why cameracalibrator.py can be used for depth camera?

[pyni]

Hi, i found you use following to calibrate the ir camera:
rosrun camera_calibration cameracalibrator.py --size 7x6 --square 0.0256 image:=/<camera_name>/ir/image camera:=/<camera_name>/ir

But cameracalibrator.py is for RGB camera. In depth image, no corners can be extracted.
I cannot understand this.