DOC PREVIEW
UW-Madison CS 559 - Transformations between color systems

This preview shows page 1-2-14-15-29-30 out of 30 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 30 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 30 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 30 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 30 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 30 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 30 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 30 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Last TimeTodayErrata: PrimariesErrata: Color MatchingComputing the MatchingGoing from RGB to XYZStandard RGB↔XYZDetermining GamutsAccurate Color ReproductionMore linear color spacesHSV Color Space (Alvy Ray Smith, 1978)HSV Color SpaceUniform Color SpacesMacAdam EllipsesCIE u’v’ SpacePaint and InkSubtractive mixingCalibrating a PrinterImage File FormatsThe Simplest FileIndexed ColorSlide 22Image CompressionGIFLZW CompressionJPEGDiscrete Cosine TransformQuantizationEntropy CodingLossless JPEG With Prediction1/29/04 © University of Wisconsin, CS559 Spring 2004Last Time•Color–Sensor response is computed from the sensor’s response curve and the incoming spectrum–Humans have three types of color sensing cones in their eyes–The principle of trichromacy says that any spectrum can be matched using three primaries (but sometimes you have to subtract a primary)–A color system consist of primaries and color matching functions that are used to determine how much of each primary is needed to match a spectrum–RGB and CIE XYZ are two color systems1/29/04 © University of Wisconsin, CS559 Spring 2004Today•Errata from last time•Transformations between color systems•Limitations of trichromatic color representations1/29/04 © University of Wisconsin, CS559 Spring 2004Errata: Primaries•Last time I implied a primary was a single wavelength•Not true – a primary can be a spectrum–Single wavelengths are just a special case400 500 600 7003 Primaries400 500 600 7003 Primariesor1/29/04 © University of Wisconsin, CS559 Spring 2004Errata: Color Matching•Last time, on the board, I implied that the matching functions are determined one primary at a time – not right•First step:–For a light of unit intensity at each wavelength, ask people to match it using some combination of R, G and B primaries–Gives you, r(), g() and b(), the amount of each primary used for wavelength –Defined for all visible wavelengths, r(), g() and b() are the RGB color matching functions1/29/04 © University of Wisconsin, CS559 Spring 2004Computing the Matching•The spectrum function that we are trying to match, E(), gives the amount of energy at each wavelength•The RGB matching functions describe how much of each primary is needed to match one unit of energy at each wavelength •Hence, if “=” means perceptually equal, then the match is: bB gG rR E dEbbdEggdErr)()()()()()(1/29/04 © University of Wisconsin, CS559 Spring 2004Going from RGB to XYZ•These are linear color spaces, related by a linear transformation•Match each primary, for example:•Substitute and equate terms:bBgGrRzZyYxXE ZzYyXxRrrrbgrzzzyyyxxxzyxbgrbgrbgr1/29/04 © University of Wisconsin, CS559 Spring 2004Standard RGB↔XYZbgrzyx9505.01192.00193.00721.07151.02126.01805.03576.04124.0zyxbgr0570.12040.00556.00416.08760.19692.04986.05374.12410.3•Note that each matrix is the inverse of the other•Recall, Y encodes brightness, so the matrix tells us how to go from RGB to grey1/29/04 © University of Wisconsin, CS559 Spring 2004Determining Gamuts•Gamut: The range of colors that can be represented or reproduced•Plot the matching coordinates for each primary. eg R, G, B•Region contained in triangle (3 primaries) is gamut•Really, it’s a 3D thing, with the color cube distorted and embedded in the XYZ gamutxyXYZ GamutRGB GamutGRB1/29/04 © University of Wisconsin, CS559 Spring 2004Accurate Color Reproduction•High quality graphic design applications, and even some monitor software, offers accurate color reproduction•A color calibration phase is required:–Fix the lighting conditions under which you will use the monitor–Fix the brightness and contrast on the monitor–Determine the monitor’s γ–Using a standard color card, match colors on your monitor to colors on the card: This gives you the matrix to convert your monitor’s RGB to XYZ–Together, this information allows you to accurately reproduce a color specified in XYZ format (and hence any other standard format)1/29/04 © University of Wisconsin, CS559 Spring 2004More linear color spaces•Monitor RGB: primaries are monitor phosphor colors, primaries and color matching functions vary from monitor to monitor•sRGB: A new color space designed for web graphics•YIQ: mainly used in television–Y is (approximately) intensity, I, Q are chromatic properties–Linear color space; hence there is a matrix that transforms XYZ coords to YIQ coords, and another to take RGB to YIQ –I and Q can be transmitted with low bandwidth1/29/04 © University of Wisconsin, CS559 Spring 2004HSV Color Space (Alvy Ray Smith, 1978)•Hue: the color family: red, yellow, blue…•Saturation: The purity of a color: white is totally unsaturated•Value: The intensity of a color: white is intense, black isn’t•Space looks like a cone–Parts of the cone can be mapped to RGB space•Not a linear space, so no linear transform to take RGB to HSV–But there is an algorithmic transform1/29/04 © University of Wisconsin, CS559 Spring 2004HSV Color SpaceHSV Color Cone Program1/29/04 © University of Wisconsin, CS559 Spring 2004Uniform Color Spaces•Color spaces in which distance in the space corresponds to perceptual “distance”•Only works for local distances–How far is red from green? Is it further than red from blue?•Use MacAdams ellipses to define perceptual distance1/29/04 © University of Wisconsin, CS559 Spring 2004MacAdam Ellipses•Scaled by a factor of 10 and shown on CIE xy color space•If you are shown two colors, one at the center of the ellipse and the other inside it, you cannot tell them apart•Only a few ellipses are shown, but one can be defined for every point1/29/04 © University of Wisconsin, CS559 Spring 2004VioletYXZYXvu943151CIE u’v’ Space•CIE u’v’ is a non-linear color space where color differences are more uniform•Note that now ellipses look more like circles•The third coordinate is the original Z from XYZ1/29/04 © University of Wisconsin, CS559 Spring 2004Paint and Ink•Paints contain


View Full Document

UW-Madison CS 559 - Transformations between color systems

Documents in this Course
Filters

Filters

14 pages

Lecture 2

Lecture 2

24 pages

Clipping

Clipping

22 pages

Modeling

Modeling

33 pages

Filters

Filters

26 pages

Dithering

Dithering

33 pages

Lecture 4

Lecture 4

20 pages

Load more
Download Transformations between color systems
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Transformations between color systems and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Transformations between color systems 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?