In HDR image creation, multiple images are taken at different exposures and then blended in order to have an image with a dynamic range higher than the dynamic range of a single image. Within this blending procedure those images with smaller exposures, which contain more information in bright areas, improve the dynamic range within these bright areas of the image, and other images with higher exposure improve the dynamic range within dark areas. Therefore by blending these images of different exposure the dynamic range of the resulting image can be increased.
However, an image sequence taken with a handheld camera contains global and local motion, wherein global motion derives e.g. from movement of the camera and local motion derives e.g. from movement of objects to be imaged. Thus, for HDR image creation those images should be aligned with each other before blending them. Hereinafter the term luminance is used equivalently to the term radiance.
It is a problem that state of the art methods for motion estimation cannot align images with different exposures efficiently, because there is no linear change in luminance between images taken with different exposures due to non-linear post processing inside the camera after an image is captured.
Conventional motion estimation does not work with non-linear luminance change, since state of the art techniques crop the images, which results in the loss of information and in artifacts, which appear in the aligned images due to motion when applying conventional techniques. Therefore, when using state of the art techniques it is not possible to preserve the dynamic range of an image in all image regions.
In conventional block based motion estimation, a block in the current image is matched with the neighboring blocks in a reference image and the block which has for example least mean absolute difference (MAD) is taken as a predicted block.
Also a method of fading images, in which a global DC of each image is subtracted from each pixel value in calculating the mean absolute difference does not work for images having different exposure because the change in exposure is luminance dependent. Therefore both the conventional method and the method for fading images do not work for the images having different exposures because the change in exposure is luminance dependent.
It has also not been possible so far to generate different exposure images from a single image or to generate different exposure frames from a single frame in HDR video.
Therefore it is an object of the invention to provide a method and unit for improving the generation of high dynamic range image based on a reference among different exposure images so that aligned images of different exposure are created for use in HDR imaging.