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CS 354 Shadow Generation; University of Texas at Austin; March 20, 2012

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1. 1. CS 354ShadowsMark KilgardUniversity of TexasMarch 20, 2012
2. 2. CS 354 2 Today’s material  In-class quiz  On GPU architecture lecture  Lecture topic  Shadow Algorithms  Course status  Mid-term graded, but not yet recorded  Project #2 on texturing, shading, & lighting is coming
3. 3. CS 354 3 My Office Hours  Tuesday, before class  Painter (PAI) 5.35  8:45 a.m. to 9:15  Thursday, after class  ACE 6.302  11:00 a.m. to 12  Randy’s office hours  Monday & Wednesday  11 a.m. to 12:00  Painter (PAI) 5.33
4. 4. CS 354 4 Last time, this time  Last lecture, we  Took the mid-term  This lecture  Shadow generation algorithms  How can we efficiently determine when and how much light is occluded by other objects during shading?
5. 5. CS 354 5 On a sheet of paper Daily Quiz • Write your EID, name, and date • Write #1, #2 followed by its answer  Multiple choice: The effective  Multiple choice: Programmable memory bandwidth of a tessellation is good for graphics processor can be higher than its raw memory a) displacement mapping bandwidth due to: b) per-pixel lighting a) more multiply-add operations per clock c) increasing the appearance of surface curvature b) not showing pixels when they are opaque d) feeding more triangles to the graphics pipeline c) domain-specific compression techniques e) a, c, and d d) off-loading work to the f) a, b, c, and d network g) b, c, and d
6. 6. CS 354 6 Shadows  Important visual cue  All real world scenes have shadows  Lack of shadows = big clue that an image is computer generated!  Occlusion from light’s point-of-view instead of viewer’s  Cue for light-object-object relationships  Artistic effect  Soft vs. hard feel, dramatic or spooky feel
7. 7. CS 354 7 Shadows Provide Cues About Depth  Two scenes  Difference is in shadows Balls resting on ground Balls floating off surface
8. 8. CS 354 8 Computed Shadows aren’t Necessarily Realistic Shadows of Past – Lyubomir Bukov
10. 10. CS 354 10 Ray Tracing Approach to Shadows  Shadows = global lighting effect  Involves more than light + surface
11. 11. CS 354 11 Ray Tracing Shadow Algorithm  Cast primary rays from eye  Determine surface intersection  Then, cast a secondary (shadow) ray  From surface  To each light source
12. 12. CS 354 12 Interactive Lighting Models  Typically lack shadow support  So-called “local” lighting models ignore occlusion of light sources that create shadows  Except trivial self-shadowing (i.e., clamping L•N)  Global lighting models account for shadows  Examples: radiosity, ray tracing  But too expensive for interactive use  Interactive shadow algorithms typically operate independent of local lighting models
13. 13. CS 354 13 Interactive Shadow Simplications  True shadows are expensive, cut corners  Simple geometry tricks  Projection  Volume intersection  Pre-compute static shadow results where possible  Make simplifying assumptions  Treat area light sources as points  Exploit hardware features such as stencil and depth textures
15. 15. CS 354 15 Common Real-time Shadow Techniques Projected Shadow planar volumes shadows Hybrid approaches Light maps
17. 17. CS 354 17 Shadow Mapping  Image-space shadow determination  Lance Williams published the basic idea in 1978  By coincidence, same year Jim Blinn invented bump mapping (a great vintage year for graphics)  Completely image-space algorithm  means no knowledge of scene’s geometry is required  must deal with aliasing artifacts  Well known software rendering technique  Pixar’s RenderMan uses the algorithm  Leverages GPU hardware  Depth buffering + texture mapping  Multi-pass algorithm: render depth maps, then project depth maps as textures for eye view
18. 18. CS 354 18 Shadow Mapping References  Important SIGGRAPH papers  Lance Williams, “Casting Curved Shadows on Curved Surfaces,” SIGGRAPH 78  William Reeves, David Salesin, and Robert Cook (Pixar), “Rendering antialiased shadows with depth maps,” SIGGRAPH 87  Mark Segal, et. al. (SGI), “Fast Shadows and Lighting Effects Using Texture Mapping,” SIGGRAPH 92  OpenGL extensions  ARB_shadow, ARB_depth_texture  Standardized in OpenGL 1.4 (2002)
19. 19. CS 354 19 Shadow Map Example  Pixar’s Luxo Jr. – state-of-the-art for 1986  Running in real-time – GeForce3 in Japan, 2001 3 light sources = 3 depth map renders per frame
21. 21. CS 354 21 The Shadow Mapping Concept (1)  Depth testing from the light’s point-of-view  Two pass algorithm  First, render depth buffer from the light’s point-of-view  the result is a “depth map” or “shadow map”  essentially a 2D function indicating the depth of the closest pixels to the light  This depth map is used in the second pass
22. 22. CS 354 22 The Shadow Mapping Concept (2)  Shadow determination with the depth map  Second, render scene from the eye’s point-of- view  For each rasterized fragment  determine fragment’s XYZ position relative to the light  this light position should be setup to match the frustum used to create the depth map  compare the depth value at light position XY in the depth map to fragment’s light position Z
23. 23. CS 354 23 The Shadow Mapping Concept (3)  The Shadow Map Comparison  Two values  A = Z value from depth map at fragment’s light XY position  B = Z value of fragment’s XYZ light position  If B is greater than A, then there must be something closer to the light than the fragment  then the fragment is shadowed  If A and B are approximately equal, the fragment is lit
24. 24. CS 354 24 Shadow Mapping with a picture in 2D  The A < B shadowed fragment case depth map image plane depth map Z = A light source eye position eye view image plane, aka the frame buffer fragment’s light Z = B
25. 25. 25 Shadow MappingCS 354 with a picture in 2D  The A ≅ B unshadowed fragment case depth map image plane depth map Z = A light source eye position eye view image plane, aka the frame buffer fragment’s light Z = B
26. 26. CS 354 26 Shadow Mapping with a picture in 2D  Note image precision mismatch! The depth map could be at a different resolution from the framebuffer This mismatch can lead to artifacts
27. 27. 27 Avoiding Artifacts withCS 354 Depth Map Bias  OpenGL supports glPolygonOffset that provides window-space depth bias  Used during shadow map construction  How much polygon offset bias depends Too little bias, Too much bias, shadow everything begins to starts too far back shadow Just right
28. 28. CS 354 28 Visualizing the Shadow Mapping Technique (1)  A fairly complex scene with shadows the point light source
30. 30. CS 354 30 Visualizing the Shadow Mapping Technique (3)  The scene from the light’s point-of-view FYI: from the eye’s point-of-view again
31. 31. CS 354 31 Visualizing the Shadow Mapping Technique (4)  The depth buffer from the light’s point-of-view FYI: from the light’s point-of-view again
32. 32. CS 354 32 Visualizing the Shadow Mapping Technique (5)  Projecting the depth map onto the eye’s view FYI: depth map for light’s point-of-view again
33. 33. CS 354 33 Shadow Map Eye Linear Texture Coordinate Transform glTexGen automatically xe xo applies this when modelview Eye matrix contains just the eye ye view Modeling yo view transform ze = (look at) matrix zo matrix we wo s 1/2 1/2 Inverse xe Light Light t 1/2 1/2 frustum view eye ye r = (projection) (look at) view ze 1/2 1/2 (look at) matrix matrix q matrix we 1 Supply this combined transform to glTexGen or shader
34. 34. CS 354 34 Projective Texturing  Mimic perspective divide but for texture  (s,t,r,q) → (s/q,t/q,r/q)  Light’s view frustum needs perspective  An intuition for projective texturing  The slide projector analogy Source: Wolfgang Heidrich [99]
35. 35. CS 354 35 Visualizing the Shadow Mapping Technique (5)  Projecting the depth map onto the eye’s view FYI: depth map for light’s point-of-view again
36. 36. CS 354 36 Visualizing the Shadow Mapping Technique (6)  Projecting light’s planar distance onto eye’s view  Essentially r/q in the [0,1] range  Analogous to window-space depth for depth buffering
37. 37. CS 354 37 Visualizing the Shadow Mapping Technique (7)  Comparing light distance to light depth map Green is where the light planar distance and Non-green is the light depth where shadows map are should be approximately equal
38. 38. CS 354 38 Visualizing the Shadow Mapping Technique (8)  Scene with shadows Notice how Notice how specular curved highlights surfaces cast never appear shadows on in shadows each other
39. 39. CS 354 39 More Examples  Smooth surfaces with object self- shadowing Note object self-shadowing
40. 40. CS 354 40 More Examples  Complex objects all shadow
41. 41. CS 354 41 Shadow Mapping Artifacts  Even the floor casts shadow, but… Note shadow leakage due to infinitely thin floor Could be fixed by giving floor thickness
42. 42. CS 354 42 Shadow Map Precision Artifacts  Resolution of rendered depth map determines shadow mapping precision low-resolution sufficient shadow map blockiness shadow map resolution
43. 43. CS 354 43 Four Images of Dueling Frusta Case Eye’s View with projection of color-coded Eye’s mipmap levels View from light: Red = minification Blue = magnification Light’s Light’s View with View re-projection of above image from the eye
44. 44. CS 354 44 Interpretation of the Images of the Dueling Frusta Case Region that is smallest in the light’s view is a region Eye’s that is very large in the View eye’s view. This implies that it would require a very high-resolution shadow map to avoid obvious blocky shadow edge artifacts. Light’s View
45. 45. CS 354 45 Dueling Frusta Blocky Shadow Edge Artifacts Notice that shadow edge is well defined in the distance. Light position out here pointing towards the viewer. Blocky shadow edge artifacts.
46. 46. CS 354 46 Shadow Acne  Ambiguous shadow map comparisons cause very unsightly artifacts  Moiré patterns form; animates poorly “shadow acne” polygon offset depth comparison is provides sufficient depth offset ambiguous for unambiguous depth comparisons
47. 47. CS 354 47 Planar Projected Shadows  So-called “fake” shadows  Uses projective geometry trick  Flattens 3D model into ground plane  Shadow effect only can be applied to planes  Supports either positional or directional lights
48. 48. CS 354 48 Projected Planar Shadow Example
49. 49. CS 354 49 Projected Planar Shadows  Classic computer graphics trick  Given  Ground plane equation, Ax + By + Cz + D = 0  Light position (x, y, z, w)  Construct projective transform that “squishes” vertices into ground plane based on light position  Concatenate this with view transform  Rendering 3D object creates shadow-like pile of polygons in ground plane
50. 50. CS 354 50 Projected Planar Shadow Matrix  4x4 Projective Matrix P - Lx A -Ly A -Lz A -Lw A -Lx B P - Ly B -Lz B -Lw B -Lx C -Ly C P - Lz C -Lw C -Lx D -Ly D -Lz D P - Lw D where P = Lx A + Ly B + Lz C + Lw D
51. 51. CS 354 51 Careful about Reverse Projection  Projection can cast a shadow of an object “behind” the light w.r.t. the plane  Make sure occluders are between light and ground plane Good Bad, reverse projection
52. 52. CS 354 52 Projected Planar Shadow Issues  Shadow must be cast on infinite planes  Stencil can limit shadow to finite planar region  “Pile of polygons” creates Z-fighting artifacts  Polygon offset fixes this, but disturbs Z values  Stencil testing is better solution  Difficult to blend shadow with ground texture  Just blending creates “double blends”  But stencil testing can eliminate “double blends”  Specular highlights can show in shadow
53. 53. CS 354 53 Planar Projected Shadow Artifacts Bad Good extends off ground region Z fighting double blending
54. 54. CS 354 54 Quick Tutorial on Stencil Testing  An extra test for fine-grain pixel control  Standard OpenGL and Direct3D feature  Per-pixel test similar to depth buffering  Tests fragment against pixel’s stencil value, rejects fragments that fail  Also, can modify pixel’s stencil buffer value based on stencil/ depth test results  Hardware accelerates stencil testing  Typically allocated with depth buffer  8-bit stencil value paired with 24-bit depth value  Invented by Kurt Akeley and Jim Foran (SGI)
55. 55. CS 354 55 Stencil Testing in the Fragment Pipeline Fragment Pixel + Scissor Alpha Ownership Test Test Associated Test Data Depth Stencil Test Test Blending Dithering Logic Frame Op buffer
56. 56. CS 354 56 Stencil Testing  Similar to Depth Testing but  Compares current reference value to pixel’s stencil buffer value  Same comparison functions as depth test:  NEVER, ALWAYS  LESS, LEQUAL  GREATER, GEQUAL  EQUAL, NOTEQUAL  Stencil bit mask controls comparison  (( ref & mask ) op ( svalue & mask )
57. 57. CS 354 57 Stencil Operations  Way stencil buffer values are controlled  Stencil side effects of  Stencil test fails  Depth test fails  Depth test passes  Possible operations  Increment, Decrement (saturates)  Increment Wrap, Decrement Wrap (a.k.a. modulo)  Keep, Replace, Zero, Invert
58. 58. CS 354 58 OpenGL API  Stencil function  Controls when stencil test passes or fails  glStencilFunc(GLenum stencil_function, GLint reference_value, GLuint read_mask)  Stencil operations  Controls how the stencil buffer is updated  glStencilOp(GLenum stencil_fail, GLenum depth_fail, GLenum depth_pass)  Stencil mask  Controls bit-mask for writing stencil  glStencilMask(GLuint write_mask)  Also two-sided stencil API  Provides different stencil operations for front- and back-facing polygons  glStencilFuncSeparate, glStencilOpSeparate, glStencilMaskSeparate
59. 59. CS 354 59 The Shadow Volume Concept  Volumetric shadows, not just planar  A single point light source splits the world in two  shadowed regions  unshadowed regions  A shadow volume is the boundary between these shadowed and unshadowed regions  First described by [Crow 77]  Professor at University of Texas!  Discrete framebuffer support  Shadow count in the framebuffer from Fournier and Dr. Fussell (1988)  Translates natrually to the stencil buffer…
60. 60. CS 354 60 Visualizing Shadow Volumes  Occluder and light projects a shadow volume
61. 61. CS 354 61 Shadow Volume Result  Objects within the volume are shadowed
62. 62. CS 354 62 Shadow Volume Algorithm  High-level view of the algorithm  Given the scene and a light source position, determine the shadow volume (harder than it sounds)  Render the scene in two passes  Draw scene with the light enabled, updating only fragments in unshadowed region  Draw scene with the light disabled, updated only fragments in shadowed region  But how to control update of regions?
64. 64. CS 354 64 Tagging Pixels as Shadowed or Unshadowed  Using stencil testing  High-level algorithm does not say how to update only either pixels in or out of the shadow volume!  The stenciling approach  Clear stencil buffer to zero and depth buffer to 1.0  Render scene to leave depth buffer with closest Zs  Render shadow volume into frame buffer with depth testing but without updating color and depth, but inverting a stencil bit  This leaves stencil bit set within shadow!
65. 65. CS 354 65 Stencil Inverting of Shadow Volume  Why it works shadowing light object source two inverts, left zero one invert, left one eye zero inverts, left zero position
66. 66. CS 354 66 Visualizing Stenciled Shadow Volume Tagging Shadowed scene Stencil buffer red = stencil value of 1 green = stencil value of 0
67. 67. CS 354 67 Computing Shadow Volumes  Harder than you might think  Easy for a single triangle, just project out three infinite polygons from the triangle, opposite the light position  But shadow volume polygons should not intersect each other for invert trick to work  This makes things hard  For complex objects, projecting object’s 2D silhouette is a good approximation (flat objects are easy)  Static shadow volumes can be pre-compiled
68. 68. CS 354 68 For Shadow Volumes With Intersecting Polygons  Use a stencil enter/leave counting approach  Draw shadow volume twice using face culling  1st pass: render front faces and increment when depth test passes  2nd pass: render back faces and decrement when depth test passes  This two-pass way is more expensive than invert  And burns more fill rate drawing shadow volumes  Inverting is better if no polygon intersections
69. 69. CS 354 69 Why Eye-to-Object Stencil Counting Approach Works Light Shadowing object source zero +1 zero Example in 2D +2 +2 Eye +1 +3 position
70. 70. CS 354 70 Illuminated, Behind Shadow Volumes (Zpass) Light Shadowing object source zero +1 zero + + + - - - Unshadowed object +2 +2 Eye +1 +3 position Shadow Volume Count = +1+1+1-1-1-1 = 0
71. 71. CS 354 71 Shadowed, Nested in Shadow Volumes (Zpass) Light Shadowing object source zero +1 zero + + + - +2 +2 Shadowed Eye +1 object +3 position Shadow Volume Count = +1+1+1-1 = 2
72. 72. CS 354 72 Illuminated, In Front of Shadow Volumes (Zpass) Light Shadowing object source zero +1 zero +2 +2 Shadowed Eye +1 object +3 position Shadow Volume Count = 0 (no depth tests pass)
73. 73. CS 354 73 Shadow Volume Issues  Practical considerations [Bergeron 86]  If eye is in shadow volume, need to determine this and flip enabled & disabled lighting passes  Shadow volume only as good as its tessellation  Shadows tend to magnify limited tessellation of curved surfaces  Open models and non-planar polygons  Must cap the shadow volume’s intersection with the near clipping plane
74. 74. CS 354 74 Problem Created by Near Clip Plane (Zpass) Missed shadow volume intersection due to near clip plane clipping; leads to mistaken count Far clip plane zero +1 +1 +2 zero +3 +2 Near clip plane
75. 75. CS 354 75 Alternative Approach: Zfail  Render scene to initialize depth buffer  Depth values indicate the closest visible fragments  Use a stencil enter/leave counting approach  Draw shadow volume twice using face culling  1st pass: render back faces and increment when depth test fails  2nd pass: render front faces and decrement when depth test fails
76. 76. CS 354 76 Illuminated, Behind Shadow Volumes (Zfail) Light Shadowing object source zero +1 zero Unshadowed object +2 +2 Eye +1 +3 position Shadow Volume Count = 0 (zero depth tests fail)
77. 77. CS 354 77 Shadowed, Nested in Shadow Volumes (Zfail) Light Shadowing object source zero +1 zero + + +2 +2 Shadowed Eye +1 object +3 position Shadow Volume Count = +1+1 = 2
78. 78. CS 354 78 Illuminated, In Front of Shadow Volumes (Zfail) Light Shadowing object source zero +1 zero - - - + + + +2 +2 Shadowed Eye +1 object +3 position Shadow Volume Count = -1-1-1+1+1+1 = 0
79. 79. CS 354 79 Problem Created by Far Clip Plane (Zfail) Far clip plane Missed shadow volume intersection due to far clip plane clipping; zero leads to mistaken count +1 zero +1 +2 +2 +3 Near clip plane
80. 80. CS 354 80 Problem Solved by Eliminating Far Clip zero +1 +1 +2 +2 +3 Near clip plane
81. 81. CS 354 81 Avoiding Far Plane Clipping  Usual practice for perspective GL projection matrix  Use glFrustum (or gluPerspective)  Requires two values for near & far clip planes  Near plane’s distance from the eye  Far plane’s distance from the eye  Assumes a finite far plane distance  Alternative projection matrix  Still requires near plane’s distance from the eye  But assume far plane is at infinity  What is the limit of the projection matrix when the far plane distance goes to infinity?
82. 82. CS 354 82 Standard glFrustum Projection Matrix  2 × Near Right + Left   Right − Left 0 0  Right − Left    2 × Near Top + Bottom  0 0 P= Top − Bottom Top − Bottom   Far + Near 2 × Far × Near   0 0 − −   Far − Near Far − Near    0 0 −1 0    Only third row depends on Far and Near
83. 83. CS 354 83 Limit of glFrustum Matrix as Far Plane is Moved to Infinity  2 × Near Right + Left   Right − Left 0 0  Right − Left   2 × Near Top + Bottom lim P = Pinf = 0 0 Far →∞  Top − Bottom Top − Bottom   0 0 −1 − 2 × Near      0 0 −1 0    First, second, and fourth rows are the same as in P  But third row no longer depends on Far  Effectively, Far equals ∞
84. 84. 84 Verifying Pinf Will Not ClipCS 354 Infinitely Far Away Vertices (1)  What is the most distant possible vertex in front of the eye?  Ok to use homogeneous coordinates  OpenGL convention looks down the negative Z axis  So most distant vertex is (0,0,-D,0) where D>0  Transform (0,0,-D,0) to window space  Is such a vertex clipped by Pinf?  No, it is not clipped, as explained on the next slide
85. 85. 85 Verifying Pinf Will Not ClipCS 354 Infinitely Far Away Vertices (2)  Transform eye-space (0,0,-D,0) to clip-space  2 × Near Right + Left  0 0  xc   xc   Right − Left Right − Left   0   y  y   2 × Near Top + Bottom   0   c  =  c =  0 0    − D   zc   Top − Bottom Top − Bottom  − D       0 0 −1 − 2 × Near     − D   wc     0    0 0 −1 0    Then, assuming glDepthRange(0,1), transform clip- space position to window-space position zc −D z w = 0.5 × + 0.5 = 0.5 × + 0.5 = 1 wc −D  So ∞ in front of eye transforms to the maximum window-space Z value, but is still within the valid depth range (i.e., not clipped)
86. 86. CS 354 86 Robust Shadow Volumes sans Near (or Far) Plane Capping  Use Zfail Stenciling Approach  Must render geometry to close shadow volume extrusion on the model and at infinity (explained later)  Use the Pinf Projection Matrix  No worries about far plane clipping  Losses some depth buffer precision (but not much)  Draw the infinite vertices of the shadow volume using homogeneous coordinates (w=0)
87. 87. CS 354 87 Visualize Net Stencil Rendering  Non-zero stencil state marks shadowed pixels w.r.t. the light source Fully shaded scene Final stencil state
88. 88. CS 354 88 Stencil Volume Rendering Animation
89. 89. CS 354 89 Shadow Volumes Add “Invisible” Geometric Complexity Visible geometry Scene with multiple light sources Shadow volume geometry Abducted game images courtesy Joe Riedel at Contraband Entertainment
90. 90. CS 354 90 Examples of Possible Silhouette Edges for Quake2 Models An object viewed from the same basic direction that the light is shining on the object has an identifiable light-view silhouette An object’s light-view silhouette appears quite jumbled when viewed form a point-of-view that does not correspond well with the light’s point-of-view
91. 91. CS 354 91 Multiple Lights and Shadow Volumes  Requires still more rendering passes, but can work! Shadows from different light sources overlap correctly
92. 92. CS 354 92 Shadow Volumes from Area Light Sources  Make soft shadows  Shadow volumes work for point light sources  Area light sources are common and make soft shadows  Model an area light source as a collection of point light sources [Brotman & Badler 84]  Use accumulation buffer or additive blending to accumulate soft shadow  Linear cost per shadow volume sample
93. 93. CS 354 93 Area Light Sources Cast Soft Shadows area light source umbra
94. 94. CS 354 94 Soft Shadow Example  Eight samples (more would be better) Note the banding artifacts
95. 95. CS 354 95 Combined Shadow Approaches  Two Approaches at Once just logo shadow volume logo lacks shadow on the teapot just planar projected teapot lacks shadows shadow on the floor combined approach
96. 96. CS 354 96 Pre-computed Shadow Textures  Lightmaps are static shadow textures  Compute lightmaps off-line with radiosity solver  local lighting models evaluated on tight grid can work well too  Lightmaps with soft shadows can be built faster with convolution approach, accelerated using the Fast Fourier Transform [Soler & Silion 98]  Pre-computed shadow textures work well for building interior with lots of diffuse surfaces
97. 97. CS 354 97 Light maps can encode Static Pre-computed Shadows decal only × (modulate)light maps only = * Id Software’s Quake 2 combined scene circa 1997
98. 98. CS 354 98 Rendering Soft Shadow Textures  Fast soft shadows [Heckbert & Herf 96]  Render a shadow texture  For each shadowed polygon, project all light-source- occluding polygons into the shadowed polygon’s plane * Similar to projected planar shadows * Model area light sources as multiple jittered points * Use additive blending to accumulate all contributions  Copy resulting image to the shadow texture  Draw polygon in final scene with shadow texture
99. 99. CS 354 99 Rendered Soft Shadow Texture Example  Shadow Texture and Final Result Shadow texture, accumulation Final resulting scene, shadow of nine jittered occluder texture modulated with projections wooden floor
100. 100. CS 354 100 Next Class  Mid-term  Return graded mid-term  Review class statistics  Project 2 assigned  On texturing, shading, & lighting  Study GLSL  Next lecture  Scene graphs  How do applications organize data structures for efficient rendering of scenes?
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