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Understanding Color Models in Computer Graphics: RGB and YUV Explained

This document explores the assignment and modeling of color attributes in computer graphics, focusing on RGB and YUV color models. It discusses the definition of colors using primary colors, how colors are assigned to polygon vertices, and different color representations, including CMYK. Additionally, it highlights the importance of brightness, saturation, and hue, and compares the YUV model's advantages in data compression and compatibility with black-and-white television. The intersection of art and technology in color representation is analyzed for improved visual output.

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Understanding Color Models in Computer Graphics: RGB and YUV Explained

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  1. Computer Graphics SS 2014 Color Rüdiger Westermann Lehrstuhl für Computer Graphik und Visualisierung

  2. Appearancemodelling • How do weassignappearanceattributestotheobject, forinstancecolor? • First, specifyhowtodefine a colorA colorisdefinedby a superpositionof 3 primarycolorsRedGreenBlue(0 0 0) -> Black(0 1 1) -> Cyan(0.5 0.5, 0.0) -> Yellowish … • Colors areassignedtopolygonverticesin themodelingstageas additional properties White Cyan Magenta Blue Green Yellow blacK Red

  3. Appearancemodelling • Triangle mesh representation • Shared vertex („indexed face set“) representation (OBJ, OFF files) • Array of coordinates, normals and colors of all vertices • Array of triangles with indices into the vertex list

  4. RGB color model Cyan White • Additive system: Black + [r,g,b] • Used in color displays • With 8 bits per primary=> components from 0-255 => 16M different colors=> 24 bits per pixel=> 1024x1024 image consumes 3 MBytes Magenta Blue Green Yellow blacK Red

  5. RGB color model

  6. CMY(K) color model • Complementary colors to RGB • Cyan, magenta, yellow • Mix to white in additive model • Mix to black in subtractive model • Subtractive color system for printers • Additional black ink Cyan White Magenta Blue Green Yellow blacK Red

  7. Colors in RGB/CMYK • Not all possiblecolorscanberepresented in RGB and CMYK

  8. HSV color space • Alternative representation of color • Visual effect of chromatic (“bunt”) light, i.e. its light spectrum, can also be characterized by • Hue (Farbton): dominant wavelength • Saturation (Sättigung): pureness, inverse proportional to the amount of white light • Value/Brightness (Helligkeit): intensity of light • Achromatic (“unbunt”) light is only characterized by brightness

  9. More color models • The chromaticity diagram • http://en.wikipedia.org/wiki/CIE_1931_color_space • http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html • The YUV color model • http://en.wikipedia.org/wiki/YUV

  10. YUV color model • In the YUV color model, a color is represented by a luminance (brightness) value Y and 2 chrominance (hue) values UV • i.e., separates brightness from chrominance • Used in JPEG image compression • The human eye is less sensitive to chrominance than luminance; ie. by separating chrominance it can be compressed more aggressively.

  11. YUV color model • RGB vs. YUV

  12. YUV color model • Advantage of YUV color model • Compatible with black and white TV • Gives rise to data compression, because the human eye can perceive higher difference of luminosity than difference of colorsi.e., store Y in full resolution but U and V in a quarter resolution ⇒ 1024x1024 RGB image ⇒ 1024x1024 Y image + 512x512 U image + 512x512 V image

  13. YUV color model • YUV in full resolution (left), Y in full but U and V in a quarter resolution (right)

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