CG T8 Colour and Light

Transcription

1 CG T8 Colour and Light L:CC, MI:ERSI Miguel Tavares Coimbra (course and slides designed by Verónica Costa Orvalho)

2 What is colour?

3 Light is electromagnetic radiation Optical Prism dispersing light Visible colour spectrum Slide by Miguel Coimbra, U.Porto

4 Visible Spectrum Electromagnetic Radiation. Same thing as FM Radiowaves! Slide by Miguel Coimbra, U.Porto

5 Colour Spectrum Pure colour Energy concentrated into a unique frequency of the visible spectrum Composite colour Energy spread among various frequencies This is what happens in reality Incandescent Light Sunlight Slide by Miguel Coimbra, U.Porto

6 How do we see colour? Cones We have three types of cones in our retina Red Green Blue We also typically model digital images using three colour primitives Slide by Miguel Coimbra, U.Porto

7 Colour is a human creation Human Colour Sensors: Cones 65% Red cones 33% Green cones 2% Blue cones Slide by Miguel Coimbra, U.Porto

8 Primary Colours Not a fundamental property of light Based on the physiological response of the human eye Form an additive colour system Slide by Miguel Coimbra, U.Porto

9 Example: Television Three types of phosphors very close together The components are added to create a final colour Slide by Miguel Coimbra, U.Porto

10 Colour Space The purpose of a color model is to facilitate the specification of colours in some standard, generally accepted way Colour space Coordinate system Subspace: One colour -> One point Gonzalez & Woods Slide by Miguel Coimbra, U.Porto

11 RGB Red Green Blue Defines a colour cube. Additive components. Great for image capture. Great for image projection. Poor colour description. Slide by Miguel Coimbra, U.Porto

12 HSI Hue Saturation Intensity Defines a colour cone Great for colour description. Slide by Miguel Coimbra, U.Porto

13 Chromaticity Diagram Axis: Hue Saturation Outer line represents our visible spectrum. No three primaries can create all colours! Slide by Miguel Coimbra, U.Porto

14 RGB to HSI G B G B H / ) )( ( ) ( ) ( ) ( cos B G B R G R B R G R ),, min( ) ( 3 1 B G R B G R S ) ( 3 1 B G R I Hue: Saturation Intensity Slide by Miguel Coimbra, U.Porto

15 HSI to RGB Depends on the sector of H 120 <= H < 240 H H 120º R I(1 S) G I 1 S cos H cos(60º H ) B 3I ( R B) 0 <= H < 120 B I(1 S) S cos H R I 1 cos(60º H ) G 3I ( R B) 240 <= H < 360 H H 240º G I(1 S) S cos H B I 1 cos(60º H ) R 3I ( R B) Slide by Miguel Coimbra, U.Porto

16 What determines illumination?

17 Illumination: main concepts light sources emit light. color spectrum. position and direction

18 Illumination: main concepts light sources emit light. color spectrum. position and direction surfaces reflect light. reflectance. geometry. transmission. absortion

19 Illumination: main concepts light sources emit light. color spectrum. position and direction surfaces reflect light. reflectance. geometry. transmission. absortion specular diffuse transparency

20 Illumination: main concepts Illumination determined by the interaction of the light source + the surface

21 Illumination: types of lights ambient directional point indirect illumination sun light bulb

22 How does light interact with a surface?

23 Three types of interactions Slide by Ron Fedkiw, Stanford University

24 Slide by Ron Fedkiw, Stanford University

25 Slide by Ron Fedkiw, Stanford University

26 Paint vs. Milk Slide by Ron Fedkiw, Stanford University

27 The lighting equation Slide by Ron Fedkiw, Stanford University

28 How to colour a pixel? Slide by Ron Fedkiw, Stanford University

29 Light energy Slide by Ron Fedkiw, Stanford University

30 Solid Angle vs. Area Slide by Ron Fedkiw, Stanford University

31 Radiant Intensity Slide by Ron Fedkiw, Stanford University

32 Irradiance Slide by Ron Fedkiw, Stanford University

33 Phong Reflection Model - Diffuse Slide by Ron Fedkiw, Stanford University

34 Phong Reflection Model - Ambient Slide by Ron Fedkiw, Stanford University

35 Phong Reflection Model - Specular Slide by Ron Fedkiw, Stanford University

36 Specular Shininess Slide by Ron Fedkiw, Stanford University

37 Next class: Simpler models for illumination in CG

38 Summary Light is electromagnetic radiation Colour is a human creation describing the frequency components of light We can describe colours using colour spaces Illumination is determined by the interaction of the light source with the surface Reflection, absortion, transmittance Complex physics lead to model simplifications for CG Diffuse, ambient, specular, etc.