Realistic colorization via the structure tensor

Colorization is a user-assisted color manipulation mechanism for changing grayscale images into colored ones. Several colorization algorithms have been constructed, and these methods are able to produce appropriately colorized images given a surprisingly sparse set of hints supplied by the user. But these color images may not in fact look realistic. Moreover, the contrast in the colorized image may not match the gradient perceived in the original grayscale image. We argue that it is this departure from the original gradient that contributes to the un-real appearance in some colorizations. To correct this, we make use of the Di Zenzo gradient of a color image derived from the structure tensor, and adjust the colorized correlate such that the Di Zenzo definition of the maximum-contrast gradient agrees with the gradient in the original gray image. This tends to result in more natural-looking images in color. In particular, "hotspots" of un-realistic color are subdued into regions of more realistic color.