Videos are generally represented on a limited number of bits (for instance 8, 10, 12 or more bits), corresponding to a limited range of values to represent the luminance signal. Videos represented in such way are called videos of low dynamic range or, shortly, LDR videos. However the human visual system is able to perceive a wider range of luminance. The limited representation most often does not allow for reconstructing correctly small signal variations, in particular in extremely dark or bright video image areas i.e. areas of high or low luminance. The HDR (High Dynamic Range) format consists in significantly extending bit-depth of signal representation to integer representation with more bits e.g. 20 to 64 bits, or even to floating representation, in order to keep a high accuracy of the signal on its entire luminance range.
HDR images or videos can be captured in various ways. For instance, Digital Single Lens Reflex cameras can use bracketing technique to capture successive images of the same scene with different exposures wherein exposure is the total density of light allowed to fall on the imaging medium (photographic film or image sensor) during the process of taking an image. Those images of different exposures are represented as LDR images. Under-exposed images capture details in the bright areas whereas over-exposed images capture details in the dark areas, as exemplarily depicted in FIG. 1 for different exposure values EV.
By fusion of these differently exposed LDR images an HDR image/video can be produced with a floating point representation, the produced HDR image/video containing all details those in dark areas as well as those in bright areas.
An HDR image/video cannot be used in its source format with devices designated for use with LDR images, e.g. set-top-boxes, PVR, and legacy displays. Anyway, a process called Tone Mapping, allows representing the image while ensuring a good restitution of the different signal intensity segments, in particular, in high and low intensity ranges. Tone Mapping creates, from an HDR image, a LDR image where all the elements are correctly exposed. The LDR image is much more detailed both in dark areas and in white areas. This is exemplarily depicted in FIG. 2.
HDR is used, in particular, in post-production. Most if not all special effects tools are dealing with HDR images with a floating point representation. The mixing being natural scene and special effects is also realized in HDR representation. At the end of the post-production process Tone Mapping is commonly applied to create a standard, e.g. 8/10/12-bit, master under the control of the Director of Photography.
The Tone Mapping applied in post processing is commonly an unknown one.
In United States Patent Application 2008/0175494 it is described a method for predicting a high dynamic range image element, said method comprising: receiving low dynamic range image data; receiving high dynamic range image data comprising prediction data and an HDR residual image element; extracting an LDR image value from said LDR image data; modifying said LDR image value based on said prediction data; and combining said modified LDR image value with said HDR residual image element to form an HDR image element.