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Introduction to Image-Space Methods in Computer Graphics

This chapter explores digital image manipulation techniques, including position changes, compositing, segmentation, and texture synthesis, using image-space methods. It also discusses the differences between real and digital images and the core concepts of digital image representation.

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Introduction to Image-Space Methods in Computer Graphics

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  1. CS 395: Adv. Computer Graphics Introduction to Image-Space Methods Watt & Watt: Chapter 8, "Pixel is Not a Little Square!" –AR Smith Jack Tumblin jet@cs.northwestern.edu

  2. Image Space Methods • A ‘Digital Image’ = a data structure for displayed images • ?What can it do, besides display? what can we change? • Position; offset, stretch, squeeze, smear, cut,…Try It! (Java Applets) http://www.mrl.nyu.edu/~hertzman/nudge/ • Combine: +/-*, Compositing, (depth images?) • Separate: matte or ‘segmenting’ • Elaborate: make ‘more of the same’ picture

  3. ‘Real’ Images: Lens-Focused Light • 3D2D Light Intensity MapI(x,y) • Angle(,)  Position(x,y) • ‘Blurring’—sharpness set by focus, lens quality I(x,y) Image Plane Intensity Viewed Scene Angle(,) Position(x,y)

  4. ‘Digital’ Images: 2D Grid of Numbers • NO intrinsic meaning, but ... • Widely assumed to represent • Point Samples of a “smoothed” 2D intensity surface • Uniform sampling pattern (but not always) (!weasel-word!) y x

  5. Real vs. Digital • Real Image == 2D Light Intensity map: I(x,y) • Digital Image == 2D grid of numbers: I(m,n) (pixels) I(x,y) I(m,n) x,y m,n

  6. Digital Images As Vectors I(m,n) m,n • ‘Stack up’ pixel values: VERY LONG vector • 1 digital image == 1 point in N-dim. Space • Nearby points == Similar images • All possible digital images: a grid of N-D points • Space of all practical digital images: • ~8Meg dimensions (2Mpix * RGBA) • discrete, quantized I00 I01 I02 … I10 I11 I12 … I = I00 I01 I02 I03 I04 I05

  7. Digital Images As Vectors • Sensible element-by-element operations: • Add, subtract, scale two images: 0.5 + (I1 - I2 ) = out - = I1 I2 out

  8. Digital Images As Vectors • Sensible element-by-element operations: ‘Alpha blending’ or ‘dissolve’:-Weighted sum of two images: 0 <  < 1 + =  I1 (1-) I2 out

  9. Digital Images As Vectors Compositing: : Use an image as ‘opacity’: = + out (1-) .* I2  .* I1

  10. Extended Compositing Methods • Environment Mattes Alpha Matte Environment Matte Photograph

  11. http://grail.cs.washington.edu/projects/envmatte/

  12. Digital Images As Vectors Exploration of the space-of-all-images: • Image shift, scale, rotation, warp, etc.as vector operation? awkward... • ‘Video Textures’ (Schodl et al., SIGG2000)http://www.gvu.gatech.edu/perception/projects/videotexture/ • ‘Face Space’, ‘eigenfaces’, etc: • Gather many example images of facesUse PCA to find desirable axes: gender, age, facial expression... • Depth images: is this still 2D?

  13. Texture Synthesis: Learned Vectors Input Sample Synthesized Result • Texture Synthesis/elaborationEfros98, Wei/Levoy99, Ashikhmin2000,...

  14. Texture Synthesis: Learned Vectors • Core Idea: • for each pixel, make a vector of neighborhood pixel values: • Make a ‘dictionary’ of these vectors:given a vector , • what are the most likely values of ? Dictionary: crude, local pattern learning • Use dictionary to build a new image from ‘seed’ pixels/regions chosen at random. I00 I01 I02 … I10 I11 I12 … I =

  15. Texture Synthesis: Learned Vectors • Refinements & Extensions: • Faster, simpler methods (Ashkhmin: real-time!) • http://www.cs.utah.edu/~michael/ts/ • Hertzmann(2001): Image analogieshttp://mrl.nyu.edu/projects/image-analogies/ • Given images A,B,C, ‘learn’ AB mapping. then find D Solves ‘A is to B’ as ‘C is to D’ problems • Recreates some local effects—brush strokes, blur, etc. • Wider variety of user-guided reconstruction.. • Recent extensions: video textures, depth, curves, shapes, textures on surfaces, colorizing B/W images...

  16. Texture Synthesis: Modified Vectors • ‘Image Inpainting’ –repair damaged images http://www.ece.umn.edu/users/marcelo/restoration.html • ‘Reaction/Diffusion’ textures: http://www.gvu.gatech.edu/people/faculty/greg.turk/reaction_diffusion/reaction_diffusion.htmliteratively compute new pixel intensity from neighbors + rules. Image evolves over time • Anisotropic Diffusion, Curvature Flow: iterative rules (PDEs) for progressive edge-preserving image smoothing

  17. ! NOT EQUIVALENT ! ? Digital image: pixel-by-pixel operations ? ? Real Image: point-by-point operations ? Source of much frustration & error! Digital Image == Real Image Also see Alvy Ray Smith’s article entitled:“A pixel is NOT a little square, a pixel is NOT a little square, a pixel is NOT a little square!”found here.

  18. Real Digital •  • Image Intensity • Image Spectral Distribution (‘color’) • Image Geometry (Position,Angle, Distortion, …) • Image Resolution (‘sharpness’ or ‘focus’) Real Images:Smooth, Continuous, Variable, Complete Unlimited… •  •  •  •  Digital Images: Sampled, Discrete, Quantized, Fixed Limited …

  19. Conversion Method? I(m,n) I(x,y) x,y m,n • Real Image == 2D Light Intensity map: I(x,y) • Digital Image == 2D grid of numbers: I(m,n) • ?

  20. Digital  Real : ‘Reconstruction’ 1.0 -0.5 0 0.5 A pixel sets strength of a real Basis function(aka ‘reconstruction filter’s impulse response’) ‘Box’ I(m,n) I(x,y) m,n x,y

  21. Digital  Real : ‘Reconstruction’ A pixel sets strength of a real Basis function(aka ‘reconstruction filter’s impulse response’) ‘Linear’ 1.0 I(m,n) I(x,y) -1.0 0 1.0 m,n x,y

  22. Digital  Real : ‘Reconstruction’ A pixel sets strength of a real Basis function(aka ‘reconstruction filter’s impulse response’) ‘Cubic’ 1.0 I(m,n) I(x,y) -1.0 0 1.0 2.0 m,n x,y

  23. RealDigital: ‘Sampling’ I(x,y) x,y • If I(x,y) is ‘smooth enough’ already, • ?Why not just grab I(x,y) values at (m,n) ? I(m,n) •  m,n

  24. ‘Smooth enough’ to Sample? • Because ‘smooth enough’ is undefined; • Samples may hit spurious peaks, valleys I(x,y) I(m,n) BAD! x,y m,n

  25. Real  Digital : ‘Pre-filter’ m,n ‘Smoothing’ Defined : Linear Pre-filter Pixel = local weighted average of real image around pixel sample point 1.0 Weight ‘Cubic’ weights I(x,y) 1.0 -1.0 0 1.0 2.0 x,y

  26. Pre-Filter functions 1.0 1.0 -0.5 0 0.5 -1.0 0 1.0 1.0 -1.0 0 1.0 2.0 • Pre-filters == anti-aliasing in Comp Graphics • SHOULD be done by continuous integration, • USUALLY done by super-sampling ? Quality Measures ? ‘Box’ filter ‘Bi-linear’ filter ‘Bi-cubic’ filter

  27. ‘Smoothness Measures’ • Sine wave magic: • ALL(continuous)functions are a (possibly infinite)weighted sum of sinusoids: f(x) = A0sin(w0x) + B0cos(w0x)+ A1sin(w1x) + B1cos(w0x)+… • For ALL FILTERS(box, linear, cubic,…, anything…)sinusoid in  scaled, shifted sinusoid out THUS– compute response to ALL signals from sinusoids • High-Frequency Sinusoids in a signal hold ALL of its sampling/aliasing troubles!

  28. 1-D Aliasing Example Real Real image: I(x,y) =sin(2fx) f=3.0 Aliasing: when bad conversion scrambles the frequencies of sinusoids

  29. 1-D Aliasing Example Real Sample Real image: I(x,y) =sin(2fx) f=3.0 Sample at: x(n) = n / 36.0 (~12 samples per cycle)

  30. Aliasing Example Real Sample Display Real image: I(x,y) =sin(2fx) f=3.0 Sample at: x(n) = n / 36.0 (~12 samples per cycle) Reconstruct: (use ‘box’ filter)  approximates f=3.0

  31. Aliasing Example Real Sample Display Real Real image: I(x,y) =sin(2fx) f=3.0 Sample at: x(n) = n / 36.0 (~12 samples per cycle) Reconstruct: (use ‘box’ filter)  approximates f=3.0 Another Real Image: I(x,y) = sin(2fx) where f= 39.0

  32. Aliasing Example Real Sample Display Sample Real Real image: I(x,y) =sin(2fx) f=3.0 Sample at: x(n) = n / 36.0 (~12 samples per cycle) Reconstruct: (use ‘box’ filter)  approximates f=3.0 Another Real Image: I(x,y) = sin(2fx) where f= 39.0 Sample at: x(n) = n / 36.0 (~0.92 samples per cycle)

  33. Aliasing Example Real Sample Display Display Sample Real Real image: I(x,y) =sin(2fx) f=3.0 Sample at: x(n) = n / 36.0 (~12 samples per cycle) Reconstruct: (use ‘box’ filter)  approximates f=3.0 Another Real Image: I(x,y) = sin(2fx) where f= 39.0 Sample at: x(n) = n / 36.0 (~0.92 samples per cycle) Reconstruct: (use ‘box’ filter) approximates f=3.0 !!

  34. Aliasing Example Real Sample Display Display Sample Real Real image: I(x,y) =sin(2fx) f=3.0 Sample at: x(n) = n / 36.0 (~12 samples per cycle) Reconstruct: (use ‘box’ filter)  approximates f=3.0 Another Real Image: I(x,y) = sin(2fx) where f= 39.0 Sample at: x(n) = n / 144.0 ( 4.0 samples per cycle) Reconstruct: (use ‘box’ filter) approximates f=39.0

  35. Aliasing Example Real Sample Display Display Sample Real • ? How much sampling is enough? • Depends on quality of reconstruction filter • Rule-of-Thumb: at least 4 samples per cycle of the highest frequency sinusoid • Nyquist Criterion: 2 samples/cycle is theoretical min.

  36. Conclusion: • Digital images are compact, indirect, ambiguous descriptions of light distributions • Digital images are vectors • Digital image patterns can be learned • Image-space manipulations must explicitly address real-digital (discrete-continuous) conversions to avoid aliasing artifacts.

  37. Aliasing Example Real Sample Display Real Sample Display Real image: I(x,y) =sin(2fx) f=3.0 Sample at: x(n) = n / 36.0 (~12 samples per cycle) Reconstruct: (use ‘box’ filter)  approximates f=3.0 Another Real Image: I(x,y) = sin(2fx) where f= 39.0 Sample at: x(n) = n / 36.0 Reconstruct: (use ‘box’ filter) approximates f=3.0

  38. Image Space Methods • Intro: What can we do with images? • Re-sampling:Continuous & Discrete Filters • Interpolation: What’s ‘in-between’ a pixel? • Antialiasing: What makes a good sampling? • Math Rigor: the Fourier Transform • Examples: box, triangle, Mitchell-Netravali • Compositing: Transparency • Un-Compositing: Matte Separations • Further extensions: • Environment Matte • Polynomial Texture Map • Texture Elaboration, Image Inpainting(Sigg2000)

  39. OLD • ‘Image’==A 2D map of light intensities from a lens • ‘Digital Image’==a 2D grid of numbers (pixels) • PROBLEMS: • What is between all those points? • Why go digital? Why is it better than film? • How can I combine the best of many pictures? Edit easily? GOAL: • More Flexibility; let me do more than just display!

  40. Image-Space Techniques GOAL: More flexibility! • Compositing /Matte: cut-and-paste, transparency , the ‘digital optical bench’ (Pixar`84)… • Warp: image as a ‘rubber sheet’, you can cut, stretch, and change at will • Environment Matte (Salesin99…) • Video Textures(Schodl 2000) • Polynomial Texture Map (Malzburg2001)

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