1. A computer implemented method for determining low dynamic range image data from high dynamic range image data, the method comprising: receiving or accessing the high dynamic range image data; and determining the low dynamic range image data based on the high dynamic range image data by optimising a degree to which the low dynamic range image data satisfies a local contrast constraint and a global consistency constraint, wherein the local contrast constraint is based on a local contrast in a perception space; the global consistency constraint is based on a relationship between points in the high dynamic range image data.