What is HDR?HDR is high-dynamic-rangeimaging (HDRI or just HDR)
Why HDR?With HDR technology, we can have agreater dynamic range of luminancebetween the lightest and darkestareas of an image than currentstandard digital imaging techniquesor photographic methods (Wikipedia)
The world is high dynamic range Luminance Type of light (candela/m^2) Light from a star 0.001Light from the moon 0.1 Inside a house 100Light from the sun 100000
The Dynamic Range of an Image The Dynamic Range of an image is the capacity of this image to restitute the correct intensity scales we can observe in the real world We often express the dynamic using the “stops” unit
So how to reserve information?capturing the entire dynamic of thescene with a very limited sensor interms of dynamic rangeVarious techniques were invented toachieve this goalthe most used is to take severalpictures of the same scene usingdifferent exposure values
radiance E falling on sensor locationi and the exposure time Δt to producevalue C at pixel i Ci = f (Ei ∆t)
g the natural log of the invertible camera function Zmin and Zmax pixel value boundary N number of samples P number of photographs w a ‘hat’ weighting function (encourages the smoothness of g ) N P Zmax −1O= w(Cij )[g(Cij − ln Ei − ∆tj ]2 + λ w(z)g n (z) i j z=Zmin +1 minimize to find range of g
Once the matrices of coefficientshave been initialized, two lines ofMATLAB code solves this system ofequations using Gaussian elimination:X=Ab;g=X(1:256);
With g fully determined by a lookup table reconstruct the radiance map by combining the exposures, using a weighted average of the camera response function of the pixels in each exposure (N total pixels in image) P N j=1 i=1 w(Cij )[g(Cij ) − ln ∆tj ]ln Ei = P j=1 w(Cij )
HDR Images FormatAvailable high dynamic-range formats:-Radiance 32-bit RGBE and XYZE pictures-TIFF 48-bit integer and 96-bit floatformats-SGI 24-bit and 32-bit LogLuv TIFF-ILM OpenEXR format-JPEG-HDR formatProposals and extensions:-HDR extension to MPEG from MPI[Mantluk et al. 2004]-HDR extension to JPEG 2000 from UFL[Xu et al. 2005]-HDR texture compression[two paper at SIGGRAPH 2006]
HDR Displays 1Y Axis 0 -6 -4 -2 0 2 4 6 8 starlight moonlight indoor lighting sunlight X Axis Human Simultaneous Luminance Vision Range Today’s Devices
HDR Displays 1Y Axis 0 -6 -4 -2 0 2 4 6 8 starlight moonlight indoor lighting sunlight X Axis 5 orders of magnitude Human Simultaneous Luminance Vision Range Today’s Devices
HDR Displays 1Y Axis 0 -6 -4 -2 0 2 4 6 8 starlight moonlight indoor lighting sunlight X Axis 5 orders of magnitude Human Simultaneous Luminance Vision Range Today’s Devices 2-3 orders
Design principle for HDR Devices Requirements: 1. High Dynamic Range 2. Compatibility New devices need to function in 8-bit environment and still deliver significant benefit New devices need to be usable in stand- alone mode 3. Cost Ideally no extra cost compared to 8-bit devices If extra cost is necessary then in line with benefit
Display Technology Conventional LCDLCD backlight is provided by CCFLtubes (fluorescent light)Light is spread evenly behind LCDpanel and does not vary with imagecontentImage control is limited to 8 bitsingle to Red, Green and Blue colourchannels (255 steps of control)
Display Technology – HDR Display LCD backlight is provided by an array of LED’s Each LED is controlled with 8 bit (255 step) signal Brightness is adjusted to level demanded by source image LCD panel provides additional 8 bits of brightness control LED and LCD panel combine optically to deliver 16 bit performance LED’s provide greater brightness
*Dual Modulation*Low/high resolution andcorrection*Veiling luminance*Implementation indisplay and projection
8A<-2):+64200,5-)B7-6+,/.A HDR Image LED array LCD with correction Output image!"#$"#%%& #%%())*+,-./0,12)324.5676-,20)8549)):+6;+,2/<+=)>)?65@,125/,<7)) C%
Display Technology ReviewCompatibility*Based on commercially availablecomponents (LCD, LED)*Legacy support through Reverse ToneMapping and Saturation Extension*Small number of LEDs allows encoding ofLED data in conventional video signalCost*LED cost money (less every day)*Significant power reduction (~25% ofcomparable constant backlight LCD onaverage)
HDR devicesHDR Displays are availableHDR Projectors are comingHDR Output Devices can providebenefit to legacy contentHDR Software / Input Devices areleading the way
Tone MappingTone mapping reduces the dynamic range, orcontrast ratio, of the entire image, whileretaining localized contrast (between neighboringpixels), tapping into research on how the humaneye and visual cortex perceive a scene, trying torepresent the whole dynamic range while retainingrealistic color and contrast.Images with too much tone mapping processing havetheir range over-compressed, creating a surreallow-dynamic-range rendering of a high-dynamic-range scene.
tone-mapping is one way to representing HDRimages on LDR displays
TechniquesOnly a few fundamentally differentapproaches to tone reproduction exist-Based on image formation* Frequency domain* Gradient domain-Based on the human visual system* Global operators* Local operators
Rendering HDR Images Tone Mapping Image Based Lighting (IBL)
Image Based Lighting Image-based lighting (IBL) is a 3D rendering technique which involves plotting an image onto a dome or sphere that contains the primary subject. The lighting characteristics of the surrounding surface are then taken into account when rendering the scene, using the modeling techniques of global illumination. This is in contrast to light sources such as a computer-simulated sun or light bulb, which are more localized.
Rendering with Natural Light SIGGRAPH 98 Electronic Theater
HDR RenderingIn 3D computer graphics, high-dynamic-rangerendering (HDRR or HDR rendering), also known ashigh-dynamic-range lighting, is the rendering ofcomputer graphics scenes by using lightingcalculations done in a larger dynamic range. Thisallows preservation of details that may be lostdue to limiting contrast ratios. Video games andcomputer-generated movies and special effectsbenefit from this as it creates more realisticscenes than with the more simplistic lightingmodels used.Graphics processor company NVIDIA summarizes themotivation for HDRR in three points:1) bright things can be really bright2) dark things can be really dark3) details can be seen in both.
Applications in computer entertainmentCurrently HDRR has been prevalent ingames, primarily for PCs, MicrosoftsXbox 360, and Sonys PlayStation 3. Ithas also been simulated onthePlayStation 2, GameCube, Xbox andAmiga systems. Sproing Interactive Mediahas announced that their new Athena gameengine for the Wii will support HDRR,adding Wii to the list of systems thatsupport it.
GPUs that support HDRR the minimum requirement for HDR rendering is Shader Model 2.0 (or in this case DirectX 9), any graphics card that supports Shader Model 2.0 can do HDR rendering. However, HDRR may greatly impact the performance of the software using it if the device is not sufficiently powerful.