Tutorial 3 intro: Underwater Camera Calibration with the Pinax model: a new camera model allowing accurate in-air calibration of cameras in flat pane underwater housings
A new calibration process for underwater cameras is presented that is very easy and convenient to use while yielding very accurate results. It is based on the novel pinax model that allows in-air calibration of underwater cameras. The focus is on a hands-on usage of the model and related software for calibration. The pinax refraction correction is derived from an analysis of the axial camera model for underwater cameras, which is among others computationally hard to tackle. But by exploiting realistic constraints on the distance of the camera to the window, aspects of a virtual pinhole model with the projection function from the axial camera model can be combined. This allows the pre-computation of a lookup-table for very fast refraction correction of the flat-pane with high accuracy. The model takes the refraction indices of water into account, especially with respect to salinity, and it is therefore sufficient to calibrate the underwater camera only once in air. It can be demonstrated by experiments in different salt and sweet water conditions that the new pinax model outperforms standard methods. Among others, it can be shown that the pinax model leads to accurate results with single in-air calibration and even with just estimated salinity values. The pinax model is introduced and explained in detail in the following article (available with open accesss): T. Łuczyński, M. Pfingsthorn and A. Birk. The Pinax-Model for Accurate and Efficient Refraction Correction of Underwater Cameras in Flat-Pane Housings. Ocean Engineering, Vol. 133, pp. 9-22, March 2017