CSE452 Computer Graphics

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1 CSE45 Computer Graphics Lecture 8: Computer Projection CSE45 Lecture 8: Computer Projection 1

2 Review In the last lecture We set up a Virtual Camera Position Orientation Clipping planes Viewing angles Orthographic/Perspective We are ready to project! U P d n d f L q w q h d f w h d n L U P CSE45 Lecture 8: Computer Projection

3 Preview CSE45 Lecture 8: Computer Projection 3 Near plane Film plane Far plane Viewpor t

4 Preview The perspective view frustum (i.e., a truncated pyramid) is non-trivial to clip against CSE45 Lecture 8: Computer Projection 4

5 How do you clip? CSE45 Lecture 8: Computer Projection 5

6 How do you clip? Check against six planes? Project to a film plane and check against near and far planes? An elegant solution that does clipping and also projection together CSE45 Lecture 8: Computer Projection 6

7 Preview We first transform the frustum to a canonical volume CSE45 Lecture 8: Computer Projection 7

8 Canonical View Volume X CSE45 Lecture 8: Computer Projection 8

9 Canonical View Volume X 1 CSE45 Lecture 8: Computer Projection 9

10 Canonical View Volume {-1,1,1} {-1,1,0} Far Clipping Plane {-1,-1,1} {1,1,1} {1,-1,1} X 1 {1,1,0} {1,-1,0} Near Clipping Plane Projectors (parallel to ) CSE45 Lecture 8: Computer Projection 10

11 Canonical View Volume Canonical view volume makes things easy: Easy clipping: Clip against the coordinates range X -1<x<1, -1<y<1, 0<z<1 1 CSE45 Lecture 8: Computer Projection 11

12 Canonical View Volume Canonical view volume makes things easy: Easy clipping: Clip against the coordinates range -1<x<1, -1<y<1, 0<z<1 X 1 Easy projecting: drop the coordinate! (because viewing plane is the X plane, and projectors are parallel to axis) (Xc, c, c) Coordinates in the canonical volume (Xc, c) Projected D coordinates CSE45 Lecture 8: Computer Projection 1

13 Viewing Transformation The transformation that warps the perspective frustum to the canonical view volume Transforms world coordinates {x w,y w,z w } into canonical coordinates {x c,y c,z c } U L q w P d n {x w,y w,z w } q h {x c,y c,z c } X d f 1 CSE45 Lecture 8: Computer Projection 13

14 Camera Coordinate System First, let s setup a coordinate system for the camera Origin at the camera Three axes: right (u), straight-up (v), negative look (n) Unit vectors forming an orthonormal, right-hand basis n u v P U L CSE45 Lecture 8: Computer Projection 14

15 Camera Coordinate System Computing n Opposite to look vector L, normalized n = - L L n L P CSE45 Lecture 8: Computer Projection 15

16 Camera Coordinate System Computing v Projection of up vector U onto the camera plane, normalized n v P U L CSE45 Lecture 8: Computer Projection 16

17 Camera Coordinate System Computing v Projection of up vector U onto the camera plane, normalized n v P U L CSE45 Lecture 8: Computer Projection 17

18 Camera Coordinate System Computing u Cross product of v and n n u v P U L CSE45 Lecture 8: Computer Projection 18

19 Camera Coordinate System Computing u Cross product of v and n n u v P U L CSE45 Lecture 8: Computer Projection 19

20 Camera Coordinate System Summary Three axes, computed from look vector L and up vector U: n v U L u,v,n form a camera coordinate system u P CSE45 Lecture 8: Computer Projection 0

21 Computing Viewing Transformation Two steps Step 1: align camera coordinate system P,u,v,n with world coordinate system O,X,, Step : scale and stretch the frustum to the cuboid CSE45 Lecture 8: Computer Projection 1

22 Step1 X 1 v n u CSE45 Lecture 8: Computer Projection

23 Step1 v 1 X n u CSE45 Lecture 8: Computer Projection 3

24 Step1 n v u 1 X CSE45 Lecture 8: Computer Projection 4

25 Step 1 First, translate the eye point P to the origin Let P have coordinates (p x,p y,p z ) O (P) CSE45 Lecture 8: Computer Projection 5

26 Step 1 (cont) Then, rotate the three axes u,v,n to X,, Let s set up the equation to solve for the rotation matrix (R): Note the homogenous coordinates for a vector ends with 0! In matrix form: R u x v x n x 0 u y v y n y 0 u z v z n z = CSE45 Lecture 8: Computer Projection 6

27 Step 1 (cont) This is a matrix inversion problem: u x v x n x 0 R = M - 1 u where M = y v y n y 0 u z v z n z Since M is an orthonormal matrix: R = M T CSE45 Lecture 8: Computer Projection 7

28 Step 1 - Done Eye point at origin, looking down negative z axis (v) (n) (P) (u) X 1 CSE45 Lecture 8: Computer Projection 8

29 Step X 1 CSE45 Lecture 8: Computer Projection 9

30 Step Some preparations First, make width/height angles to be π/ Non-uniform scaling in X, coordinates q h h h 1 A look down the X axis CSE45 Lecture 8: Computer Projection 30

31 Step Some preparations First, make width/height angles to be π/ Non-uniform scaling in X, coordinates q h h h 1 A look down the X axis CSE45 Lecture 8: Computer Projection 31

32 I hate cotangent, arctan, cosecant... Why call arctan? According to wikipedia the arc whose cosine is x the angle whose cosine is x is the same as CSE45 Lecture 8: Computer Projection 3

33 Step (cont) Some preparations Next, push the far plane from =-d f to =-1 Uniform scaling in all three coordinates S xyz = 1 df df df d f 1 CSE45 Lecture 8: Computer Projection 33

34 Step (cont) Where we are now: 1 -d n /d f -1-1 A look down the X axis (same picture when looking down ) CSE45 Lecture 8: Computer Projection 34

35 Step (cont) Perspective transformation Stretching and flipping the truncated pyramid to the cuboid Change range: ([-d n /d f,-1] -> [0,1]) Stretch in X plane: Non-uniform stretching based on z d n /d f -1 A look down the X axis CSE45 Lecture 8: Computer Projection 35

36 Homogeneous coordinate revisited is deep and has interesting meanings... In short (x, y, z, 1) and (x, y, z, ) are the same point. (x, y, z, 1) and (-3x, -3y, -3z, -3) are the same point. CSE45 Lecture 8: Computer Projection 36

37 Step (cont) Perspective transformation D = k k k- 1, where k = d n d f Applying D to homogeneous coordinates: Converting from homogeneous coordinates {x,y,z,w} to Cartesian coordinates (divide x,y,z by w) CSE45 Lecture 8: Computer Projection 37

38 Step (cont) Perspective transformation 1 -k A look down the X axis CSE45 Lecture 8: Computer Projection 38

39 Putting Together Translation: Rotation: Scaling: T R S xy, S xyz Perspective transformation: D World-to-camera transformation (v) (P) (n) (u) T, R S xy, S xyz, D X 1 CSE45 Lecture 8: Computer Projection 39

40 Putting Together Complete viewing transformation to bring a point canonical volume: q' = D S xyz S xy R T q q to the (v) T, R (n) (P) (u) S xy, S xyz, D X 1 CSE45 Lecture 8: Computer Projection 40

41 Clipping After transformation into the canonical volume, each object will be clipped against 6 cuboid faces. Point clipping: checking coordinates range Edge clipping: computing line/plane intersections We will discuss a D version in next lecture. Triangle clipping: can be done by line/plane intersections CSE45 Lecture 8: Computer Projection 41

42 Projecting Dropping z coordinate Resulting points have range: -1<=x<=1, -1<=y<=1 X 1 CSE45 Lecture 8: Computer Projection 4

43 Viewport Transform Get viewport (pixel) coordinates Viewport coordinate {0,0} is at top-left corner (0,0) X 1 (0,b-1) (a-1,0) (a-1,b-1) If the viewport is a pixels wide and b pixels high, what is the pixel coordinates for a projected point {x,y}? CSE45 Lecture 8: Computer Projection 43

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