Real-Time Rendering

1. Real-Time Rendering SPEACIAL EFFECTS Lecture 03 Marina Gavrilova

2. Brief Outline • Image based Rendering • Lens Effects • Billboarding • Particle Effects • Reflection • Shadow mapping • Edge Effects • Volumetric rendering

3. The Rendering Spectrum • Effects can be produced by manipulation of images or geometry or both

4. Image Based Rendering • Early games used sprites (image object) • Move/scale and draw sprite on screen • Animated texture  animated sprite • Layered Rendering with transparency  • Assign depth to sprite • Render sprites in particular order • Use transparency • Talisman architecture renders objects into layers and composite them for several frames • Lumigraph rendering  Render using image interpolation

5. Billboarding • Create complex objects using texture and simple geometry • Used extensively in particle rendering systems • Use faceted quads  normal of the quad equals camera direction • Used frequently in games • High performance

6. Types of billboarding • Screen-aligned billboard • Facted quads/sprites • Flare/glow/smoke • World aligned billboard • Aligned facing camera • Has it’s own up vector • Scenery/complex objects • Axial Billboard • One rotation axis • Tree/objects on terrain

7. Particle system • Point based sprites • Additive or other blending • Physically based modeling • Realistic volumetric effect

8. Lenz Effects Effects that makes a scene natural By mimicking physical properties of eye/camera

9. Lens Flare • Glow of light source • Halo around the source • Streaks and bloom formed around the source • Secondary lens effects (internal reflection and refraction)

10. Motion Blur • Use accumulation buffer to render object with interpolated coordinate • Requires several rendering passes • Realistic effect • Slow • Texture based motion blur • Other pseudo motion blur techniques are used in games

11. Reflection • Method 1: Use Environment mapping • Method 2: Flat mirror • Render scene from mirrors point of view • Darken the scene • Use that as a texture

12. Exposure control • Method 1: render multiple scene and blend with exposure parameter • Method 2: Define per texture luminance • Render using High Dynamic Range luminance (HDR)

13. Depth of Field • Hold camera target stationery • Vary camera source • Render several pass • Accumulate scene

14. Shadow mapping Without darkness, light cannot exist Shadow mapping makes a scene significantly more realistic

15. Shadow in a scene

16. Planar Shadow • When occluders cast shadow on planar objects (i.e. floor) • Fully shadowed region: umbra • Partially shadowed region: Penumbra • Hard shadow  Projection algorithm • Soft shadow  Shadow mapping

17. Project the vertices of an object on a plane Render the transformed object with dark color (without lighting) Assume light source=l, vertex projected=v, projected vertex=p In matrix form for plane y=0  (p=Mv): For any arbitrary plane: Matrix M= Projection Shadow

18. Problems with projective shadow • Hard edge  Less realistic • Rendering on a plane tricky: • Offset from the plane to avoid z buffer overlap • Different offset for different view angle • Anti-shadow problem:

19. Soft Projective Shadow • Generate a texture (render into texture) • Render the texture as a plain • Only recompute texture if light source changes • Method 1: Render into accumulation buffer with varying light source • Use perspective projection on a plane • Light source = a • Receiver parallelogram: one vertex b and edge ex,ey • Advantage: Free from Anti-shadows and false shadows

20. Soft Projective Shadow • Method 2: Use spherical light source • Less expensive computationally • Looks better • Problem: Visual artifact when object touching the plane Hard Soft

21. Shadow on arbitrary object: Shadow Volume • Uses the stencil buffer • Extend a pyramid from a light source through vertices of a triangle • Shadow Volume: volume under the pyramid • Consider a ray from a pixel to an object: if number of times intersecting front face of shadow volume>0 then vertex is under shadow

22. Shadow Volume Technique • Render the scene with ambient and emission • Compute the faces of shadow volume • Clear the stencil buffer and set to increment • Render the front faces of shadow volumes into stencil • Set Stencil to decrement • Render the back faces of shadow volumes into stencil • Render the scene with light only where stencil=0 • Shadow volume is slow for complex geometry

23. Shadow Volume

24. Shadow Mapping • Render the z-buffer from the lights point of view • Shadow map = content of this z-buffer • Render scene using viewer • If distance between a rendered point and light is greater than projected z value then it is under shadow • Use texture mapping hardware

25. Shadow Mapping Using Texture • First generate the shadow map • Render from viewer using ambient lighting only • Convert shadow map z value to viewers coordinate system • Shadow test: Test z values of point with z values in the shadow map

26. Shadow mapping steps… • Shadow mapping for a complex scene the pointlight source

27. Shadow mapping steps… • Scene from light’s point of view

28. Shadow mapping steps… • Scene from light’s point of view

29. Shadow mapping steps… • Scene from viewer with shadow map (z values)

30. Shadow mapping steps… • Test Shadows: non green areas are shadows

31. Shadow mapping steps… • Finally the scene can be rendered with the shadow

32. Other Rendering Concerns • Edge highlighting: Draw edge in different color (engineering app) • Polygon Edge rendering: • Render the filled polygon (all buffer active) with z buffer replacement=off • Render the polygon edges (all active) • Render the polygon only to the z buffer

33. Other Rendering Concerns • Hidden line rendering: Draw filled polygon to z-buffer and lines only on the color buffer (no hidden lines drawn) • Haloing: front-face lines halo the back faced lines • Height field and volumetric rendering: Use of voxels and 3D textures 

34. Height fields and volumes

35. End of Lecture 03 Questions?