Intensity & Color (Color vs. Grayscale) PDF

Intensity & Color (or Color vs. Grayscale) Original Notes by Dr. Minglun Gong Edition by Dr. Oscar Meruvia-Pastor © Copyright Outline Intensity (a.k.a. Luminosity, Brightness, Luminance) Range Quantization Color Color space Color models...

Intensity & Color (or Color vs. Grayscale) Original Notes by Dr. Minglun Gong Edition by Dr. Oscar Meruvia-Pastor © Copyright Outline Intensity (a.k.a. Luminosity, Brightness, Luminance) Range Quantization Color Color space Color models Gong & Meruvia © 2  Range of the Intensity The visible light emitted/reflected from an object varies from very dark to very bright The luminous intensity of a candle is ~1 candela The sunlight is about 100,000 candela per m 2 In nature, there is no upper limit to brightness Relatively hard to capture/represent intensity variations in the whole intensity range The ratio between the maximum and minimum recorded intensities in an image considered is called “dynamic range” Gong & Meruvia © 3 Examples of Dynamic Range Intensity variation smaller than the min is ignored (lack of details in shadowed regions) Intensity value larger than the max is truncated (blow out highlights) Gong & Meruvia © 4  What is High Dynamic Range? Gong & Meruvia © 5 https://en.wikipedia.org/wiki/High-dynamic-range_imaging Quantization of Intensity How to sample the intensity range using a set of discrete numbers? Arithmetic sequence: 𝑰𝟎 = 𝒂, 𝑰𝟏 = 𝒂 + 𝒌, 𝑰𝟐 = 𝒂 + 𝟐𝒌, 𝑰𝟑 = 𝒂 + 𝟑𝒌, … Geometric sequence: 𝑰𝟎 = 𝒂, 𝑰𝟏 = 𝒂 ⋅ 𝒌, 𝑰𝟐 = 𝒂 ⋅ 𝒌𝟐 , 𝑰𝟑 = 𝒂 ⋅ 𝒌𝟑 , … min max Gong & Meruvia © 6 Defining colors WHAT IS RED? (MARY O'NEILL) Red is a sunset For a scientist, color is a Blazing and bright. Red is feeling brave vector quantity, and for With all your might. a computer scientist red Red is a sunburn is, in most cases, Spot on your nose. Sometimes red represented by the Is a red, red rose. vector (255,0,0) in the Red squiggles out RGB color model! When you cut your hand Red is a brick And the sound of a band. Red is hotness You get inside When you're embarrassed And want to hide. Fire-cracker, fire-engine Fire clicker red - Gong & Meruvia © 7  Color A color can be defined using red, green, & blue wavelength combinations Some colors need negative weight for red CIE (Commission Internationale de l’Éclairage) defined 3 standard primaries: X, Y, & Z Can match all visible colors using only positive weights Gong & Meruvia © 8 CIE Chromaticity Diagram and Color Spaces Chromaticity diagram is the 𝑿 + 𝒀 + 𝒁 = 𝟏 plane in the CIE space Can be considered as the colors of lights that have the same total amount of energy Gong & Meruvia © 9 Monitor Gamut The range of colors that can be shown on a specific display device is called gamut The gamut of a typical monitor does not cover the entire CIE space The corners of the triangle depend on the emittance of the phosphors of the monitor Certain colors cannot be shown Gong & Meruvia © 10 Color Model Families RGB: Red, green, & blue CMY & CMYK: Cyan, magenta, yellow (& black) HSV & HSL: Hue, saturation, & value (lightness) YUV & YIQ Gong & Meruvia © 11 RGB Model & the Color Cube Additive color model: Red + Blue = Magenta Blue + Green = Cyan Green + Red = Yellow Red + Blue + Green = White 0.5Red+0G+0.2B = 0.5, 0, 0.2 = ? Used by most light emitting displays such as monitors and projectors Gong & Meruvia © 12 RGB Color Channels Decomposition Gong & Meruvia © 13  sRGB vs. Adobe RGB sRGB (standard RGB) is an RGB color space created by HP & Microsoft Matches what CRT monitors can display Adobe RGB is an RGB color space developed by Adobe in 1998 Has a larger gamut than sRGB https://fstoppers.com/pictures/adobergb -vs-srgb-3167 Gong & Meruvia © 14 CMY Model Subtractive color mode is based on pigment reflectance, as materials only reflect the light they do not absorb: Cyan (C) absorbs red Magenta (M) absorbs green wavelengths Yellow (Y) absorbs blue Used by color printers and reflective media systems Gong & Meruvia © 15 Converting between RGB & CMY RGB → CMY C=1–R M=1–G Y=1–B CMY → RGB R=1–C G=1–M B=1–Y Gong & Meruvia © 16  CMYK Model Adding the 4th ink to color: Black (K) Use black ink directly instead of mixing color inks Convert from CMY to CMYK K = min(C, M, Y) C´ = C – K M´ = M – K Y´ = Y – K Gong & Meruvia © 17 HSV/HSB vs. HSL Model User-oriented color models: Hue – Dominant wavelength Saturation – Excitation purity from white to Hue Value – Luminance from 0 to HS HSL model is similar: Uses Lightness instead of Value Chroma/Saturation is the transition from greyscale to the chosen Gong & Meruvia © 18 hue  Converting RGB to HSV or RGB to HSL Max = max(R, G, B); Max = max(R, G, B); Min = min(R, G, B); Min = min(R, G, B); V = Max; L = (Max + Min) / 2; if (Max == 0) if (Min == Max) S = 0; S = 0; else else if (L ≤ ½) S = 1 – Min / Max; S = (Max – Min) / 2L; … else S = (Max – Min) / (2 – Converting HSB/HSV to HSL 2L); https://stackoverflow.com/questions/34 … 23214/convert-hsb-hsv-color-to-hsl PYTHON RGB AND HSV CONVERSION RGB to HSL conversion formula (PYTHON RECIPE BY MICHAEL @ RapidTables.com FOGLEMAN @ ACTIVE STATE) Gong & Meruvia © 20 YUV Model Human visual system (HVS)-based color model One luminance channel (Y) Two chrominance channels Used in PAL analog Chrominance is defined video & digital video as the difference between a color and a Human eyes are more reference white at the sensitive to luminance same luminance than to chrominance 5.5 MHz for Y 1.8 MHz each for U & V Gong & Meruvia © 21 YIQ Model Aligns with human’s color perception sensitivities: Y is the brightness I is the orange-blue axis: Used in NTSC color TV I = V*cos33° - broadcasting U*sin33° Y is the brightness Q is the purple-green Eyes are most sensitive axis: to Y, then to I, then to Q Q= V*sin33° + 4.2 MHz for Y U*cos33° 1.5 MHz to I 0.55 MHz to Q Gong & Meruvia © 22 Converting RGB to YUV or RGB to YIQ Y = 0.299*R + 0.587*G Y = 0.299*R + 0.587*G + 0.114*B + 0.114*B U = 0.492 * (B - Y) I = 0.596*R - 0.275*G - V = 0.877 * (R - Y) 0.321*B Q = 0.212*R - 0.523*G + 0.311*B Gong & Meruvia © 23 But… Why do we have so many color models? Some are older and some are newer. Some are more convenient and intuitive to use for artists (HSV & HSL) Some color models are closely related to the display hardware (RGB). Some are more designed to fit the some of the characteristics of the Human Visual System (YIQ & YUV). Some were developed in different regions of the world at roughly the same time (YIQ/YUV). Some are more convenient for software developers (RGB) Gong & Meruvia © 24 Homework: Check out these Online Resources Color Models & their Meanings! (11m19s) https://www.youtube.com/watch?v=CF4wuPLBaAA Color tutorial: Understanding color spaces | lynda.com (6m) https://www.youtube.com/watch?v=KKX08oOTMkk JPEG Color spaces by Computerphile (7m 30s) https://www.youtube.com/watch?v=LFXN9PiOGtY The HSV color model by Thales Sehn Körting (2mins) https://www.youtube.com/watch?v=NAw2_NtGNaA Beginning Graphic Design: Color (6m 32s) https://www.youtube.com/watch?v=_2LLXnUdUIc HSV Color Theory For Digital Artists (6’) https://www.youtube.com/watch?v=6tyumG7W1lQ Gong & Meruvia © 25 TopHat-evaluated Homework: THE NATURE OF THINGS’ Living Colour (44’) Link available on lecture slides for: 01_visual_perception_OM Evaluation on next Monday! Gong & Meruvia © 27