  Near-infrared gray images captured together with corresponding visible color
images have recently proven useful for image restoration and classification.
This paper introduces a new coloring method to add colors to near-infrared gray
images based on a contrast-preserving mapping model. A naive coloring method
directly adds the colors from the visible color image to the near-infrared gray
image; however, this method results in an unrealistic image because of the
discrepancies in brightness and image structure between the captured
near-infrared gray image and the visible color image. To solve the discrepancy
problem, first we present a new contrast-preserving mapping model to create a
new near-infrared gray image with a similar appearance in the luminance plane
to the visible color image, while preserving the contrast and details of the
captured near-infrared gray image. Then based on the proposed
contrast-preserving mapping model, we develop a method to derive realistic
colors that can be added to the newly created near-infrared gray image.
Experimental results show that the proposed method can not only preserve the
local contrasts and details of the captured near-infrared gray image, but
transfers the realistic colors from the visible color image to the newly
created near-infrared gray image. Experimental results also show that the
proposed approach can be applied to near-infrared denoising.
