GammaLib

An important and time consuming part of the cube response computation is the determination of the distance between the model centre and the observed event direction. The relevant code is shown below:

double GCTAResponseCube::irf_ptsrc(const GEvent&       event,
                                   const GSource&      source,
                                   const GObservation& obs) const
{
    ...
    double delta = bin->dir().dir().dist(srcDir);
    ...
}
double GCTAResponseCube::irf_radial(const GEvent&       event,
                                    const GSource&      source,
                                    const GObservation& obs) const
{
    ...
    double rho_obs = model->dir().dist(obsDir);
    ...
}
double GCTAResponseCube::irf_elliptical(const GEvent&       event,
                                        const GSource&      source,
                                        const GObservation& obs) const
{
    ...
    double rho_obs      = model->dir().dist(obsDir);
    double posangle_obs = model->dir().posang(obsDir); // Celestial
    ...
}

Since for a cube the observed event direction is known a priori (the spatial cube pixels), the distances can be cached for each model. The cache would be a sky map for each model, with the sky map definition defined by the event cube. The sky map can be preset with negative values (for example -1), signaling that the distance was not yet computed.