add a wrapper around getPerspectiveTransform

ext/opencv/cvmat.cpp

@@ -352,6 +352,7 @@ void define_ruby_class()
   rb_define_method(rb_klass, "resize", RUBY_METHOD_FUNC(rb_resize), -1);
   rb_define_method(rb_klass, "warp_affine", RUBY_METHOD_FUNC(rb_warp_affine), -1);
   rb_define_singleton_method(rb_klass, "rotation_matrix2D", RUBY_METHOD_FUNC(rb_rotation_matrix2D), 3);
+  rb_define_singleton_method(rb_klass, "get_perspective_transform", RUBY_METHOD_FUNC(rb_get_perspective_transform), 2);
   rb_define_method(rb_klass, "warp_perspective", RUBY_METHOD_FUNC(rb_warp_perspective), -1);
   rb_define_singleton_method(rb_klass, "find_homography", RUBY_METHOD_FUNC(rb_find_homograpy), -1);
   rb_define_method(rb_klass, "remap", RUBY_METHOD_FUNC(rb_remap), -1);
@@ -4053,6 +4054,40 @@ rb_rotation_matrix2D(VALUE self, VALUE center, VALUE angle, VALUE scale)
   return map_matrix;
 }
 
+/*
+ * call-seq:
+ *   CvMat.get_perspective_transform(<i>from_points,to_points</i>) -> cvmat
+ *
+ * Calculates a perspective transform from four pairs of the corresponding points.
+ * Returns a matrix suitable for use with warp_perspective
+ */
+VALUE
+rb_get_perspective_transform(VALUE self, VALUE source, VALUE dest)
+{
+  Check_Type(source, T_ARRAY);
+  Check_Type(dest, T_ARRAY);
+
+  int count = RARRAY_LEN(source);
+
+  CvPoint2D32f* source_buff = ALLOCA_N(CvPoint2D32f, count);
+  CvPoint2D32f* dest_buff = ALLOCA_N(CvPoint2D32f, count);
+
+  for (int i = 0; i < count; i++) {
+    source_buff[i] = *(CVPOINT2D32F(RARRAY_PTR(source)[i]));
+    dest_buff[i] = *(CVPOINT2D32F(RARRAY_PTR(dest)[i]));
+  }
+
+  VALUE map_matrix = new_object(cvSize(3, 3), CV_MAKETYPE(CV_32F, 1));
+
+  try {
+    cvGetPerspectiveTransform(source_buff, dest_buff, CVMAT(map_matrix));
+  }
+  catch (cv::Exception& e) {
+    raise_cverror(e);
+  }
+  return map_matrix;
+}
+
 /*
  * call-seq:
  *   warp_perspective(<i>map_matrix[,flags = CV_INTER_LINEAR | CV_WARP_FILL_OUTLIERS][,fillval=0])</i>) -> cvmat

ext/opencv/cvmat.h

@@ -175,6 +175,7 @@ VALUE rb_quadrangle_sub_pix(int argc, VALUE *argv, VALUE self);
 VALUE rb_resize(int argc, VALUE *argv, VALUE self);
 VALUE rb_warp_affine(int argc, VALUE *argv, VALUE self);
 VALUE rb_rotation_matrix2D(VALUE self, VALUE center, VALUE angle, VALUE scale);
+VALUE rb_get_perspective_transform(VALUE self, VALUE source, VALUE dest);
 VALUE rb_warp_perspective(int argc, VALUE *argv, VALUE self);
 VALUE rb_find_homograpy(int argc, VALUE *argv, VALUE self);
 VALUE rb_remap(int argc, VALUE *argv, VALUE self);

test/test_cvmat_imageprocessing.rb

@@ -450,6 +450,40 @@ class TestCvMat_imageprocessing < OpenCVTestCase
     # snap mat0, mat1, mat2, mat3, mat4
   end
 
+  def test_get_perspective_transform
+    from = [
+        OpenCV::CvPoint2D32f.new(540, 382),
+        OpenCV::CvPoint2D32f.new(802, 400),
+        OpenCV::CvPoint2D32f.new(850, 731),
+        OpenCV::CvPoint2D32f.new(540, 731),
+    ]
+    to = [
+      OpenCV::CvPoint2D32f.new(0, 0),
+      OpenCV::CvPoint2D32f.new(233, 0),
+      OpenCV::CvPoint2D32f.new(233, 310),
+      OpenCV::CvPoint2D32f.new(0, 310),
+    ]
+    transform = OpenCV::CvMat.get_perspective_transform(from, to)
+    assert_equal 3, transform.rows
+    assert_equal 3, transform.columns
+    expected = [
+      0.923332154750824,
+      0.0,
+      0.0,
+      1.4432899320127035e-15,
+      0.0,
+      0.0,
+      -498.599365234375,
+      0.0,
+      0.0,
+    ]
+    3.times do |i|
+      3.times do |j|
+        assert_in_delta(expected.shift, transform[i][j], 0.001)
+      end
+    end
+  end
+
   def test_rotation_matrix2D
     mat1 = CvMat.rotation_matrix2D(CvPoint2D32f.new(10, 20), 60, 2.0)
     expected = [1.0, 1.73205, -34.64102,