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Lecture 11: Output 2

Lecture 11: Output 2. UI Hall of Fame or Shame?. Today’s Topics. Alpha compositing Transforms Clipping Painting tricks. Transparency. Alpha is a pixel ’ s transparency from 0.0 (transparent) to 1.0 (opaque) 32-bit RGBA pixels: each pixel has red, green, blue, and alpha values

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Lecture 11: Output 2

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  1. Lecture 11: Output 2 6.831 UI Design and Implementation

  2. UI Hall of Fame or Shame? 6.831 UI Design and Implementation

  3. Today’s Topics • Alpha compositing • Transforms • Clipping • Painting tricks 6.831 UI Design and Implementation

  4. Transparency • Alpha is a pixel’s transparency • from 0.0 (transparent) to 1.0 (opaque) • 32-bit RGBA pixels: each pixel has red, green, blue, and alpha values • Uses for alpha • Antialiasing • Nonrectangular images • Translucent components 6.831 UI Design and Implementation

  5. Antialiasing Antialiased Simple 6.831 UI Design and Implementation

  6. Alpha Compositing • Compositing rules control how source and destination pixels are combined • Source • Image • Stroke drawing calls • Destination • Drawing surface 6.831 UI Design and Implementation

  7. Porter-Duff Alpha Compositing Rules Source pixel [Rs Gs Bs As ] Destination pixel [ Rd Gd Bd Ad ] • Premultiply RGB by A [RGB]s = [RGB]sAs [RGB]d = [RGB]dAd • Compute weighted combination of source and destination pixel [RGBA]d = [RGBA]sfs + [RGBA]dfd for weights fs, fd determined by the compositing rule • Postdivide RGB by A [RGB]d = [RGB]d / Ad unless Ad == 0 6.831 UI Design and Implementation

  8. Simple Copying clear fs=0, fd=0 [RGBA]d = 0 src fs=1, fd=0 [RGBA]d = [RGBA]s dst fs=0, fd=1 [RGBA]d = [RGBA]d 6.831 UI Design and Implementation

  9. Layering src over dst fs=1, fd=1-As [RGBA]d = [RGBA]s + [RGBA]d(1-As) dst over src fs=1-Ad, fd=1 [RGBA]d = [RGBA]d + [RGBA]s(1-Ad) 6.831 UI Design and Implementation

  10. Masking src in dst [RGBA]d = [RGBA]sAd dst in src [RGBA]d = [RGBA]dAs src out dst [RGBA]d = [RGBA]s(1-Ad) dst out src [RGBA]d = [RGBA]d(1-As) 6.831 UI Design and Implementation

  11. Other Masking src atop dst [RGBA]d = [RGBA]sAd + [RGBA]d(1-As) dst atop src [RGBA]d = [RGBA]s(1-Ad) + [RGBA]dAs src xor dst [RGBA]d = [RGBA]s(1-Ad) + [RGBA]d(1-As) 6.831 UI Design and Implementation

  12. Coordinate Transforms • Translation • moves origin by dx, dy • Scaling • multiplies coordinatesby sx, sy • Rotation • rotates by θ around origin 6 6 6.831 UI Design and Implementation

  13. sxx syy x y x y cosθ sinθ -sinθ cosθ sx 0 0 sy Matrix Representation • Normally points in 2D are represented by a two-element vector [x,y] • Transformations are 2x2 matrices • But translation can’t be represented this way Scaling Rotation = 6.831 UI Design and Implementation

  14. x+dx y+dy 1 x y 1 x y 1 x y 1 sxx syy 1 cosθ sinθ 0 -sinθ cosθ 0 0 0 1 1 0 dx 0 1 dy 0 0 1 sx 0 0 0 sy 0 0 0 1 Homogeneous Transforms • We can represent all three transforms as matrices if points are three-element vectors [x,y,1] Translation = Scaling Rotation = 6.831 UI Design and Implementation

  15. Common Mistakes in Using Transforms • Transforms affect later drawing, not the current contents of the drawing surface drawImage(“bunny.jpg”) scale(2, 2) • Transforms are not commutative translate(50,50) scale(2,2) scale(2, 2) translate(25,25) 6.831 UI Design and Implementation

  16. x y 1 x y 1 1 0 -ox 0 1 -oy 0 0 1 sx 0 -sxox+ox/sx 0 sy -syoy+oy/sy 0 0 1 1 0 ox/sx 0 1 oy/sy 0 0 1 sx 0 0 0 sy 0 0 0 1 Combining Multiple Transforms • Scaling around a point (ox,oy) 1. Move the point back to the origin translate(-ox,-oy) 2. Scale relative to the new origin scale(sx, sy) 3. Move the point back (using the new scale) translate(ox/sx, oy/sy) = 6.831 UI Design and Implementation

  17. Some Applications of Transforms • Clock face draw circle(0,0,2r,2r) translate(r, r) for i = 0 to 11 { draw line (r-k, 0, r, 0) rotate(2π/12) } 6.831 UI Design and Implementation

  18. Some Applications of Transforms • Standard Cartesian origin translate(0, height) scale(1, -1) 6.831 UI Design and Implementation

  19. Some Applications of Transforms • Drawing in inches rather than pixels dpi = pixels per inch scale(dpi, dpi) 6.831 UI Design and Implementation

  20. Clipping • Rectangular clipping regions setClip(x,y,w,h) drawString(“hello”) • Stroke-based clipping setClip(new Circle(x, y, w, h)) drawString(“hello”) • Pixel-based clipping drawImage(“bunny.jpg”) setComposite(src in dst) drawString(“hello”) hello hello hello 6.831 UI Design and Implementation

  21. Component Model Effects • Changing Graphics passed to children • Transforms: rotation, zooming • Clipping: setting new clipping regions • Wrapping Graphics passed to children • Intercept child calls and modify or capture them • Painting onto offscreen images and then transforming the images • Blur, shimmer, masking • Using components as rubber stamps • Table, list, and tree cell renderers 6.831 UI Design and Implementation

  22. Scene Graphs • Traditional 2D toolkits are limited in many ways • View hierarchy is a tree (can’t share views) • Parents must enclose descendents (and clip them) • Parents translate children, but don’t otherwise transform them • Piccolo toolkit (designed for zooming user interfaces) • View hierarchy is actually a graph, not merely a tree • Components can translate, rotate, scale their children • Parents transform but don’t clip their children by default • Input events and repaint requests are transformed too 6.831 UI Design and Implementation

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