11/4/2004 Tone Reproduction Luminance levels A media based - - PDF document

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A media based framework for color and tone reproduction in Computer Graphics

Joe Geigel

Department of Computer Science Rochester Institute of Technology jmg@cs.rit.edu

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Tone Reproduction

• Luminance levels

Sky = 12400 nits Trees = 64 nits

Range of lighting in real world Range of lighting in display environment

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High Dynamic Range (HDR) I maging

• high dynamic range imaging is a set of

techniques that allow a far greater dynamic range of exposures than normal digital imaging techniques.

• The intention is to accurately represent the

wide range of intensity levels found in real scenes, ranging from direct sunlight to the deepest shadows.

Wikipedia

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I mage Synthesis in Computer Graphics camera synthetic image camera model

processing

photo processing tone reproduction real scene 3D models Photographic print Photography: Computer Graphics:

Reinterpretation of scene

• ptimized for viewing

(24 bit RGB) Scene luminance

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• Simple Linear tone reproduction

Light source = firefly Light source = Searchlight

[Tumblin 1993]

The need for Tone Reproduction

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Tone Reproduction in Computer Graphics

[Ferwerda 1998]

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Approaches to Tone Reproduction in CG

• Perceptual Based Operators

– Based on Perceptual Studies and Models of Human Perception – Goal: Tone Mapping to reproduce how humans perceive scene

• Media Based Operator

– Observation 1: Tone Reproduction problem is not unique to CG

• Over a century of advanced study
• Scientists and artists have devised processes and techniques
• Techniques optimized for particular display devices

– Observation 2: We may not always want what we see!

• Artistic photography (Ansel Adams)
• Compositing with content on existing media
• Parameter space of practioners

– Our Goal:

• Achieve tone reproduction via simulation of existing processes
• Perception not ignored, rather already considered

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Media Based Tone Reproduction As viewed

• n a

given piece of media Simulated response of that media

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Media Based Tone Reproduction

• Simulation

– Model and simulate response of light entering the system

• Capture (Camera)
• Media Response

– Brightness – Spectral – Acuity – Noise / Grain

• Media Presentation
• Color Management

– Definition: A system for ensuring that colors remain the same regardless of the device or medium used to display the colors. – Model the device on which the image will be displayed

• Brightness Capabilities
• Color Gamut
• Viewing Environment

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color management simulation Media Based Tone Reproduction Framework

Camera Media Presentation Device

Image Appearance

HDR Image Output device

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Example: Print Photography

• Follow the path of light from scene to photo to viewer!

scene camera film enlarger print viewer

Radiance / luminance exposure Radiance / luminance exposure

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Example: Print Photography

• Print photography process

Camera Film

Process Process Negative

Print Paper Printer

Optics Photographic Material Processed Photographic Material

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Photographic Units

exposure

E = It

I = Illuminance (lux) t = time (sec) E = exposure (lux-sec) density

D = log (O)

O = opacity = 1 / T T = transmission = It / Io It = transmitted light Io = incident light

defines the amount of light hitting the photographic material at each point A logarithmic means for describing transparency or reflectance once the material is developed

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color management simulation Media Based Tone Reproduction Framework

Camera Media Presentation

Device Image Appearance

HDR Image Output device

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set by photographer

Camera Model

Scene luminance to exposure

time shutter eff. flare scene from luminance

4 2

) cos 4 ( ) ( t I n L x Exposure

f

η θ π τ + = ′

Lens transmittance

Aperture / f-stop

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color management simulation Media Based Tone Reproduction Framework

Camera Media Presentation

Device Image Appearance

HDR Image Output device

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Photographic Response: Brightness

• A typical brightness response / characteristic curve

Log Exposure D e n s i t y

I II III IV I - toe II - straight line section III - shoulder IV - area of solarization

γ

gamma - slope of region

• II. gives contrast range

speed - indicates sensitivity to light S

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Photographic Response: Brightness

Original 100 Speed Film 400 Speed Film 800 Speed Film Original Low Contrast Medium Contrast High Contrast

Effects of speed Effects of gamma

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Photographic Response: Spectral

100 200

panchromatic

100

300 400 500 600

blue sensitive

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Photographic Response: Acuity

modulation transfer function point spread function

20 40 60 80 100 40 80 120 spatial freq. (cycles/mm) (%)

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Photographic Response: Grain Graininess increases with magnification No magnification 2X zoom 4X zoom

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Modeling Photographic Response

exposure density conversion

expose spectral sensitivity resolution density response granularity convert to transmission/ reflection input image

negative

• r

print

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Color Photography

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color management simulation Media Based Tone Reproduction Framework

Camera Media Presentation

Device Image Appearance

HDR Image Output device

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Photography: Presentation

• Photographic Model provides:

– Reflective densities for each emulsion layer – Spectral qualities of dyes on each layer

• Image isn’t viewed unless illuminated

– Illuminant

• Brightness
• Spectral Qualities

– Viewing Environment

• Brightness
• Spectral Qualities
• After presentation, we have specification of

– Image as viewed by a given light in a given environment – Must drive display device to match perceptual response

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color management simulation Media Based Tone Reproduction Framework

Camera Media Presentation

Device Image Appearance

HDR Image Output device

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Device Modeling

• Device Characterization and Calibration

– Non-linear response (gamma) – Device independent color specification – Gamut mapping

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color management simulation Media Based Tone Reproduction Framework

Camera Media Presentation Device

Image Appearance

HDR Image Output device

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Viewing Environment

• Viewing environment can affect image perception
• Adaptation
• The process by which the visual mechanism adjusts to the

conditions under which the eyes are exposed to radiant energy.

– General brightness adaptation

• Adjustments in response to the overall level of stimulus exposed

– Lateral brightness adaptation

• Adjustments in response due to stimulus in adjacent areas of the

retina

– Chromatic adaptation

• Adjustments in response to the average chromaticity in the

stimulus.

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General Brightness Adaptation

• Note also…differences in acuity

[Ferwerda 1996]

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Lateral Brightness / Chromatic Adaptation Chromatic Adaptation Applet Lateral Brightness (Surround Effect)

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Color and I mage Appearance Models

• Color Appearance Models

– Used to predict color appearance – Accounts for changes in viewing enviornment

• Color of illuminant
• Illumination level
• Surround luminance
• Image Appearance Models

– Incorporates spatial and temporal properties of human vision. – iCAM (Munsell Color Science Lab)

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color management simulation Media Based Tone Reproduction Framework

Camera Media Presentation Device

Image Appearance

HDR Image Output device

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Other Applications: High Dynamic Range I magery [Debevec 2001]

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Traditional and Digital Photography camera

processing

photo processing real scene Photographic print Traditional Photography:

Reinterpretation of scene optimized for viewing

camera Processing performed by camera real scene Digital image Digital Photography:

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HDR in Computer Graphics [Ward 2001]

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Rethinking Digital Photography

processing

Digital Photography:

Reinterpretation of scene

• ptimized for viewing

(24 bit RGB) Scene luminance

HDR Photography: camera Processing performed by camera real scene Digital image camera Processing performed as post process real scene Digital image

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Other Application: Film Archiving

• Goal: Present films as they were originally shown

color management simulation

Camera Media Presentation Device

Image Appearance

HDR Image Output device

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Conclusion

• Framework for Tone / Color Reproduction

– Based on Simulation of Media Response – Include Color Management

• Applications

– Computer Generated Images – High Dynamic Range Images – Presentation of Archival Images

• Questions?