Tone Reproduction Definition: Compressing the dynamic Photographic Tone Reproduction range of a scene’s luminances/radiances so that it can be displayed on a given device in such a way that minimizes the perceptual difference between viewing the scene and viewing the rendering of the scene. Photographic Response Photographic Response An alternative to modeling visual response Why bother with photographic model? directly. Far better understood than human visual Instead, models response to photographic system. materials (film/paper). Optimized for human viewing Artistic photography Composition of CG elements with scenes captured on film. Photographic Pipeline Lighting Units Follow the path of light from scene to photo Units: to viewer! Radiance – light hitting a surface from a given direction (light traveling along a ray) Luminance – photometric equivalent of radiance enlarger film (radiance scaled by luminous efficiency curve) Irradiance – light hitting a surface from all directions Illuminance – photometric equivalent of irradiance (irradiance scaled by luminous efficiency curve) print viewer camera scene 1
Photographic Units Photographic Units Exposure exposure density E = It D = log (O) Essentially defines the amount of light hitting the photographic material at each I = Illuminance (lux) O = opacity = 1 / T point t = time (sec) T = transmission Density E = exposure (lux-sec) = I t / I o A logarithmic means for describing I t = transmitted light transparency once the material is I o = incident light developed Photographic Response Step 1: Calculate exposure Print photography process Follow the path of light from scene to photo to viewer! Radiance / luminance Film Camera Process Negative enlarger film Print Printer Paper Process print viewer camera Processed scene Photographic Optics Photographic Material Radiance / luminance exposure exposure [Geigel97] Material Luminance to exposure Luminance to exposure To get irradiance at a given point on the film plane, we must Things to consider when figuring out integrate radiance values over a circle representing the exit exposure. pupil. Irradiance from scene radiance Vignetting Transmittance (formerly called transmission) Flare Shutter efficiency A bit more than the basic pinhole camera! 2
Luminance to exposure Step 2: Simulate film response Final model We now know how much exposure is present on each point in our film plane: viewer π enlarger film 4 ′ = τ θ + η Exposure ( x ) ( L cos I ) t f 2 4 n illuminanc e from scene flare shutter time eff. print camera scene set by photographer Photographic Materials Photographic Response Comprised of microscopic grains of Brightness Response - high level response of an emulsion to light silver halide in a gelatin (emulsion) Spectral Sensitivity - Response of a material Latent image formed when exposed to to different wavelengths of light light Acuity - Level at which material can Silver halide converted to metallic silver reproduce spatial details during processing. Graininess - Observed variation due to grain Converted silver results in opacity 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 IV section t i s III - shoulder sensitometric measure. n II e γ IV - area of D solarization I γ - gamma Log Exposure [Geigel97] 3
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 Spectral Response for Three Types of Film Photographic Response - Gamma 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: 2 2 = 1 G = (2A) σ N Σ ( Δ D i ) σ Δ D i = deviation of sample A = area of scanning i from the mean Original Panchromatic Blue Sensitive aperture Indication of sample uniformity Measure of granularity [Geigel97] 4
Photographic Response - Grain Photographic Response – Acuity (Resolution) point spread function modulation transfer function 100 80 60 (%) 40 20 0 0 40 80 120 spatial freq. (cycles/mm) [Geigel97] Photographic Response Photographic Response - Acuity High level description of photographic response Model can process at grain level, but impractical to do so. All sensiometic measurements are available Without MTF With MTF With MTF & Grain for photo materials from the manufacturer. [Geigel97] Modeling Photographic Response Modeling Photographic Response input image Uses sensitometric measures to model characteristics of photo materials spectral expose resolution sensitivity Physically based exposure Built using an imaging pipeline where each module in the pipe represents an convert to density image processing operation. granularity transmission/ response reflection negative density conversion or print [Geigel97] 5
Okay, where are we? Step 3: Create the print We now know how transparent our negative is To create the print: at each point in our film plane: Negative is placed in an enlarger viewer Light is shown through the negative onto enlarger film photographic paper (which contains an emulsion) Paper is exposed and then developed print Note that the enlarger has its own lens camera scene system. Photographic Units Modeling Photographic Response Exposure Must run thru pipeline twice, once for Essentially defines the amount of light hitting the capture on film and once for printing photographic material at each point Result of model Density Image of floats [0, 1] A logarithmic means for describing reflection once the material is developed Represents transmission or reflection values For photographic paper, reflective density is calculated. Reflective density = fraction of light that goes through the emulsion on the paper, hits the paper base and reflects back to the viewer. Step 4: View the print Modeling Photographic Response Prints are reflective media Follow the path of light from scene to photo to viewer! Are not visible unless illuminated Values from model must be modified to enlarger film account for the luminance / color characteristic of the assumed print illumination print viewer camera scene 6
Modeling Photographic Response Modeling Photographic Response Virtual Darkroom Applet Some nice factoids http://www.jogle.com/Research/vdr/java/vdr.html Photographic engineers have spent an awful lot of time and energy in designing films and papers to assure, to the best of their power: A photo viewed using “normal” or “typical” lighting will be a nice perceptual match with the scene photographed. The luminance range of CRTs approximates normal interior viewing conditions fairly well. Scaling reflectances to CRT luminaces produces a decent picture Issues with Tone Reproduction Issues with Tone Reproduction Tone, not color Tone, not color Viewing /display conditions generally Most tone reproduction operators are applied equally to RGB. not considered Not necessarily the way to gain best Real time tone reproduction results. As an example, look at color film. Photographic Response Photographic Response So what about color? Additive Color (light) Color Materials have multiple emulsion Primaries (red, green, blue) layers, each sensitive to a certain range (red, green, blue) of wavelength. 7
Photographic Response Photographic Response Note that in additive system: Subtractive color (dyes) White - Red = Cyan primaries (magenta, cyan, yellow) White - Green = Magenta White - Blue = Yellow In subtractive system Cyan dye absorbs red light Magenta dye absorbs green light Yellow dye absorbs blue light Photographic Response Photographic Response Color Materials Color Brightness Response Each layer has Each emulsion layer will have its own it’s own characteristic curve spectral sensitivity Photographic Pipeline (back in the day) Photographic Response Color Grain and Acutity Follow the path of light from scene to photo to viewer! Each layer will have its own MTF and grain characteristics. enlarger film Applying same TR to each color channel may not be the best approach. print Questions. viewer camera scene 8
Recommend
More recommend
