static Rect rgbExpandMatch(RGBImage a, RGBImage b, Rect r1, Rect r2, float ppt) {
  assertEquals(r1.w, r2.w);
  assertEquals(r1.h, r2.h);
  int w = a.w(), h = a.h();
  //print("Comparing " + r1 + " and " + r2);
  repeat {
    Rect r = r1;
    
    // left
    if (r2.x > 0 && r1.x > 0 && rgbImageSectionsIdentical_ppt(a, new Rect(r1.x-1, r1.y, 1, r1.h), b, new Rect(r2.x-1, r2.y, 1, r2.h), ppt)) {
      r1 = growRectLeft(r1, 1); r2 = growRectLeft(r2, 1);
    }
    
    // right
    if (r2.x2() < w && r2.x2() < w && rgbImageSectionsIdentical_ppt(a, new Rect(r1.x2(), r1.y, 1, r1.h), b, new Rect(r2.x2(), r2.y, 1, r2.h), ppt)) {
      r1 = growRectRight(r1, 1); r2 = growRectRight(r2, 1);
    }
  
    // top
    if (r2.y > 0 && r1.y > 0 && rgbImageSectionsIdentical_ppt(a, new Rect(r1.x, r1.y-1, r1.w, 1), b, new Rect(r2.x, r2.y-1, r2.w, 1), ppt)) {
      r1 = growRectTop(r1, 1); r2 = growRectTop(r2, 1);
    }
    
    // bottom
    if (r2.y2() < h && r2.y2() < h && rgbImageSectionsIdentical_ppt(a, new Rect(r1.x, r1.y2(), r1.w, 1), b, new Rect(r2.x, r2.y2(), r2.w, 1), ppt)) {
      r1 = growRectBottom(r1, 1); r2 = growRectBottom(r2, 1);
    }
    
    if (eq(r, r1)) break;
  }
 
  ret r1;
}