CS 352: Computer Graphics

1
CS 352 Computer Graphics
Chapter 7 Discrete Techniques
2
(No Transcript)
3
(No Transcript)
4
(No Transcript)
5
Applications

• How do you do all those nice object textures?
• Brick pattern on a walkway. How to give it some
  depthmodel the bricks? Model the roughness of
  the brick surface?
• Pool-like water patterns, caustics
• Fog. How?
• 3D scan of artifacts. Digital flattening
• Data an image and many (x, y, z, u, v) points
• How to digitally flatten an artifact such as a
  crumpled vellum leaf?
• How to undo the lighting?

6
Overview

• Texture mapping
• Bump mapping
• Reflection or environment mapping
• Bit and pixel operations in OpenGL
• Compositing
• Blending
• Antialiasing
• Depth cueing and fog
• Using the accumulation buffer

7
The Joy of Frame Buffers

• For decades, and until recently, shape only no
  access to frame buffer
• Access to frame buffer and operations on
  fragments have made enormous difference in
  interactive graphics
• Applications texture mapping, antialiasing,
  compositing, transparency, fog, accumulation
  buffering,
• Silicon Graphics/GL first real texture-mapping
  systems. Major advance.

8
OpenGL frame buffer

• The OpenGL frame buffer actually consists of
  many separate buffers

9
Fragment processing

• These various buffers are accessed during
  fragment processing

10
Reading and writing buffers

• Typically done in rectangular bit blocks with
  bit-block transfer operations (bitblt), such as
  write_block(source, n, m, x, y, destination, u, v)

11
Setting active buffers

• OpenGL lets you sent the present drawing buffer
  and present reading buffer
• glDrawBuffer() // default back color buffer
• glReadBuffer()
• Pixels can be read or written
• glReadPixels(x, y, w, h, format, type, image)
• glDrawPixels(w, h, format, type, image)
• glCopyPixels(x, y, w, h, type)

12
Drawing an image

• Native byte order and padding may not match
  OpenGL
• may have to pack or unpack
• or alter OpenGLs format with glPixelStore
• To draw an image into a buffer
• QImage gl_buffer convertToGLFormat(buffer)
• glRasterPos2i(0,0)
• glDrawPixels(buffer.width(), buffer.height(),
  GL_RGBA, GL_UNSIGNED_BYTE,
  gl_buffer.bits())
• glFlush()

13
Mapping methods

• Imagine modeling a piece of carved wood, or a
  golf ball, or a spoon, or a tree
• Rather than modeling the geometry at a very fine
  level of detail, we may prefer discrete methods
• texture mapping
• bump mapping
• environment mapping

14
Texture mapping

• Texture mapping applying images to rendered
  polygons.
• A separate texture buffer on the graphics card
  stores texture images
• Texture pixels are called texels
• Textures may be 1-, 2-, or 3- (or 4-!)
  dimensional (mapped onto 1-, 2-, or 3-dimensional
  surfaces)
• The need for texture images is why you have 256MB
  graphics cards (and repetitive patterns!)

15
Mapping geometry

• Texture mapping rendering polygons by applying
  images with mapped geometry
• (s,t) texture coordinates
• T(s,t) texture
• x(s,t), y(s,t), z(s,t) texture map (a mapping
  of texture onto world coords)
• To render, we really want the inverse map
  s(x,y,z) and t(x,y,z)
• These can be hard to find in practice!

16
Mapping geometry

• Also have to map screen coordinates (xs,ys) into
  world coordinates before you can map texture
  coordinates onto world coordinates

17
Other difficulties

• Consider examples
• Rendering a face on a sphere how to map?
• Leaf picture water droplets rubber deformations
• Zooming way in or way out on a texture-mapped
  object
• Have to handle
• Geometry mapping
• Modifying the geometry map (rubber example)
• Extreme magnification or minification
• Antialiasing
• Render modes decal, replace, modulate (), blend

18
(No Transcript)
19
Texture mapping examples

• hplantin/public/352/OpenGL/Examples
• Redbook
• Check
• Wrap
• Mipmap
• Texbind
• Texturesurf
• Examples
• Mjkwarp
• Mjkshift
• Dinoshade

• Advanced
• Projtex
• Textile
• Texwinalign
• Texext

20
Texture mapping overview

• Assume texture image has coordinates (s,t) from
  (0,0) to (1,1)
• Specify texture coordinates for each vertex
• To render polygon, find texture colors by
  interpolation over the texture image region
• Mix texture color with object color
• Replace just use the texture map color
• Decal apply lighting to texture mapped point
• Modulate multiply object color by texture map
• Blend alpha-blend colors

21
Pixel color from texture map?

• How do we get the pixels color from the texture
  map?
• We could map the center of the pixel to the
  texture map and use the color we find
• But this can result in severe aliasing and
  flickering
• Better to map the whole pixel and average. But
  wont that be slow?

22
Mipmaps

• Make pre-filteredimages of size1x1, 2x2, 4x4,
  , nxn
• Choose the best

23
Texture mapping in OpenGL

• Steps
• Create texture object and specify texture image
  for the object
• Indicate how the texture is to be applied to each
  pixel (set texture mapping parameters)
• Enable texture mapping
• Draw the scene, supplying both texture and
  geometric coordinates for each vertex
• Works only in RGBA mode
• Example checker.c

24
Creating a texture

• Synthesize
• Glubyte checkImagewidthheight3
• glPixelStorei(GL_UNPACK_ALIGNMENT, 1)
• set values
• or read
• Use QTs load image and convert-to-gl-format
  functions
• Define as OpenGL texture
• glTexImage2D(GL_TEXTURE_2D, 0, 3, width, height,
  0, GL_RGB, GL_UNSIGNED_BYTE, checkImage000)
  

25
glTexImage2D parameters

• glTexImage2D(GL_TEXTURE_2D, 0, 3, width, height,
  0, GL_RGB, GL_UNSIGNED_BYTE, checkImage000)
  
• glTexImage2D(target, level, internalFormat,
  width, height, border, format, type, texels)
• Target texture or texture proxy for size
  testing
• Level 0, unless you are mipmapping
• internalFormat 0..3 or one of 38 formats, e.g.
  GL_LUMINANCE6_ALPHA2, GL_RGB (3)
• Border 0 (no border) or 1 (border)
• Width, height must be 2m 2b
• Format, type of texture image

26
Texture mapping parameters

• Wrapping glTexParameterf
• (GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP)
  // or GL_REPEAT
• Filtering
• glTexParameterf(GL_TEXTURE_2D, GL_MAG_FILTER,
  GL_LINEAR) // or GL_NEAREST (severe artifacts)
• Perspective correction glHint(GL_PERSPECTIVE_CORR
  ECTION_HINT, GL_NICEST) // or GL_FASTEST
• Texture mode
• glTexEnvi(GL_TEX_ENV, GL_TEX_ENV_MODE,
  GL_DECAL)// or GL_REPLACE, GL_MODULATE,
  GL_BLEND
• glEnable(GL_TEXTURE_2D)

27
Draw the scene

• glBegin(GL_QUADS)
• glTexCoord2f(0.0, 0.0) glVertex3f(-2.0, -1.0,
  0.0)
• glTexCoord2f(0.0, 1.0) glVertex3f(-2.0, 1.0,
  0.0)
• glTexCoord2f(1.0, 1.0) glVertex3f(0.0, 1.0,
  0.0)
• glTexCoord2f(1.0, -1.0) glVertex3f(0.0, -1.0,
  0.0)
• glEnd()

28
Mipmaps

• You can create mipmap levels individually if you
  really want to (see mipmap.c)
• Or, create them automatically using
• gluBuild2DMipmaps(target, iformat, width, height,
  format, type, texels)
• // parameters same as glTexImage2d()
• Additional glTexParameter types
• GL_NEAREST_MIPMAP_NEAREST
• GL_LINEAR_MIPMAP_NEAREST
• GL_NEAREST_MIPMAP_LINEAR
• GL_LINEAR_MIPMAP_LINEAR

29
Handling multiple textures

• Using multiple texture objects
• Generate texture names
• Bind texture objects to texture data (including
  wrap mode and filtering parameters)
• See if you have enough high performance texture
  space (or establish texture priorities)
• Bind the texture you want to use, then draw
• Example texbind.c

30
Texture map fonts

• If you have many shapes to draw, such as a font
  face, you can store as texture maps

31
Environment Maps

• A form of texture mapping
• Compute image of environment
• Map to surface of object
• OpenGL supports autogeneration of
  texturemapping coordinates
• glTexGenfv(GL_S, GL_SPHERE_MAP, 0)
• glTexGenfv(GL_T, GL_SPHERE_MAP, 0)
• glEnable(GL_TEXTURE_GEN_S)
• glEnable(GL_TEXTURE_GEN_T)

32
Bump Maps

• Simulate localvariations in shape
• Vary lighting byperturbing the surface normals
• Lighting does the rest
• Bump map an image or array of normal
  variations (in two directions)

33
Buffer reads and writes

• You can read and write buffers
• Color (normal drawing buffers)
• Depth (for hidden surface removal)
• Stencil (masking e.g. screendoor transparency)
• Accumulation (antialiasing, image processing)
• 16 buffer write modes set, clear, copy, or, and,
  xor, nand, invert, copy_inverted, etc.
• Can magnify, reduce, flip, fog, blend, gamma
  correct, modify colors, dither, histogram, add,
  multiply, do screendoor transparency

34
Anti-Aliasing

• Render an image multiple times with a jittered
  viewpoint
• Average them in the accumulation buffer

35
Screen-door transparency

• Alpha-blending and z-buffering dont work
  together
• blending depends on order of polygons rendering
• Z-buffering allows arbitrary polygon order
• How to get quick transparency effects?
• Screen door approach put a pattern with a
  set percent coverage into the stencil buffer
  and render.
• Even works with many layers (but use different
  patterns)

36
Colors for Picking

• How can you determine from a click on an image
  which object was selected?
• One method pseudo-colored image
• Render image of scene in off-screen bitmap
• Use false colors unique solid color for each
  object
• Determine color of point clicked this gives
  object

37
Compositing

• Forest Gump how?
• Motion capture actors for crowd scenes
• Actors are worried about losing work
• Compositing the general term for combining
  different images or video sequences using alpha
  blending, mattes, etc.

38
Blending in OpenGL

• Steps
• Use colors with alpha values
• Enable blending
• Set a blending function
• glEnable(GL_BLEND)
• glBlendFunc(source_factor, dest_factor)
• Possible values for source or destination factor
  GL_ONE, GL_ZERO, GL_DST_COLOR, GL_SRC_ALPHA,
  GL_DST_ALPHA, GL_ONE_MINUS_DEST_ALPHA,
  GL_CONSTANT_ALPHA, etc.

39
Blending example

• To get blend based on alpha of second picture
• Set source to GL_ONE and dest to GL_ZERO draw
  first picture
• Set source to GL_SRC_ALPHA and dest to
  GL_ONE_MINUS_SRC_ALPHA draw second picture

40
More uses of blending

• Paint program airbrush tool
• Simulate colored filter set R, G, and B alpha
  components separately
• Billboarding
• Draw two intersecting rectangles at right angles
• Texture map with tree image
• Use alpha to cut away hollow parts