3D Rendering and Ray Casting

Transcription

1 3D Rendering and Ray Casting Michael Kazhdan ( /657) HB Ch. 13.7, 14.6 FvDFH 15.5, 15.10

2 Rendering Generate an image from geometric primitives Rendering Geometric Primitives (3D) Raster Image (2D)

3 3D Rendering Example What issues must be addressed by a 3D rendering system?

4 Overview 3D scene representation 3D viewer representation What do we see? How does it look?

5 Overview 3D scene representation 3D viewer representation What do we see? How does it look? How is the 3D scene described in a computer?

6 3D Scene Representation Scene is usually approximated by 3D primitives Point Line segment Polygon Polyhedron Curved surface Solid object etc.

7 3D Point Specifies a location Origin

8 3D Point Specifies a location Represented by three coordinates Infinitely small struct Point3D { float x, y, z; }; (x, y, z) Origin

9 3D Vector Specifies a direction and a magnitude

10 3D Vector Specifies a direction and a magnitude Represented by three coordinates Magnitude v = dx 2 + dy 2 + dz 2 Has no location struct Vector3D { float dx, dy, dz; }; v = (dx, dy, dz)

11 3D Vector Specifies a direction and a magnitude Represented by three coordinates Magnitude v = dx 2 + dy 2 + dz 2 Has no location Dot product of two 3D vectors v 1, v 2 = dx 1 dx 2 + dy 1 dy 2 + dz 1 dz 2 v 1, v 2 = v 1 v 2 cos θ Cross product of two 3D vectors v 1 v 2 = Vector normal to plane v 1, v 2 v 1 v 2 = v 1 v 2 sin θ θ v 1 = (dx 1, dy 1, dz 1 ) v 2 = (dx 2, dy 2, dz 2 )

12 Cross Product: Review Let v 1 = v 2 v 3 : dx 1 = dy 2 dz 3 dz 2 dy 3 dy 1 = dz 2 dx 3 dx 2 dz 3 dz 1 = dx 2 dy 3 dy 2 dx 3 v w = w v (remember right-hand rule) We can show: v w = v w sin θ n, where n is unit vector normal to v and w v v = 0

13 3D Line Segment Linear path between two points Origin

14 3D Line Segment Use a linear combination of two points Parametric representation:» p t = p 1 + t (p 2 p 1 ), (0 t 1) p 2 struct Segment3D { Point3D p1, p2; }; p 1 Origin

15 3D Ray Line segment with one endpoint at infinity Parametric representation:» p t = p 1 + t v, (0 t < ) struct Ray3D { Point3D p1; Vector3D v; }; p 1 v Origin

16 3D Line Line segment with both endpoints at infinity Parametric representation:» p t = p 1 + t v, ( < t < ) struct Line3D { Point3D p1; Vector3D v; }; p 1 v Origin

17 3D Plane A linear combination of three points p 2 p 3 p 1 Origin

18 3D Plane A linear combination of three points Implicit representation:» Φ p = ax + by + cz d = 0» Φ p = p, n d = 0, or struct Plane3D { Vector3D n; float d; }; p 2 n is the plane normal» (May be) unit-length vector» Perpendicular to plane d is the signed (weighted) distance of the plane from the origin. p 1 n = (a, b, c) d p 3 Origin

19 3D Polygon Area inside a sequence of coplanar points Triangle Quadrilateral Convex Star-shaped Concave Self-intersecting struct Polygon3D { Point3D *points; int npoints; }; Points are in counter-clockwise order Holes (use > 1 polygon struct)

20 3D Sphere All points at distance r from center point c = (c x, c y, c z ) Implicit representation:» Φ p = p c 2 r 2 = 0 Parametric representation:» x φ, θ = r cos φ sin θ + c x» y φ, θ = r cos φ sin θ + c y» z θ, φ = r sin φ + c z c r struct Sphere3D { Point3D center; float radius; }; Origin

21 Other 3D primitives Cone Cylinder Ellipsoid Box Etc.

22 3D Geometric Primitives More detail on 3D modeling later in course Point Line segment Polygon Polyhedron Curved surface Solid object etc.

23 Overview 3D scene representation 3D viewer representation What do we see? How does it look? How is the viewing device described in a computer?

24 Camera Models The most common model is pin-hole camera All captured light rays arrive along paths toward focal point without lens distortion (everything is in focus) View plane Other models consider... Depth of field Motion blur Lens distortion Eye position (focal point)

25 Camera Parameters What are the parameters of a camera?

26 Camera Parameters Position Eye position: Point3D eye Orientation View direction: Vector3D view Up direction: Vector3D up Aperture Field of view: float xfov, yfov Film plane (View plane normal) back right Up direction Eye Position View Plane Look at Point

27 Other Models: Depth of Field Close Focused Distance Focused P. Haeberli

28 Other Models: Motion Blur Mimics effect of open camera shutter Gives perceptual effect of high-speed motion Generally involves temporal super-sampling Brostow & Essa

29 Traditional Pinhole Camera The film sits behind the pinhole of the camera. Rays come in from the outside, pass through the pinhole, and hit the film plane. Film Plane Pinhole

30 Traditional Pinhole Camera The film sits behind the pinhole of the camera. Rays come in from the outside, pass through the pinhole, and hit the film plane. Photograph is upside down Film Plane Pinhole

31 Virtual Camera The film sits in front of the pinhole of the camera. Rays come in from the outside, pass through the film plane, and hit the pinhole. Pinhole Film Plane

32 Virtual Camera The film sits in front of the pinhole of the camera. Rays come in from the outside, pass through the film plane, and hit the pinhole. Photograph is right side up Pinhole Film Plane

33 Overview 3D scene representation 3D viewer representation Ray Casting Where are we looking? What do we see? How does it look?

34 Ray Casting For each sample Where: Construct ray from eye through view plane What: Find first surface intersected by ray through pixel How: Compute color sample based on surface radiance

35 Ray Casting Simple implementation: Image RayCast(Camera camera, Scene scene, int width, int height) { Image image = new Image(width, height); for( int i=0 ; i<width ; i++ ) for( int j=0 ; j<height ; j++ ) { Ray ray = ConstructRayThroughPixel( camera, i, j ); Intersection hit = FindIntersection( ray, scene ); image[i][j] = GetColor( hit ); } } return image; }

36 Ray Casting Where? Image RayCast(Camera camera, Scene scene, int width, int height) { Image image = new Image(width, height); for( int i=0 ; i<width ; i++ ) for( int j=0 ; j<height ; j++ ) { Ray ray = ConstructRayThroughPixel( camera, i, j ); Intersection hit = FindIntersection( ray, scene ); image[i][j] = GetColor( hit ); } } return image; }

37 Constructing a Ray Through a Pixel Up direction View Plane back p 0 right v p i [j]

38 Constructing a Ray Through a Pixel Up direction View Plane back p 0 right v The ray has to originate at p 0 (the position of the camera). So the equation for the ray is: Ray t = p 0 + t v p i [j]

39 Constructing a Ray Through a Pixel Up direction View Plane back p 0 right v If the ray passes through the point p i [j], then the solution for v is: v = p i [j] p 0 p i [j] p 0 p i [j]

40 Constructing a Ray Through a Pixel Up direction View Plane back p 0 right v p i [j] If p i [j] represents the (i, j)-th pixel of the image, what is its position?

41 Constructing Ray Through a Pixel 2D Example: Side view of camera at p 0 Where is the i-th pixel, p[i]? (i [0, height)) θ = frustum half-angle (given), or field of view d = distance to view plane (arbitrary = you pick) up p 0 θ towards d

42 Constructing Ray Through a Pixel 2D Example: Side view of camera at p 0 Where is the i-th pixel, p[i]? (i [0, height)) θ = frustum half-angle (given), or field of view d = distance to view plane (arbitrary = you pick) p 2 p 1 = p 0 + d towards d tan θ up p 2 = p 0 + d towards + d tan θ up p 0 up θ towards d 2 d tan θ p 1

43 Constructing Ray Through a Pixel 2D Example: Side view of camera at p 0 Where is the i-th pixel, p[i]? (i [0, height)) θ = frustum half-angle (given), or field of view d = distance to view plane (arbitrary = you pick) p 2 p 1 = p 0 + d towards d tan θ up p 2 = p 0 + d towards + d tan θ up p 0 up θ towards d 2 d tan θ p i = p 1 + = p 1 + i height i height p 2 p 1 2 d tan θ up p[i] p 1

44 Constructing Ray Through a Pixel 2D Example: The ray passing through the i-th pixel is defined by: Ray t = p 0 + t v p 0 : camera position v: direction to the i-th pixel: v = p i p 0 p i p 0 p 0 θ p[i]: i-th pixel location: i p i = p 1 + p height 2 p 1 p 1 and p 2 are the endpoints of the view plane: p 1 = p 0 + d towards d tan θ up p 2 = p 0 + d towards + d tan θ up up towards d p[i] p 2 p 1 2 d tan θ

45 Constructing Ray Through a Pixel Figuring out how to do this in 3D is assignment 2 p 2 p 0 up θ towards d 2 d tan θ p[i] p 1

46 Ray Casting Where? Image RayCast(Camera camera, Scene scene, int width, int height) { Image image = new Image(width, height); for( int i=0 ; i<width ; i++ ) for( int j=0 ; j<height ; j++ ) { Ray ray = ConstructRayThroughPixel( camera, i, j ); Intersection hit = FindIntersection( ray, scene ); image[i][j] = GetColor( hit ); } } return image; }

47 Ray Casting What? Image RayCast(Camera camera, Scene scene, int width, int height) { Image image = new Image(width, height); for( int i=0 ; i<width ; i++ ) for( int j=0 ; j<height ; j++ ) { Ray ray = ConstructRayThroughPixel( camera, i, j ); Intersection hit = FindIntersection( ray, scene ); image[i][j] = GetColor( hit ); } } return image; }

48 Ray-Scene Intersection Intersections with geometric primitives Sphere Triangle

49 Ray-Sphere Intersection Ray: p t = p 0 + t v, (0 t < ) Sphere: Φ p = p c 2 r 2 = 0 p p p 0 v r c

50 Ray-Sphere Intersection Ray: p t = p 0 + t v, (0 t < ) Sphere: Φ p = p c 2 r 2 = 0 Substituting for p(t), we get: Φ t = p 0 t v c 2 r 2 = 0 p 0 v p r c p

51 Ray-Sphere Intersection Ray: p t = p 0 + t v, (0 t < ) Sphere: Φ p = p c 2 r 2 = 0 Substituting for p(t), we get: Φ t = p 0 t v c 2 r 2 = 0 Solve quadratic equation: a t 2 + b t + c = 0 where: a = 1 b = 2 v, p 0 c c = p 0 c 2 r 2 p 0 v p r c p

52 Ray-Sphere Intersection Ray: p t = p 0 + t v, (0 t < ) Sphere: Φ p = p c 2 r 2 = 0 Substituting for p(t), we get: Φ t = p 0 t v c 2 r 2 = 0 Solve quadratic equation: a t 2 + b t + c = 0 where: a = 1 b = 2 v, p 0 c p 0 v Generally, there are two solutions to the c = quadratic p 0 c 2 equation, r 2 giving rise to points p and p. Want to return the first positive hit. p r c p

53 Ray-Sphere Intersection Need normal vector at intersection for lighting calculations: p c n = p c n p 0 v p r c

54 Ray-Scene Intersection Intersections with geometric primitives Sphere» Triangle

55 Ray-Triangle Intersection First, intersect ray with plane Then, check if point is inside triangle p v p 0

56 Ray-Plane Intersection Ray: p t = p 0 + t v, (0 t < ) Plane: Φ p = p, n d = 0 Substituting for P, we get: Φ t = p 0 + t v, n d = 0 Algebraic Method Solution: t = p 0, n d v, n v p n p 0

57 Ray-Triangle Intersection I Check if point is inside triangle algebraically: Generate triangles by connecting the ray source to each edge Check if the point of intersection is above each of these triangles For each side of triangle v 1 = T 1 p 0 v 2 = T 2 p 0 n 1 = v 2 v 1 if ( p p 0, n 1 < 0 ) return FALSE; v 1 T 1 n 1 v 2 p T 3 T 2 p 0

58 Ray-Triangle Intersection II Check if point is inside triangle parametrically A point p is inside the triangle iff. it can be expressed as the weighted average of the corners: p = α T 1 + β T 2 + γ T 3 where: 0 α, β, γ 1 α + β + γ = 1 α T 1 p γ T 3 β T2

59 Ray-Triangle Intersection II Check if point is inside triangle parametrically Solve for α, β, γ such that: p = α T 1 + β T 2 + γ T 3 And α + β + γ = 1 γ T 3 Check if the point is in the triangle: 0 α, β, γ 1 T 1 α p β v T2 p 0