High-dynamic-range rendering
High-dynamic-range rendering (HDRR or HDR rendering), also known as high-dynamic-range lighting, is the rendering of computer graphics scenes by using lighting calculations done in high dynamic range (HDR). This allows preservation of details that may be lost due to limiting contrast ratios. Video games and computer-generated movies and special effects benefit from this as it creates more realistic scenes than with more simplistic lighting models.
Graphics processor company Nvidia summarizes the motivation for HDR in three points: bright things can be really bright, dark things can be really dark, and details can be seen in both.[1]
History
The use of
In 1990, Nakame, et al., presented a lighting model for driving simulators that highlighted the need for high-dynamic-range processing in realistic simulations.[4]
In 1995, Greg Spencer presented Physically-based glare effects for digital images at SIGGRAPH, providing a quantitative model for flare and blooming in the human eye.[5]
In 1997, Paul Debevec presented Recovering high dynamic range radiance maps from photographs[6] at SIGGRAPH, and the following year presented Rendering synthetic objects into real scenes.[7] These two papers laid the framework for creating HDR light probes of a location, and then using this probe to light a rendered scene.
HDRI and HDRL (high-dynamic-range image-based lighting) have, ever since, been used in many situations in 3D scenes in which inserting a 3D object into a real environment requires the light probe data to provide realistic lighting solutions.
In gaming applications,
Examples
One of the primary advantages of HDR rendering is that details in a scene with a large contrast ratio are preserved. Without HDR, areas that are too dark are clipped to black and areas that are too bright are clipped to white. These are represented by the hardware as a floating point value of 0.0 and 1.0 for pure black and pure white, respectively.
Another aspect of HDR rendering is the addition of perceptual cues which increase apparent brightness. HDR rendering also affects how light is preserved in optical phenomena such as reflections and refractions, as well as transparent materials such as glass. In LDR rendering, very bright light sources in a scene (such as the sun) are capped at 1.0. When this light is reflected the result must then be less than or equal to 1.0. However, in HDR rendering, very bright light sources can exceed the 1.0 brightness to simulate their actual values. This allows reflections off surfaces to maintain realistic brightness for bright light sources.
Limitations and compensations
Human eye
The human eye can perceive scenes with a very high dynamic contrast ratio, around 1,000,000:1. Adaptation is achieved in part through adjustments of the iris and slow chemical changes, which take some time (e.g. the delay in being able to see when switching from bright lighting to pitch darkness). At any given time, the eye's static range is smaller, around 10,000:1. However, this is still higher than the static range of most display technology.[citation needed]
Output to displays
Although many manufacturers claim very high numbers,
Some increase in dynamic range in LCD monitors can be achieved by automatically reducing the backlight for dark scenes. For example, LG calls this technology "Digital Fine Contrast";[10] Samsung describes it as "dynamic contrast ratio". Another technique is to have an array of brighter and darker LED backlights, for example with systems developed by BrightSide Technologies.[11]
Light bloom
Light blooming is the result of scattering in the human lens, which human brain interprets as a bright spot in a scene. For example, a bright light in the background will appear to bleed over onto objects in the foreground. This can be used to create an illusion to make the bright spot appear to be brighter than it really is.[5]
Flare
Flare is the diffraction of light in the human lens, resulting in "rays" of light emanating from small light sources, and can also result in some chromatic effects. It is most visible on point light sources because of their small visual angle.[5]
Typical display devices cannot display light as bright as the Sun, and ambient room lighting prevents them from displaying true black. Thus HDR rendering systems have to map the full dynamic range of what the eye would see in the rendered situation onto the capabilities of the device. This tone mapping is done relative to what the virtual scene camera sees, combined with several full screen effects, e.g. to simulate dust in the air which is lit by direct sunlight in a dark cavern, or the scattering in the eye.
Tone mapping and blooming shaders can be used together to help simulate these effects.
Tone mapping
Tone mapping, in the context of graphics rendering, is a technique used to map colors from high dynamic range (in which lighting calculations are performed) to a lower dynamic range that matches the capabilities of the desired display device. Typically, the mapping is non-linear – it preserves enough range for dark colors and gradually limits the dynamic range for bright colors. This technique often produces visually appealing images with good overall detail and contrast. Various tone mapping operators exist, ranging from simple real-time methods used in computer games to more sophisticated techniques that attempt to imitate the perceptual response of the human visual system.
Applications in computer entertainment
 | This section's Sproing Interactive Media has announced that their new Athena game engine for the Wii will support HDRR, adding Wii to the list of systems that support it.
In floating point formats. This is useful irrespective of the aforementioned limitations in some hardware.
Development of HDRR through DirectXComplex shader effects began their days with the release of Shader Model 1.0 with DirectX 8. Shader Model 1.0 illuminated 3D worlds with what is called standard lighting. Standard lighting, however, had two problems:
On December 24, 2002, GeForce FX series of graphics processing units.
On August 9, 2004, Microsoft updated DirectX once more to DirectX 9.0c. This also exposed the Shader Model 3.0 profile for NVIDIA states that contrast ratios using Shader Model 3.0 can be as high as 65535:1 using 32-bit lighting precision. At first, HDRR was only possible on video cards capable of Shader-Model-3.0 effects, but software developers soon added compatibility for Shader Model 2.0. As a side note, when referred to as Shader Model 3.0 HDR, HDRR is really done by FP16 blending. FP16 blending is not part of Shader Model 3.0, but is supported mostly by cards also capable of Shader Model 3.0 (exceptions include the GeForce 6200 series). FP16 blending can be used as a faster way to render HDR in video games.
Shader Model 4.0 is a feature of DirectX 10, which has been released with Windows Vista. Shader Model 4.0 allows 128-bit HDR rendering, as opposed to 64-bit HDR in Shader Model 3.0 (although this is theoretically possible under Shader Model 3.0). Shader Model 5.0 is a feature of DirectX 11. It allows 6:1 compression of HDR textures without noticeable loss, which is prevalent on previous versions of DirectX HDR texture compression techniques. Development of HDRR through OpenGLIt is possible to develop HDRR through GLSL shader starting from OpenGL 1.4 onwards.
Game engines that support HDR rendering
See alsoReferences
External links
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