1. Discrete Techniques

2. Contents Buffers Compositing and blending Bitmaps and images
Mapping techniques

3. Discrete techniques I

4. Overview
Buffers
Compositing and blending

5. Discrete Techniques Which section of the pipeline?
Geometric models, lighting models, camera models, etc.
Modeling Transformations
Lighting
Which section of the pipeline?
Done in screen space, dealing with fragments
Buffer operations, per-pixel processing, etc.
Viewing Transformations
Clipping
Projection
Screen space
Rasterisation
Pixels for display
Fragment Processing

6. Why fragments? After surface rasterised, no longer notion of a polygon
Surface essentially been ‘broken up’ or ‘discretised’ into small pieces
Each of which is at most the size of one pixel, called fragments
Conceptually fragments can be smaller than one pixel
i.e. more than one fragment contributes to colour of pixel

7. Buffers Per-pixel data is stored in buffers
Color Buffer (front and back)
Depth Buffer
Accumulation Buffer
Stencil Buffer
Others (overlay planes, auxiliary buffers, color indices)

8. Buffers Discrete Limited resolution, spatially and depth wise
Can define a buffer as a block of memory with
k m x n bit-planes (usually hundreds of bits, a pixel refers to all the k elements resolution)
k can range from a few hundreds of bits, a pixel refers to all the k elements at a particular location
Depth, bits per pixel

9. Buffers The frame buffer
Consists of a variety of buffers, collectively known as the frame buffer
The term frame
actually refers to the
total display area
But often when people
refer to the frame
buffer, they are really
talking about the
colour buffer
OpenGL frame buffer

10. Buffers The colour buffer Where the RGBA pixel values are stored
Generally need two separate colour buffers
One for displaying, the other for rendering
In double buffering referred to as front and back buffers
Double buffering
To prevent flickering and other undesirable artifacts that will appear if an image that is currently being displayed is updated

11. Buffers

12. Buffers The depth-buffer (z-buffer) Use for visibility determination
Tests can be enabled or disabled
Certain situations need disable depth-buffer tests (e.g. when rendering transparent polygons)

13. Buffers The stencil buffer
General purpose buffer for doing things not possible with colour and depth buffer alone
E.g. for creating reflective surfaces and essential in many shadow rendering techniques, like shadow values
‘Tag’ pixels in one rendering pass to control their update in subsequent rendering passes
Can specify different rendering operations like
Stencil test fails
Stencil test passes & depth test fails
Stencil test passes & depth test passes

14. Buffers Stencil buffer to reflection Without stencil buffer
With stencil buffer

15. Buffers Stencil buffer for reflection (cont.) Basic algorithm
Clear buffers
Disable depth buffer and draw mirror
surface to stencil buffer
Enable depth buffer, draw reflected
geometry where stencil buffer passes
Disable stencil buffer, draw normal
scene

16. Buffers Stencil buffer for reflection (cont.) Another way
Clear buffers
Draw all non-mirror geometry to frame & depth buffers
Draw mirror to stencil buffer, where depth buffer passes
Set depth to infinity, where stencil buffer passes
Draw reflected geometry to frame & depth buffer, where stencil buffer passes

17. Buffers Shadows The umbra, area that is completely in shadow
The penumbra, area partly illuminated and partly occluded

18. Buffers
Shadows (cont.)
Hard Shadows
Soft Shadows

19. Buffers Shadow volumes Volume of space in shadow
Pyramid with point light as
the apex
Polygons inside a shadow
volume will not be
illuminated by that particular
light
Shadow test similar
to clipping

20. Buffers Shadow volumes with the stencil buffer Scene with shadows
Stencil buffer contents
• green = stencil value of 0
• red = stencil value of 1
• darker reds = stencil value > 1

21. Buffers Shadow volumes (cont.) Limitations
Introduces a lot of new geometry and computations
Can be computationally expensive
Only generates hard shadows

22. Buffers The accumulation buffer The idea is to accumulate
multiple images
Common uses
Compositing
Anti-aliasing
Motion blur
Depth of field
Soft shadows

23. Compositing and Blending
Opacity and transparency
Opaque surfaces permit no light to pass through
Transparent surfaces permit all light to pass
Translucent surfaces pass some light

24. Compositing and Blending
Dealing with translucency in a physically correct manner is difficult due to
The complexity of the internal interactions of light and matter
Using a pipeline renderer

25. Compositing and Blending
Alpha blending
Alpha channel
The ‘A’ component in the RGBA (or RGB) colour mode
When blending enabled
Value of α determines how the RGB values are written into the frame buffer
Objects blended or composited blended together
Because fragments from multiple objects can contribute to the colour of the same pixel
Useful for rendering objects with a translucent component (e.g. water, glass, etc.)

26. Discrete techniques II

27. Overview Bitmaps and images Mapping techniques Texture mapping
Other mapping techniques

28. Digital images Image data
Generally work with images that are arrays of pixels
Can be of a variety of sizes and data types
For example
If working with RGB images, each colour component usually represented as one byte
For a 512 x 512 image
Or allocate memory to

29. Bitmaps and images Both take the form of rectangular arrays of pixels
Some differences
A bitmap (note this is different from the image format)
Consists of a single bit of information about each pixel (a map of bits)
Used as mask to overlay another image
Image data
Typically include several pieces of data per pixel (e.g. complete RGBA colour components)
Simply overwrites or is blended with whatever data is in the frame buffer

30. Bitmaps in OpenGL
OpenGL treats 1-bit pixels (bitmaps) differently from multi-bit pixels (pixelmaps)
Rectangular array of 0s and 1s
Mask that determines if the corresponding pixel in the frame buffer is drawn with the present raster colour
Most commonly used for drawing character
E.g.

31. Bitmaps in OpenGL Data stored in chunks that are multiples of 8 bits
Start at bottom left
To draw the bitmap, use

32. Image data
Pixel format in frame buffer can be different from that of processor memory
Many kinds of frame buffer data
Many ways to store pixel information in processor memory
Various data conversion can be performed during reading, writing and copying operations
These two types of memory reside in different places
Need packing and unpacking
Drawing/reading can be slow

33. Image data Pixel packing and unpacking
Refers to the way in which pixel data is written to and read from processor memory
An image stored in memory has between one and four chunks of data, called elements
Some elements are integers, others are floating-point values (typically 0.0 – 1.0)
Floating-point values usually stored in frame buffer with lower resolution (e.g. 8-bits), exact number of bits depends on hardware
Pixel storage mode controlled using
glPixelStore*()

34. Mapping Techniques There are limitations to geometric modelling
Texture mapping
Uses images to fill inside of polygons
‘Paint’ image onto polygons
Analogy – sticking wallpaper onto a wall
Increase the visual realism

35. Texture mapping

36. Texture mapping

37. Texture mapping
2D texture
Textured 3D model

38. Texture mapping
Different texture maps can be used for same object

39. Texture mapping

40. Texture mapping
Texture coordinates are referred to as (s, t)

41. Texture mapping

42. Texture mapping Aliasing in textures The under-sampling of a signal
Looks worst when moving

43. Texture mapping Aliasing Texture sampling
Aliasing artifacts emerge as a result of discrete sampling
Texture sampling
Point sampling
Use value of closest texel
Linear filtering
Use weighted average in the neighbourhood, more work

44. Texture mapping
Size of pixel we are trying to colour may be larger or smaller than one texel

45. Texture mapping Mipmaps Create multiple resolutions of an image
Hand it 256 x 256, create 128 x 128, 64 x 64, 32 x 32, 16 x 16, 8 x 8, 4 x 4, 2 x 2, 1 x 1
Point sample used depends on polygon’s size on screen

46. Texture mapping
Mipmaps can be used in conjunction with texture filtering methods
Nearest neighbour only
Mipmaps & linear interpolation

47. Mapping techniques Point sampling Linear filtering
Mipmapped linear filtering
Mipmapped point sampling

48. Drawbacks of texture filtering
Filters work by averaging the values of pixels in some way
Higher order filters are computationally expensive
Blurring effect which might be undesirable

49. Mapping techniques Billboarding
Texture map a 2D image onto a 2D polygon
Rotate it so polygon’s normal always faces the viewer
Creates illusion that 2D image is a 3D object

50. Billboarding

51. Mapping techniques Bump mapping
Textures used to perturb the surface normal
Actual geometry of surface does not change, just shaded as if it were different shape

52. Mapping techniques
Bump mapping – outline looks smooth + =

53. Mapping techniques Environment mapping
Quick but inaccurate way of generating effects like reflections (a.k.a reflection mapping)
Image of the surrounding environment used to project reflections onto object’s surface
Common methods
Cube mapping
Spherical mapping

54. Mapping techniques Cube mapping Popular because easily constructed
Top
Cube mapping
Popular because easily constructed
Place camera in centre of box and render the 6 different views
Surrounding scene mapped onto surfaces of a cube
Left
Back
Front
Right
Bottom

55. Cube mapping

56. Cube mapping

57. Environment mapping

58. Mapping techniques Multi-texturing + =
Paint multiple textures onto the same surface + =

59. Mapping techniques Light mapping
Illumination maps / light maps were introduced in the game Quake to represent lighting effects

60. Mapping techniques Light mapping
Allows lighting to be pre-calculated and stored as 2D texture map

61. Mapping techniques
Light mapping x =

62. Mapping techniques Displacement mapping
Use texture map to actually move points on surface
Geometry displacement before visibility determined
Shadows can be generated

63. Effects using Image Processing+ =

64. Effects using Image Processing

65. Questions?
Thanks