Add example image about distortion effect.

modules/calib3d/doc/camera_calibration_and_3d_reconstruction.rst

@@ -83,7 +83,14 @@ is extended as:
 :math:`k_6` are radial distortion coefficients.
 :math:`p_1` and
 :math:`p_2` are tangential distortion coefficients.
-Higher-order coefficients are not considered in OpenCV. In the functions below the coefficients are passed or returned as
+Higher-order coefficients are not considered in OpenCV.
+
+The next figure shows two common types of radial distortion: barrel distortion (typically :math:`k_1 > 0` and pincushion distortion (typically :math:`k_1 < 0`).
+
+.. image:: pics/distortion_examples.png
+    :align: center
+
+In the functions below the coefficients are passed or returned as
 
 .. math::