HDR Comes to Replace RGB
Where is the way out then?
As far as the computer monitor is concerned, there is hardly anything you can do about it: you cannot increase the screen brightness up to the level of Sun brightness.
But if there is nothing we could do about the monitor then why don’t we give up the RGB model, especially since it can be done absolutely painlessly. Let’s describe the images with real physical values of light intensity and color, and the let the monitor display all it can, as it will hardly be worse anyway. :) This is exactly the idea behind HDRI: for pixels of the image we set the intensity and color in real physical values or values linearly proportional to them. Of course, all real (and fake) lighting parameters are now described with real numbers and not integers, so that we will not be able to cope with 8 bits per channel. This approach immediately eliminates all limitations imposed by the RGB model: the dynamic image range is not limited at all theoretically. This way the question about discreetness and the number of brightness gradations is no longer acute, and the problem of insufficient color coverage is also solved.
We could state that the introduction of HDRI for the first time allowed separating and making independent the description, as a numeric representation of the image within the HDRI model, and the presentation of this description on any technical display device, such as a PC monitor, ink-jet or photo-printer. This way, the image presentation and display turned into two independent processes, while HDRI description became hardware independent.
The display of HDR image on the monitor or its printout requires transforming the dynamic range and HDRI color range into the dynamic and color range of the output device: RGB for monitors, CMYK for printers, CIE Lab, Kodak CYY and the like. Since all these models are LDRI (Low Dynamic Range Images), this transformation cannot be performed painlessly. This process is known as tone mapping and it uses the peculiarities of the human eye to re3duce the losses during this transformation. Since there is no math1ematical model describing the human eyesight and its mechanisms fully and correctly, there is no general tone mapping algorithm, which could always ensure quality outcome.
Let’s return to the numeric representation of the HDRI description. Infinite dynamic range is a good thing, but the computer cannot process the infinity. That is why in practice, the dynamic range is usually limited from the top and bottom. A good approximation of this limitation is human eye range, i.e. from 10^6 to 10^8. So, we get a dilemma here. On the one hand, the broader is the dynamic range, the better. On the other hand, we should spare some of the computer resources, because bigger range requires more data to describe this image then. In order to solve this problem, they developed a few formats of the HDR numeric image representation, which differ only by the available range and desired size.
NVIDIA NV40 Acquires HDR
NVIDIA uses a compromise variant, the 16-bit OpenEXR format developed by Industrial Light and Magic. The 16-bit OpenEXR description devotes one bit for the sign of the exponent, five bits to store the value of the exponent and ten bits to store the mantissas of the chromatic color coordinates (u, v), five bits per coordinate. The dynamic representation range thus stretches to 9 orders of magnitude: from 6.14*10-5 to 6.41*104.
The process of constructing and outputting a HDR image with the NV40 graphics processor is divided into three steps:
- Light Transport: rendering a scene with a high lighting dynamic range and saving the information about the light characteristics of each pixel in a buffer that uses the OpenEXR floating-point data format. NVIDIA stresses the fact that the NV40 supports floating-point data representation on each step of creation of a HDR scene, ensuring the minimum quality loss:
- floating-point calculations in shaders;
- floating-point texture filtering;
- operations with buffers that use a floating-point data format.
So, the NV40 with its HPDR technology makes high-dynamic-range images available for admirers of NVIDIA products, not only to owners оf RADEONs. This is another step to bringing photorealistic graphics into computer games.