1. A computer-implemented process for tone mapping a sub-image of a wide angle, high dynamic range (HDR) image for display, comprising using a computer to perform the following process actions: computing, in advance of rendering the sub-image, a global tone map of the image; and using the global tone map during rendering of the sub-image to compute a local tone map for the sub-image based on an average luminance and a contrast of the pixels of the sub-image, said local tone map computation comprising, computing an initial luminance histogram for the pixels of the sub-image using luminance values assigned to the corresponding pixels in the global tone map, computing an initial average luminance for the sub-image from the initial luminance histogram, computing an initial luminance range for the sub-image from the initial luminance histogram, biasing the initial luminance histogram by shifting the average luminance towards middle gray using a prescribed luminance shifting function to produce a biased average luminance, wherein middle gray is defined as a luminance value falling mid-way between a largest possible luminance value and a smallest possible luminance value, stretching the shifted luminance histogram by increasing the initial luminance range to produce a stretched luminance range, if possible, thereby enhancing the contrast between the pixels, and establishing a local tone map for the sub-image by computing a new luminance value for each pixel of the sub-image using the biased average luminance and the stretched luminance range.