The shear models define a 2-dimensional map where the shear bias, presumably due to weak lensing, is defined at every point. The maps vary from completely artificial (a constant shear or single sheared region), to simple (a single point mass), to arbitrary (shear specifically defined in an input file).

The interface functions are used to calculate shear at some arbitrary point. One function returns the shear bias aligned with the x-y axis. The other returns the shear 45 deg to the x-y axis. Both take a unitless x and y dimension. Depending on the application, this coordinate can be either the coordinate in pixels or in arcsecs. The model considers everything in the same unitless space, so as long as the coordinate unites match those used in the model parameters, the coordinate systems will line up.

User interface definitions are given in the virtual ShearModel class, located in modelShear/ShearModel.h. These are:

double gamma1(double pos_x, double pos_y);	The value of gamma1 bias at the given coordinate. That is, the shear distortion aligned with x-y.
double gamma2(double pos_x, double pos_y);	The value of gamma2 bias at the given coordinate. That is, the shear distortion 45 deg to x-y.