A method for merging histograms may include generating a histogram for a region of an image, the histogram including bucket values representing a count of pixels having the same pixel value or a weighting dependent on the pixels. The method may include maintaining an array of values indicating non-zero histogram entries for a group of bucket values (e.g., a count of non-zero bucket values in the group or a bitmask indicating if each bucket value is non-zero). A sparse histogram for which such an array exists may be merged with a second histogram. Merging the histograms may include not merging any bucket values in the group if the associated array value is zero, and merging some or all of the bucket values if it is non-zero. The methods disclosed may be implemented by program instructions executing in parallel on CPU(s) or GPUs.