16 Computer Cinematography The Toy Story Lighting Model by ezw15872

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




                                                      1
Real-World Film Lighting
   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



                                                                      2
Real-World Film Lighting Trickery
    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…




                                                                                 3
Computer Graphics Trickery
   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
                                                                  4
Lighting Model
   Several independent aspects
       selection
       shape
       shadowing
       texture
       dropoff
       direction
       properties




                                  5
Basic setup




   only fill lights
                       6
One key light
 (will talk about the beam-of-light effect later)




 (will talk about the beam of light effect later)
                                                    7
Selection




   torus unaffected by key light
                                    8
Shape
   Region of space affected by light
   Real-world commonly use spotlights, barn door lights
   CG model:
       Generalized cone/pyramid
       Soft edges
       Cuton and cutoff




                                                           9
Generalized cone/pyramid




   A “superellipse” shape swept into a pyramid
        superellipse varies between circle and rectangle
        pyramid may be truncated or sheared

                                                            10
2D Superellipse
   2-dimensional superellipse:
            x   a cos n ( ) 
            y    b sin n ( )  , where 0    2 and 0  n  
                               
                  2n           2n
            x          y
                               1
            a          b
                                     n2
                   x2 n
           y  b 1    
                   a 




        n=1 is ordinary ellipse or circle
        need to handle 4 quadrants carefully:
          • avoid exponentiation of negative numbers
                         n
          • use x n   x if x  0

                                                                       11
Aside: 3D Superellipsoid



   x   a cos 1 ( )cos 2 ( ) 
                 n           n

   y    b cos n1 ( )sin n2 ( ) 
                                  
  z 
             csin ( )
                       n1
                                     
                                     
  where 0    2
   and -    
         2
                      
                      2

   and      0n




                                         12
Returning to lighting…




    A 2D superellipse swept into a pyramid
         pyramid may be truncated or sheared


                                                13
Shape: Rounded Rectangle




                           14
Shape: Sheared Barn Door




                           15
Soft Edges




   Full intensity in central pyramid
   No effect (zero intensity) outside outer pyramid
   Smooth falloff between them

                                                       16
Shape: Hard Edge




                   17
Shape: Soft Edges




                    18
Step Transitions
    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:
         Function step(s, a,b) for a  b
         Should return 0 for s  a
         Should return 1 for s  b
         Should vary smoothly between them
         Can use, e.g., to transition between two colors:
                 Lerp(step(s, a,b),Color1 ,Color2 )




                                                                       19
Linear Step
    Vary linearly between a and b:
                           0,      sa
                          s  a
                          
     linearstep(s, a,b)        , asb
                          b  a
                           1
                                   bs

         Simple, quick
         Not C1-continuous
           • Abrupt
           • Can cause Mach banding

                                           1



                                           0
                                               a   b   s

                                                           20
“SmoothStep”
   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

                                          0,               sa
                                        3             2
                              s  a         s  a
        smoothstep(s,a,b)  2            3          , asb
                                b  a
                                              b  a
                                                     
                                          1                bs
                            

                                                  1
        In RenderMan shading language library

                                                  0
                                                             a     b        s

                                                                                 21
Shape: Cutoff




    Truncate bottom of pyramid
         smooth transition zone (fairly sharp in the example)   22
Shape: Cuton




    Truncate top of pyramid
         smooth transition zone (gradual in the example)   23
Shadowing
    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




                                                                           24
Basic Shadows




                25
Shadow Selection




    Cube casts no shadow
                            26
Shadow Direction




    move shadowmap camera away from light position to control where shadows fall
       surprisingly acceptable, especially for shadows cast on distant surface     27
Shadow Sharing




    use one light’s shadowmap for other lights
       prevent other lights from washing out important shadows   28
Fake shadows




    Place a virtual shadow-casting superellipse in space. known as a “blocker”
       (Real-world, place opaque cards in front of light)                        29
Shape Trimming




    Make large blocker, place it to trim the shape of the light
       Can animate blocker for effects like door opening offscreen   30
Shadow Softening
    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




                                                                             31
Texture
    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”




                                                                                            32
Texture files




                33
Cookie changing light shape




                              34
Cookie breaking up light




                           35
Projecting a color slide




                           36
Intensity distribution
    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




                                                                         37
Uniform Intensity




                    38
Beam Distribution




                    39
Distance Falloff




                   40
Light Ray Direction
    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




                                                                      41
Light Properties
    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…




                                                                                                     42
Example 1




            43
Example 1 notes
   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




                                                                           44
Example 2




            45
Example 2 notes
    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




                                                               46
Example 3




            47
Example 3 notes
    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




                                                                      48
Example 4




            49
Example 4 notes
    Practical light source: Flashlight
         Cookie for lens ring effect
         Two light sources:
            • one illuminates character
            • one generates fog effect, with cutoff to avoid obscuring character
    “Ultraviolet” light
         white shirt responds with blue glow




                                                                                   50
Example 5




            51
Example 5 notes
    Shadow placement. Milk crate shadows adjusted so that
         “X” lands on Woody’s face
         Doesn’t obscure eyes or mouth
    Blockers darken background
         Darkens mood
         Focuses attention on face in the foreground




                                                             52
Example 6




            53
Example 6 notes
    Soft edges of key light
    Buzz’s shadow direction adjusted for compositional effect
    Grazing-angle light on desk only, accentuates its texture
    Separate lights on Buzz only for extra highlighting
    Cookie animates from one frame to next for falling-rain effect




                                                                      54
Fog in computer graphics
     Simple version: depth cueing
          After lighting computation
          Blend between lit surface color and constant fog color, based on depth
          Typically have a “near fog plane” then linear or exponential fogging with distance
          Compute in pixel shaders                             dz
          Supported by some hardware                Lerp(e ,C ,C )    lit    fog
          No beam of light effects




              QuickTime™ and a                                 QuickTime™ and a
     TIFF (Uncompressed) decompressor                TIFF (Un compressed) decompressor
                                                        are neede d to see this picture.
        are need ed to see this picture.




                                                                                                55
Atmospheric effects
    Participating Medium
         Light bounces off air particles
         Some light gets absorbed by air particles
         Full simulation of multiple scattering is possible, using monte carlo techniques
           • Gets best results for clouds, etc.


    Simple approximation: ray marching, single scattering
         (Used for examples today)
         Start at surface, with lit color
         Take incremental steps along ray towards eye
         At each step:
           •   attenuate existing color / merge towards fog color
           •   look up lighting (with shadows)
           •   compute amount of light scattered towards eye
           •   add to current color
         Can be implemented in pixel shader
         Some systems support procedural volume shaders to run after surface shading

                                                                                             56
Real-time clouds
    Various approximation techniques.
         Billboards, transparency, frustum culling, …
         Video shows some of the acceleration techniques Steve talked about last class
         [play video]




                                                                                          57
Done
   Next class:
        Guest Lecture: Daniel Maskit, Digital Domain “A View From The Trenches”




                                                                                   58

								
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