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CSCE641: Computer Graphics Image FormationAre They Images?OutlineSlide 4Color RepresentationHuman VisionSlide 7Electromagnetic SpectrumVisible LightSlide 10Slide 11Spectral Energy DistributionSlide 13Slide 14Slide 15Color Representation?Slide 17Light Detection: Rods and ConesTristimulus of Color TheorySlide 20Slide 21Slide 22Slide 23Tristimulus Color TheoryAdditive and Subtractive ColorRGB Color SpaceSlide 27Image RepresentationSlide 29Slide 30Slide 31Slide 32How Do We See the World?Pin-hole CameraCamera ObscuraSlide 36Shrinking the ApertureShrink the Aperture: DiffractionShrink the ApertureThe Reason of LensesAdding A LensChanging LensesSlide 43Projection MatrixModeling Projection: 3D->2DSlide 46Slide 47Homogeneous CoordinatesSlide 49Slide 50Perspective ProjectionSlide 52Perspective EffectsSlide 54Slide 55Parallel ProjectionWeak-perspective ProjectionSlide 58View TransformationSlide 60Viewport TransformationSlide 62Putting It TogetherSlide 64Camera ParametersSlide 66How about this image?Slide 68Plenoptic FunctionSlide 70Slide 71Color ImageDynamic SceneMoving around A Static SceneMoving around A Dynamic SceneSlide 76How to “Capture” Orthographic ImagesSlide 78Slide 79Slide 80Slide 81Multi-perspective ImagesSlide 83Slide 84Slide 85They Are All ImagesNext lectureCSCE641: Computer GraphicsImage FormationJinxiang ChaiAre They Images?Outline•Color representation•Image representation•Pin-hole Camera•Projection matrix•Plenoptic functionOutline•Color representation•Image representation•Pin-hole Camera•Projection matrix•Plenoptic functionColor Representation•Why do we use RGB to encode pixel color?•Can we use RGB to represent all colors?•What are other color representations?Human VisionModel of human visionHuman VisionModel of human visionVision components:• Incoming light• Human eyeElectromagnetic SpectrumVisible light frequencies range between:–Red: 4.3X1014 hertz (700nm)–Violet: 7.5X1014 hertz (400nm)Visible LightThe human eye can see “visible” light in the frequency between 400nm-700nmVisible LightThe human eye can see “visible” light in the frequency between 400nm-700nm400nm 700nmVisible LightThe human eye can see “visible” light in the frequency between 400nm-700nm400nm 700nm- Not strict boundary- Some colors are absent (brown, pink)12Spectral Energy DistributionThree different types of lights13Spectral Energy DistributionThree different types of lightsCan we use spectral energy distribution to represent color?14Spectral Energy DistributionThree different types of lightsCan we use spectral energy distribution to represent color?- Not really, different distribution might result in the same color (metamers)!Spectral Energy DistributionThe six spectra below look the same purple to normal color-vision peopleColor Representation?Why not all ranges of light spectrum are perceived?So how to represent color? - unique - compact - work for as many visible lights as possible 400nm 700nmHuman VisionPhotoreceptor cells in the retina: - Rods - ConesLight Detection: Rods and ConesRods: -120 million rods in retina -1000X more light sensitive than Cones - Discriminate B/W brightness in low illumination - Short wave-length sensitiveCons: - 6-7 million Cones in the retina - Responsible for high-resolution vision - Discriminate Colors - Three types of color sensors (64% red, 32%, 2% blue) - Sensitive to any combination of three colorsTristimulus of Color TheorySpectral-response functions of each of the three types of conesTristimulus of Color TheorySpectral-response functions of each of the three types of conesCan we use them to match any spectral color?Tristimulus of Color TheorySpectral-response functions of each of the three types of conesColor matching function based on RGB - any spectral color can be represented as a linear combination of these primary colorsTristimulus of Color TheorySpectral-response functions of each of the three types of conesColor matching function based on RGB - any spectral color can be represented as a linear combination of these primary colorsTristimulus of Color TheorySpectral-response functions of each of the three types of conesColor matching function based on RGB - any spectral color can be represented as a linear combination of these primary colorsTristimulus Color TheorySo, color is psychological- Representing color as a linear combination of red, green, and blue is related to cones, not physics- Most people have the same cones, but there are some people who don’t – the sky might not look blue to them (although they will call it “blue” nonetheless)- But many people (mostly men) are colorblind, missing 1,2 or 3 cones (can buy cheaper TVs)Additive and Subtractive ColorRGB color modelCMY color modelComplementary color models: R=1-C; G = 1-M; B=1-Y;White: [1 1 1]TGreen: [0 1 0];White: [0 0 0]TGreen: [1 0 1];RGB Color SpaceRGB cube–Easy for devices–Can represent all the colors?–But not perceptual–Where is brightness, hue and saturation?redgreenblueOutline•Color representation•Image representation•Pin-hole Camera•Projection matrix•Plenoptic functionImage RepresentationAn image is a 2D rectilinear array of Pixels - A width X height array where each entry of the array stores a single pixelImage RepresentationAn image is a 2D rectilinear array of Pixels - A width X height array where each entry of the array stores a single pixel A 5X5 picturepixelImage RepresentationA pixel stores color informationLuminance pixels - gray-scale images (intensity images) - 0-1.0 or 0-255 - 8 bits per pixelRed, green, blue pixels (RGB) - Color images - Each channel: 0-1.0 or 0-255 - 24 bits per pixelImage RepresentationAn image is a 2D rectilinear array of Pixels - A width X height array where each entry of the array stores a single pixel - Each pixel stores color information(255,255,255)Outline•Color representation•Image representation•Pin-hole Camera•Projection matrix•Plenoptic FunctionHow Do We See the World?Let’s design a camera: idea 1: put a piece of film in front of camera Do we get a reasonable picture?Pin-hole Camera•Add a barrier to block off most of the rays–This reduces blurring–The opening known as the aperture–How does this transform the image?Camera Obscura•The first camera–Known to Aristotle–Depth of the room is the focal length–Pencil of rays – all rays through a pointCamera ObscuraHow does the aperture size affect the


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