3D graphics eBook - Course Materials Repository
3D graphics eBook - Course Materials Repository 3D graphics eBook - Course Materials Repository
High dynamic range rendering 59 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 generally higher than the static range achievable by most display technology. Output to displays Although many manufacturers claim very high numbers, plasma displays, LCD displays, and CRT displays can only deliver a fraction of the contrast ratio found in the real world, and these are usually measured under ideal conditions. The simultaneous contrast of real content under normal viewing conditions is significantly lower [10] . Some increase in dynamic range in LCD monitors can be achieved by automatically reducing the backlight for dark scenes (LG calls it Digital Fine Contrast [11] , Samsung are quoting "dynamic contrast ratio"), or having an array of brighter and darker LED backlights (BrightSide Technologies – now part of Dolby [12] , and Samsung in development [13] ). Light bloom Light blooming is the result of scattering in the human lens, which our 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] Otherwise, HDR rendering systems have to map the full dynamic range to 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 help simulate these effects.
High dynamic range rendering 60 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 Currently HDRR has been prevalent in games, primarily for PCs, Microsoft's Xbox 360, and Sony's PlayStation 3. It has also been simulated on the PlayStation 2, GameCube, Xbox and Amiga systems. 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 desktop publishing and gaming, color values are often processed several times over. As this includes multiplication and division (which can accumulate rounding errors), it is useful to have the extended accuracy and range of 16 bit integer or 16 bit floating point formats. This is useful irrespective of the aforementioned limitations in some hardware. Development of HDRR through DirectX Complex 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: 1. Lighting precision was confined to 8 bit integers, which limited the contrast ratio to 256:1. Using the HVS color model, the value (V), or brightness of a color has a range of 0 – 255. This means the brightest white (a value of 255) is only 256 levels brighter than the darkest shade above pure black (i.e.: value of 0). 2. Lighting calculations were integer based, which didn't offer as much accuracy because the real world is not confined to whole numbers. Before HDRR was fully developed and implemented, games may have attempted to enhance the contrast of a scene by exaggerating the final render's contrast (as seen in Need For Speed: Underground 2's "Enhanced contrast" setting) or using some other color correction method (such as in certain scenes in Metal Gear Solid 3: Snake Eater). On December 24, 2002, Microsoft released a new version of DirectX. DirectX 9.0 introduced Shader Model 2.0, which offered one of the necessary components to enable rendering of high dynamic range images: lighting precision was not limited to just 8-bits. Although 8-bits was the minimum in applications, programmers could choose up to a maximum of 24 bits for lighting precision. However, all calculations were still integer-based. One of the first graphics cards to support DirectX 9.0 natively was ATI's Radeon 9700, though the effect wasn't programmed into games for years afterwards. On August 23, 2003, Microsoft updated DirectX to DirectX 9.0b, which enabled the Pixel Shader 2.x (Extended) profile for ATI's Radeon X series and NVIDIA's 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 high level shader language (HLSL). Shader Model 3.0's lighting precision has a minimum of 32 bits as opposed to 2.0's 8-bit minimum. Also all lighting-precision calculations are now floating-point based. 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
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High dynamic range rendering 59<br />
Examples<br />
One of the primary advantages of HDR rendering is that details in a scene with a large contrast ratio are preserved.<br />
Without HDR, areas that are too dark are clipped to black and areas that are too bright are clipped to white. These are<br />
represented by the hardware as a floating point value of 0.0 and 1.0 for pure black and pure white, respectively.<br />
Another aspect of HDR rendering is the addition of perceptual cues which increase apparent brightness. HDR<br />
rendering also affects how light is preserved in optical phenomena such as reflections and refractions, as well as<br />
transparent materials such as glass. In LDR rendering, very bright light sources in a scene (such as the sun) are<br />
capped at 1.0. When this light is reflected the result must then be less than or equal to 1.0. However, in HDR<br />
rendering, very bright light sources can exceed the 1.0 brightness to simulate their actual values. This allows<br />
reflections off surfaces to maintain realistic brightness for bright light sources.<br />
Limitations and compensations<br />
Human eye<br />
The human eye can perceive scenes with a very high dynamic contrast ratio, around 1,000,000:1. Adaptation is<br />
achieved in part through adjustments of the iris and slow chemical changes, which take some time (e.g. the delay in<br />
being able to see when switching from bright lighting to pitch darkness). At any given time, the eye's static range is<br />
smaller, around 10,000:1. However, this is still generally higher than the static range achievable by most display<br />
technology.<br />
Output to displays<br />
Although many manufacturers claim very high numbers, plasma displays, LCD displays, and CRT displays can only<br />
deliver a fraction of the contrast ratio found in the real world, and these are usually measured under ideal conditions.<br />
The simultaneous contrast of real content under normal viewing conditions is significantly lower [10] .<br />
Some increase in dynamic range in LCD monitors can be achieved by automatically reducing the backlight for dark<br />
scenes (LG calls it Digital Fine Contrast [11] , Samsung are quoting "dynamic contrast ratio"), or having an array of<br />
brighter and darker LED backlights (BrightSide Technologies – now part of Dolby [12] , and Samsung in<br />
development [13] ).<br />
Light bloom<br />
Light blooming is the result of scattering in the human lens, which our brain interprets as a bright spot in a scene. For<br />
example, a bright light in the background will appear to bleed over onto objects in the foreground. This can be used<br />
to create an illusion to make the bright spot appear to be brighter than it really is. [5]<br />
Flare<br />
Flare is the diffraction of light in the human lens, resulting in "rays" of light emanating from small light sources, and<br />
can also result in some chromatic effects. It is most visible on point light sources because of their small visual<br />
angle. [5]<br />
Otherwise, HDR rendering systems have to map the full dynamic range to what the eye would see in the rendered<br />
situation onto the capabilities of the device. This tone mapping is done relative to what the virtual scene camera sees,<br />
combined with several full screen effects, e.g. to simulate dust in the air which is lit by direct sunlight in a dark<br />
cavern, or the scattering in the eye.<br />
Tone mapping and blooming shaders, can be used together help simulate these effects.