CMU CS 15463 - Light & Color (5 pages)

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Light & Color



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Light & Color

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Pages:
5
School:
Carnegie Mellon University
Course:
Cs 15463 - Computational Photography
Computational Photography Documents
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Light Color OUTLINE Physics Perception Color Reproduction Physics of Light and Color It all comes from electromagnetic EM radiation The amplitude of radiation is a function of wavelength Frequency 2 EM spectrum radio infrared visible ultraviolet x ray gamma ray long wavelength short wavelength low frequency high frequency Don t confuse EM spectrum with spectrum of signal in signal image processing they re usually conceptually distinct Likewise don t confuse spectral wavelength frequency of EM spectrum with spatial wavelength frequency in image processing Notes 14 Computer Graphics 2 15 463 1 Perception of Light and Color 1 Light is EM radiation that is visible to the human eye Wavelength range of visible band approx 400 700 nanometers We re most sensitive to greens yellows not very sens to blue Humans have evolved to sense the dominant wavelengths emitted by the sun space aliens have presumably evolved differently In low light we perceive primarily with rods in the retina which have broad spectral sensitivity at night you can t see color Rods have relatively good spatial resolution in the periphery as well as fovea center In bright light rods are overloaded we perceive primarily with the three sets of cones in the retina which have narrower spectral sensitivity roughly corresponding to red green and blue wavelengths Cones are concentrated mostly in the fovea so we have low spatial resolution in periphery Green cones most populous then red then blue hence our poor blue spatial sensitivity Perception of Light and Color 2 Some people are colorblind they have one or more sets of defective or poorly wired cones Other animals have fewer or more sets of cones 1 7 Facts from neurophysiology signals cones are combined in higher levels of processing in the retina and in the brain by neurons whose receptive fields point spread functions seem to come in several types spot detectors edge detectors and line detectors And each comes at a range of scales orientations and color channels The eye can focus various depths by changing the shape of the lens The eye has a huge dynamic range 10 orders of magnitude facilitated by eyelids pupil cone rod and electro chemical adaptive mechanisms The eye and brain are remarkably good at color adaptation perceiving the color of objects under light sources of various colors and brightnesses Perception is great at qualitative analysis poor at quantitative analysis Notes 14 Computer Graphics 2 15 463 2 What is Color Color is human perception of light Our perception is imperfect we don t see P instead we see three scalars long wavelength red L P sL d middle wavelength yellow M P sM d short wavelength blue S P sS d where sL sM and sS are the sensitivity curves of our three sets of cones Color is three dimensional because any light we see is indistinguishable to our eyes from some mixture of three spectral monochromatic single wavelength primaries Two spectral distributions that appear the same to a human even though their spectra may differ greatly are called metamers Additive vs Subtractive Color When working with light we use additive primaries often but not always red green and blue since light adds by the superposition property of electromagnetism When working with pigments ink or paint on paper we use subtractive primaries often but not always yellow magenta and cyan black since pigments filter out light The process is better described as multiplicative not subtractive With paint artists say yellow blue green but what s happening physically is spectrum that looks yellow spectrum that looks blue spectrum that looks green Notes 14 Computer Graphics 2 15 463 3 The Reproduction of Color In realistic image synthesis graphic design and other settings we want WYSIWYG color what you see on the screen is what you get in your output paper video film whatever But both during design producer and during display consumer Monitors differ in brightness color contrast Printers paper inks and video projectors differ Photographic film and film processing differs NTSC video sucks discards too much chroma information Working conditions have differing light levels Displays and inks have limited gamut What do we do CALIBRATE Color Calibration Two methods Device independent calibration Use sensing devices to objectively measure spectra of various test colors black white red etc and compute a color transform to match them If receiver can calibrate too then you can transmit device independent colors this way without knowing receiver s characteristics End to end calibration Ask user to interactively adjust a color transform to match a printed color for example with a monitor color This method is more practical because it doesn t require sensing devices but is more limited since it s receiver specific In addition to color balancing you need gamma correction correction for nonlinearities too Notes 14 Computer Graphics 2 15 463 4 Color Spaces Color can be described in various color spaces spectrum allows non visible radiation to be described but usually impractical unnecessary RGB CRT oriented color space good for computer storage HSV a more intuitive color space good for user interfaces H hue the color wheel the spectral colors S saturation purity how gray V value related to brightness luminance how bright a non linear transform of RGB since H is cyclic CIE XYZ used by color scientists a linear transform of RGB other color spaces less commonly used For extremely realistic image synthesis use of four or more samples of the spectrum may be necessary but for most purposes the three samples used by RGB color space is just fine Color Space Transforms Convert from RGB to spectrum PRGB R PR G PG B PB where PR PG PB are spectra of RGB phosphors Convert from spectrum to RGB want spectrum and RGB that look the same have same L M S L M S of P L M S of PRGB 3 equations in 3 unknowns with constant coefficients two step process find L M S from P then do linear color transform with 3x3 matrix to convert to R G B Transforms between RGB and CIE very similar also expressed using 3x3 matrices Notes 14 Computer Graphics 2 15 463 5


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