q1 - 1/2 Q0
      24             : P1 = 3/2 Q2 - 1/2 Q3
      25             : If this is a degree-elevated cubic, then both equations will give the same answer for P1. Since
      26             :  it's likely not, your best bet is to average them. So,
      27             : 
      28             : P1 = -1/4 Q0 + 3/4 Q1 + 3/4 Q2 - 1/4 Q3
      29             : */
      30             : 
      31             : #include "SkPathOpsCubic.h"
      32             : #include "SkPathOpsQuad.h"
      33             : 
      34             : // used for testing only
      35           0 : SkDQuad SkDCubic::toQuad() const {
      36             :     SkDQuad quad;
      37           0 :     quad[0] = fPts[0];
      38           0 :     const SkDPoint fromC1 = {(3 * fPts[1].fX - fPts[0].fX) / 2, (3 * fPts[1].fY - fPts[0].fY) / 2};
      39           0 :     const SkDPoint fromC2 = {(3 * fPts[2].fX - fPts[3].fX) / 2, (3 * fPts[2].fY - fPts[3].fY) / 2};
      40           0 :     quad[1].fX = (fromC1.fX + fromC2.fX) / 2;
      41           0 :     quad[1].fY = (fromC1.fY + fromC2.fY) / 2;
      42           0 :     quad[2] = fPts[3];
      43           0 :     return quad;
      44             : }