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 to a high degree, 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, wherein the degree to which the low dynamic range image data satisfies a local contrast constraint is based on an error function that represents a distance of a candidate solution from the local contrast constraint in a solution space.