Assignments Checkpoint 6 Tone Reproduction Due Monday Another - PDF document

Assignments • Checkpoint 6 Tone Reproduction – Due Monday – Another extra…shadow ray • Checkpoint 7 – To be given Monday • RenderMan – Due February 16th Projects Logistics • Project feedback • Final Report • Approx 18 projects – Introduction • Listing of projects now on Web – Approach Taken • Presentation schedule – Implementation Details – Just Feb 16 th and Feb 21 st – Results – Feb 14 th – project preparation day – Appendix/Code • ALL PROJECTS HAVE BEEN SCHEDULED Computer Graphics as Virtual Photography Tone/Color Reproduction • Where are we? real camera photo Photographic Photography: scene (captures processing print – Described our scene during modeling light) – Simulated light transport during rendering processing – Captured and projected light from the scene onto a 2D plane during capture camera Computer 3D synthetic – Now we must convert this simulated light tone model Graphics: models reproduction image capture into an image for display (focuses simulated lighting) 1

Tone Reproduction Traditional Photography real photo Photographic • Luminance levels Photography: camera scene processing print processing Reinterpretation of scene optimized for Sky = 12400 nits viewing Trees = 64 nits Digital Photography Digital Photography real Processing Photography: camera scene performed • Issues Digital image by camera –Tone Reproduction is “hard coded” processing into camera –Color Management Issues Reinterpretation of • Which RGB? scene optimized for viewing • Optimized for what display? (24 bit RGB) Image Synthesis in CG Tone Reproduction real photo Photographic Definition: Compressing the dynamic range Photography: camera scene processing print of a scene’s luminances/radiances so that it can be displayed on a given device in such a processing way that minimizes the perceptual difference between viewing the scene and Computer 3D camera tone synthetic viewing the rendering of the scene. Graphics: models model reproduction image Reinterpretation of scene Scene luminance optimized for viewing (24 bit RGB) 2

Tone Reproduction - Definition What if we ignore tone Reproduction? • Simple Linear tone reproduction • Dealing with luminances / radiances • Rendering will be displayed on a given device • Minimize perceptual difference between real and created. Light source = Searchlight Light source = firefly [Tumblin93] Tone Reproduction Tone Reproduction • Radiance / Luminance • Using 0 – 1 to indicate light intensity – Flux arriving at or – What does 1 mean? leaving from a given • CG tends to use intensity space of output point or surface in a given direction . device – Radiance measured in W • Images optimized for a given output device. / m 2 /sr dA – Luminance measured in cd/m 2 (nit) Why Tone Reproduction? The Tone Reproduction Problem • What operator will create a close match between real- • Human response to light is neither simple world and display brightness sensation? nor linear. • Most display devices are not linear • Incorrect response modeling results in incorrect perception of results. [Tumblin93] 3

Tone / Color Reproduction Tone Reproduction in CG • Response / Observer – How does a system (like the human visual system or photography) respond to the collected light? • Display – How do we translate that response using a particular output device (like a CRT or printer)? [Ferwerda 1998] Response Models Response Models • Applying observer/response model will result • Image Characteristics in the luminances as seen by your display – Spectral response - how system responds to different wavelengths of light observer. – Intensity response - how system responds to – i.e., will be in luminance range of your output different intensities of light device. – Acuity - the sharpness of the image produced by • Observer/Response Models the system – Human Visual System – Noise – inherent noise in the image produced by the system – Photographic Systems Human Visual Response Human Visual Response • Pupil – Regulates the amount of light that gets to the retina • Photoreceptors – Rods • 75 - 150 million • sensitive to 10 -6 to 10 2 cd/m 2 (low light levels) • Achromatic (detects “brightness”) – Cones • 6 - 7 million • sensitive to 0.01 to 10 8 cd/m 2 (high light levels) • Responsible for color vision 4

Human Visual Response Human Visual Response • Levels of Brightness Response • Spectral response – Scotopic (Primarily rods) – Human Visual System is sensitive to light in the • 10 -6 to 10 2 cd/m 2 wavelength range of approx. 350 - 700 nm. – Photopic (Primarily cones) – Sensitivity changes dependent on illumination • 0.01 to 10 8 cd/m 2 level – Mesopic (overlap!) • 0.01 to 10 2 cd/m 2 • Both rods and cones • Little known -- active area of research Human Visual Response Human Visual System • Changes in Spectral Sensitivity • Acuity – Ability to resolve spatial detail • Snellen Chart – View from 20 ft away – Line 8 subtends 1 min of visual angle Scotopic Mesotopic Photopic – People who can read this is said to have 20/20 vision [Ferwerda96] [Ferwerda96] Human Visual System Human Visual System • Acuity also changes dependent on luminance • Response at different illumination levels level [Ferwerda96] [Ferwerda96] 5

Human Visual System Human Visual System • Adaptation • Threshold Studies – Our vision system has the ability to adapt to a – determine the threshold at which a person can given luminance level notice the change between a light sample given a certain background luminance. – Light Adaptation - from darkness to light – Dark Adaptation - from brightness to dark – Adaptation is gradual, not immediate (and is subject to age! � ) Human Visual System Human Visual System • Time course of light adaptation • Time course for light adaptation For rods For cones [Ferwerda96] [Ferwerda96] Human Visual System Human Visual System • Time course of dark adaptation • Time course of dark adaptation [Ferwerda96] [Ferwerda96] 6

Human Visual System Ward Tone Reproduction • Ferwerda’s model • Greg Ward offers a simpler – Scales luminances as to preserve perceived approach in Graphics Gems, IV contrast using psychophysical data as a guide. • L w = mL d – Different models for scotopic and photopic vision with slider to blend the two to simulate mesopic vision. • m will vary dependent upon whether scene is in scotopic, photopic, or mesopic range. “Normal” Linear Mapping Original Tumblin-Rushmeier operator • Based on “brightness”, a perceptual measure of how bright humans perceive light. [ Graphics Gems, IV ] Tumblin-Rushmeier Operator Ward Operator Results [ Graphics Gems, IV ] [ Graphics Gems, IV ] 7

Human Visual System Photographic Response • Print photography process • A good overview of CG tone reproduction operators is available from – “Tone Reproduction and Physically Based Spectral Rendering” by Devlin et al., State of the Art Report , EUROGRAPHICS 2002. Camera Film Process Negative • Note that Tone Reproduction operators are now starting to run in real time using GPU. Print Printer Paper Process • Questions? Break. Processed Photographic Optics Photographic Material [Geigel97] Material Photographic Materials Photographic Response • Illumination Response - high level response • Comprised of microscopic grains of silver of an emulsion to light halide in a gelatin (emulsion) • Spectral Sensitivity - Response of a material • Latent image formed when exposed to light to different wavelengths of light • Silver halide converted to metallic silver • Acuity - Level at which material can during processing. reproduce spatial details • Converted silver results in opacity • Graininess - Observed variation due to grain distribution Photographic Response Photographic Response • Sensitometry • A typical brightness response / characteristic curve – The science of measuring the sensitivity of III I - toe photographic materials II - straight line y – Each characteristic has its own unique t IV section i s III - shoulder n sensitometric measure. II γ e IV - area of D solarization I γ - gamma Log Exposure [Geigel97] 8

Photographic Response Photographic Response Effects of film Speed gamma - slope of region II speed - indicates gives contrast range sensitivity to light 3 3 2 2 Original 100 Speed Film γ 1 1 0 0 S -2 0 2 -2 0 2 800 Speed Film 400 Speed Film [Geigel97] Photographic Response Photographic Response - Gamma Spectral Response for Three Types of Film 100 panchromatic (Entire visible spectrum) 0 100 Original Low Contrast orthochromatic (Blue/Green sensitive) 0 100 blue sensitive (Untreated- blue/ultraviolet) 0 Medium Contrast High Contrast 300 400 500 600 [Geigel97] [Geigel97] Photographic Response Photographic Response - Grain Effects of Spectral Sensitivity Selwyn Granularity: rms deviation: N Σ ( ∆ D i ) 2 2 = 1 (2A) σ σ G = ∆ D i = deviation of sample A = area of scanning Original Panchromatic Blue Sensitive i from the mean aperture Indication of sample uniformity Measure of granularity [Geigel97] 9