#16: Computer Cinematography: The Toy Story Lighting Model

1. #16: Computer Cinematography:The Toy Story Lighting Model CSE167: Computer Graphics Instructor: Ronen Barzel UCSD, Winter 2006

2. What makes film look different from home movie? Camera Lighting Not trying for “photorealism” Lighting in cinematography Contribute to storytelling Set mood Compose image Direct viewer’s eye Film Cinematography

3. Practical lights: lights that are visible onscreen e.g. desk lamps, ceiling lamps, flashlights, fires, … rarely major contributors to illumination Main lights and spotlights placed off-camera To create desired illumination effect Standard three-point lighting setup: Key light -- provides major source of illumination & shadow Fill Light -- fills in dark areas & shadows made by key Back light -- creates subtle highlights around edges helps give sense of 3D depth separates subject from background Real-World Film Lighting

4. Cheat as necessary: Suspend sheet in front of light to soften shadows Position opaque cards or graded filters to shape light Focus narrow “tickler” lights to get extra highlights Hide “kicker” light under a desk to fill dark areas under character’s chin etc… Real-World Film Lighting Trickery

5. We’re not looking for realism real-world lighting is constrained by reality, CG isn’t Things that can’t be done in real life lights coming out of nowhere lights that act on only certain objects decoupling lights and shadows etc. Real life does some things easier than standard CG: soft shadows soft area lights global illumination techniques help this Computer Graphics Trickery

6. Several independent aspects selection shape shadowing texture dropoff direction properties Lighting Model

7. only fill lights Basic setup

8. (will talk about the beam-of-light effect later) One key light (will talk about the beam of light effect later)

9. Selection • torus unaffected by key light

10. Region of space affected by light Real-world commonly use spotlights, barn door lights CG model: Generalized cone/pyramid Soft edges Cuton and cutoff Shape

11. Generalized cone/pyramid • A “superellipse” shape swept into a pyramid • superellipse varies between circle and rectangle • pyramid may be truncated or sheared

12. 2-dimensional superellipse: n=1 is ordinary ellipse or circle need to handle 4 quadrants carefully: avoid exponentiation of negative numbers use 2D Superellipse

13. Aside: 3D Superellipsoid

14. Returning to lighting… • A 2D superellipse swept into a pyramid • pyramid may be truncated or sheared

15. Shape: Rounded Rectangle

16. Shape: Sheared Barn Door

17. Soft Edges • Full intensity in central pyramid • No effect (zero intensity) outside outer pyramid • Smooth falloff between them

18. Shape: Hard Edge

19. Shape: Soft Edges

20. Want to transition between two values E.g. in a shader, transition between two colors on surface E.g. inside/outside light shape Want function to provide control value between 0 to 1: Step Transitions

21. Vary linearly between a and b: Simple, quick Not C1-continuous Abrupt Can cause Mach banding Linear Step 1 0 s a b

22. Cubic curve: Tangent is horizontal at start and end C1-continuous Special-case of a Hermite Spline: Cubic spline defined by two interpolating points and two tangents In RenderMan shading language library “SmoothStep” 1 0 s a b

23. Shape: Cutoff • Truncate bottom of pyramid • smooth transition zone (fairly sharp in the example)

24. Shape: Cuton • Truncate top of pyramid • smooth transition zone (gradual in the example)

25. Shadows & shadow placement important for cinematography Can control shadows for artistic effect Think of shadow as “volume of darkness” illumination inhibited inside the volume Control: Shadow selection Shadow direction Shadow sharing Fake shadows Shape trimming Shadow softening Techniques can be implemented using shadowmaps & procedural shading Shadowing

26. Basic Shadows

27. Shadow Selection • Cube casts no shadow

28. Shadow Direction • move shadowmap camera away from light position to control where shadows fall • surprisingly acceptable, especially for shadows cast on distant surface

29. Shadow Sharing • use one light’s shadowmap for other lights • prevent other lights from washing out important shadows

30. Fake shadows • Place a virtual shadow-casting superellipse in space. known as a “blocker” • (Real-world, place opaque cards in front of light)

31. Shape Trimming • Make large blocker, place it to trim the shape of the light • Can animate blocker for effects like door opening offscreen

32. Effect of shadow on light doesn’t need to be binary on/off: Only partially inhibit the light (scale down rather than remove) Soften edges shadowmap edge blurring techniques smooth falloff in shadow blockers Rather than inhibiting light, lerp to a different color e.g. can be effective to lerp to dark blue default: lerp to black gives normal shadowing Shadow Softening

33. We use images to create texture on surfaces Can also use images to create texture in lighting, via projection Cookie: single-channel matte used as a “cookie cutter” (a.k.a. cucaloris) changes shape breaks up light Slide: color image used as a slide projector can blur based on distance for getting unfocused projection (e.g. from a TV) Noise: 2D procedural noise makes light “dirty” Texture

34. Texture files

35. Cookie changing light shape

36. Cookie breaking up light

37. Projecting a color slide

38. The intensity of the light can vary across the shape Beam distribution cosinen across beam Distance falloff inverse-linear, inverse-square, etc. Usually only used for practical light effects Choosing dropoff exponent parameters not intuitive Easier to adjust lighting using soft shape, cutoff, blockers Intensity distribution

39. Uniform Intensity

40. Beam Distribution

41. Distance Falloff

42. Two common CG options: radial from apex of pyramid parallel Subtle effect on shading and highlights effect more noticeable as light source is close to object Light Ray Direction

43. Properties of the “photons” that are incident on the surfaces Intensity: nominal intensity at a target point within the shape Attenuated by: shape boundary, shadows, cookies, noise, dropoff Color: nominal RGB color of the light Filtered by: slide, noise, colored shadow Effect: how much light contributes to CG illumination model Separate scale factor for ambient, diffuse, specular Fill lights typically ambient-only, to avoid unwanted specular hilights Other… depending on what the surface shading model can handle “ultraviolet”4th color channel. “fluorescent” surface responds by self-illuminating Lights that don’t actually light… Surface can react specially to specific “light” types: lights whose effect is to attenuate other lights lights that affect surface properties e.g. use a slide to project a bump map? etc… Light Properties

44. Example 1

45. Light shape/placement for image composition Barn door light frames character Nominally shines from offscreen window Placed for visual effect, not necessarily consistent with window Softened (partial opacity) shadows of bed on wall Example 1 notes

46. Example 2

47. Strips of light on the characters Not actually cast by the venetian blind slats Generated using a cookie, adjusted for best effect Slats are lit from below to get desired glow Example 2 notes

48. Example 3

49. Cookie (the one from slide 33) used for dappled leaves Separate cone-shaped spotlight w/cookie on soldier in rear Characters in foreground lit by blue light for blue highlights Practical light source: red LED Example 3 notes

50. Example 4