# PDX and PDY

The PDX and PDY terms describe the perspective distortion in the X-axis and Y-axis directions. Positive values for PDX causes the plane to recede with increasing values in the unmapped X-axis direction while negative values cause the plane to recede with decreasing values in the unmapped X-axis direction, as shown in the following figure:

Positive values for PDY causes the plane to recede with increasing values in the unmapped Y-axis direction while negative values cause the plane to recede with decreasing values in the unmapped Y-axis direction, as shown in the following figure:

When evaluating the transformation produced by the Checkerboard Calibration tool, you can use the computed PDX and PDY terms to measure the degree of perspective shift between your camera and calibration plate.