Advanced%20OpenGL%20Technique

1
Advanced OpenGL Technique

• Jian Huang, CS 594, Fall 2002
• Redbook Chapters 6.1, 9.5, 9.6, 10

2
List of Topics

• Blending
• Multi-texturing
• Frame buffers
• Stencil buffer
• A-buffer (accumulation buffer)
• Fragment Tests

3
Alpha the 4th Color Component

• Measure of Opacity
• simulate translucent objects
• glass, water, etc.
• composite images
• antialiasing
• ignored if blending is not enabled
• glEnable( GL_BLEND )

4
Compositing for Semi-transparency

• Requires sorting! (BTF vs. FTB)
• over operator - Porter Duff 1984
• a opacity

5
Fragment

• Fragment in OGL after the rasterization stage
  (including texturing), the data are not yet
  pixel, but are fragments
• Fragment is all the data associated with a pixel,
  including coordinate, color, depth and texture
  coordinates.

6
Blending in OGL

• If a fragment makes it to FB, the pixel is read
  out and blended with the fragments color and
  then written back to FB
• The contributions of fragment and FB pixel is
  specified glBlendFunc( src, dst )

7
Blending Function

• Four choices
• GL_ONE
• GL_ZERO
• GL_SRC_ALPHA
• GL_ONE_MINUS_SRC_ALPHA
• Blending is enabled using glEnable(GL_BLEND)
• Note If your OpenGL implementation supports the
  GL_ARB_imaging extension, you can modify the
  blending equation as well.

8
3D Blending with Depth Buffer

• A scene of opaque and translucent objects
• Enable depth buffering and depth test
• Draw opaque objects
• Make the depth buffer read only, with
  glDepthMask(GL_FALSE)
• Draw the translucent objects (sort those
  triangles still, but which order, FTB or BTF?)

9
How Many Buffers Are There?

• In OpenGL
• Color buffers front, back, front-left,
  front-right, back-left, back-right and other
  auxiliaries
• Depth buffer
• Stencil buffer
• Accumulation buffer
• Exactly how many bits are there in each buffer,
  depends on
• OGL implementation
• what you choose
• Each buffer can be individually cleared

10
Selecting Color Buffer for Writing and Reading

• Drawing and reading can go into any of front,
  back, front-left, front-right, back-left,
  back-right and other auxiliaries
• glDrawBuffer select buffer to be written or
  drawn into
• Can have multiple writing buffer at the same time
• glReadBuffer select buffer as the source for
• glReadPixels
• glCopyPixels
• glCopyTexImage (copy from FB to a texture image)
• glCopyTexSubImage

11
Accumulation Buffer

• Problems of compositing into color buffers
• limited color resolution (e.g. 8 bits/channel)
• clamping
• loss of accuracy
• Accumulation buffer acts as a floating point
  color buffer
• accumulate into accumulation buffer
• transfer results to frame buffer

12
Accumulation Buffer

• OGL dont directly write into A-buffer
• Typically, a series of images are rendered to a
  standard color buffer and then accumulated, one
  at a time, into the A-buffer.
• Then, the result in A-buffer has to be copied
  back to a color buffer for viewing
• A-buffer may have more bits/color

13
Accessing Accumulation Buffer

• glAccum(op, value ) operations
• within the accumulation buffer
• GL_ADD, GL_MULT
• from read buffer
• GL_ACCUM, GL_LOAD
• transfer back to write buffer
• GL_RETURN
• glAccum( GL_ACCUM, 0.5) multiplies each value in
  read buffer by 0.5 and adds to accumulation
  buffer
• How do you average N images?!

14
Accumulation Buffer Applications

• Compositing
• Full Scene Anti-aliasing
• Depth of Field
• Filtering
• Motion Blur

15
Full Scene Antialiasing

• Jittering the view (how?? )
• Each time we move the viewer, the image shifts
• Different aliasing artifacts in each image
• Averaging images using accumulation buffer
  averages out these artifacts
• Like super-sampling in ray-tracing

16
Depth of Focus

• Keeping a Plane in Focus
• Jitter the viewer to keep one plane unchanged

How do you jitter the viewer ????
Move the camera in a plane parallel to the focal
plane.
17
What else?

• Can you do soft shadows?
• Jitter the light sources
• What about motion blur
• Jitter the moving object in the scene
• glAccum(GL_MULT, decayFactor)

18
A Fragments Journey Towards FB

• Many Tests (on/off with glEnable)
• scissor test - an additional clipping test
• alpha test - a filtering test based on alpha
• stencil test - a pixel mask test
• depth test - fragment occlusion test

19
Scissor Box

• Additional Clipping test
• glScissor( x, y, w, h )
• any fragments outside of box are clipped
• useful for updating a small section of a
  viewport
• affects glClear() operations

20
Alpha Test

• Reject pixels based on their alpha value
• glAlphaFunc(func, value)
• GL_NEVER GL_LESS
• GL_EQUAL GL_LEQUAL
• GL_GREATER GL_NOTEQUAL
• GL_GEUQAL GL_ALWAYS
• For instance, use as a mask for texture
• How would you render a fence?

21
Stencil Test

• Used to control drawing based on values in the
  stencil buffer
• Fragments that failt the stencil test are
  not drawn
• Example create a mask in stencil buffer and draw
  only objects not in mask area

22
Stencil Buffer

• Dont render into stencil buffer
• Control stencil buffer values with stencil
  function
• Can change stencil buffer values with each
  fragment passing or failing the test

23
Controlling Stencil Buffer

• For each pixel, what do I do? Look
• glStencilFunc( func, ref, mask )
• compare value in buffer with (ref AND mask) and
  (stencil_pixel AND mask) using func
• func is one of standard comparison functions
• glStencilOp( fail, zfail, zpass )
• Allows changes in stencil buffer based on
• Failing stencil test
• Failing depth test
• Passing depth test
• GL_KEEP, GL_INCR, GL_REPLACE, GL_DECR, GL_INVERT,
  GL_ZERO

24
Creating a Mask

• Initialize Mask
• glInitDisplayMode( GLUT_STENCIL )
• glEnable( GL_STENCIL_TEST )
• glClearStencil( 0x0 )
• glStencilFunc( GL_ALWAYS, 0x1, 0x1 )
• glStencilOp( GL_REPLACE, GL_REPLACE,
• GL_REPLACE )

25
Using Stencil Mask

• glStencilFunc( GL_EQUAL, 0x1, 0x1 )
• draw objects where stencil 1
• glStencilFunc( GL_NOT_EQUAL, 0x1, 0x1 )
• glStencilOp( GL_KEEP, GL_KEEP, GL_KEEP )
• draw objects where stencil ! 1

26
Dithering

• glEnable( GL_DITHER )
• Dither colors for better looking results
• Used to simulate more available colors
• Say, newspapers
• OGL will modify the color of a fragment with a
  dithering table

27
Logical Operations on Fragments

• Combine fragment and pixel color using bit-wise
  logical operations glLogicOp( mode )
• Common modes
• GL_CLEAR GL_SET GL_COPY,
• GL_COPY_INVERTED GL_NOOP GL_INVERT
• GL_AND GL_NAND GL_OR
• GL_NOR GL_XOR GL_AND_INVERTED
• GL_AND_REVERSE GL_EQUIV
• GL_OR_INVERTED GL_OR_REVERSE

28
Texturing

• Texture functions glTexEnv( ), specifies how
  texture values are combined with the color values
  of each fragment
• Texture environment mode Tables 9-4,5
• GL_DECAL
• GL_REPLACE
• GL_MODULATE
• GL_BLEND

29
Multi-Texturing

• Should have at least 2 texture units if
  multi-texturing is supported
• Each texture unit
• Texture image
• Filtering parameters
• Environment application
• Texture matrix stack
• Automatic texture-coordinate generation (doesnt
  seem obvious, but very have very creative apps)

30
Texture Matrix Stack

• At least 2 matrices deep
• 4x4 matrix
• Can take texture coordinates in homogeneous space
  (s, t, r, q)

31
More extensions

• Segal 1992, need OpenGL extensions.
  Multi-texturing is made extremely interesting and
  creative. Please check nvidia.com for white
  papers on these.
• Not in the current redbook published in 1997.
• ifndef GL_VERSION_texture_env_combine
• define GL_COMBINE 0x8570
• define GL_ADD_SIGNED 0x8574
• define GL_INTERPOLATE 0x8575
• define GL_CONSTANT 0x8576
• define GL_PRIMARY_COLOR 0x8577
• define GL_PREVIOUS 0x8578
• define GL_COMBINE_RGB 0x8571
• define GL_COMBINE_ALPHA 0x8572
• define GL_SOURCE0_RGB 0x8580
• define GL_SOURCE1_RGB 0x8581
• define GL_SOURCE2_RGB 0x8582
• define GL_SOURCE0_ALPHA 0x8588
• define GL_SOURCE1_ALPHA 0x8589
• define GL_SOURCE2_ALPHA 0x858A
• define GL_OPERAND0_RGB 0x8590
• define GL_OPERAND1_RGB 0x8591

32
OpenGL MultitextureExtension

• Standardized and widely available
• Introduces concept of multiple texture units
• Two or more texture units
• Existing texture API extended to implicitly
  useactive texture unit glActiveTextureARB(GL_TE
  XTUREn_ARB)
• Each unit has its own complete and independent
  OpenGL texture state
• Texture image, filtering modes,
  textureenvironment, texture matrix

33
OpenGL Multitexture Quick Tutorial

• Configuring up a given texture unitglActiveTextu
  reARB(GL_TEXTURE1_ARB)glBindTexture(GL_TEXTURE_2
  D, texObject)glTexImage2D(GL_TEXTURE_2D,
  )glTexParameterfv(GL_TEXTURE_2D,
  )glTexEnvfv(GL_TEXTURE_ENV, )glTexGenfv(GL_S
  , )glMatrixMode(GL_TEXTURE)glLoadIdentity()
• Setting texture coordinates for a
  vertexglMultiTexCoord4f(GL_TEXTURE0_ARB, s0,
  t0, r0 ,q0)glMultiTexCoord2f(GL_TEXTURE1_ARB,
  s1, t1)glMultiTexCoord3f(GL_TEXTURE2_ARB, s2,
  t2, r2)glVertex3f(x, y, z)

Sets active texture unit
Implicitlyupdate state of active texture unit
34
OpenGL Multitexture Texture Environments

• Chain of Texture Environment Stages

Pre-texturing colorglColor3f(r,g,b)
0
Lookup filter
glMultiTexCoord2f( GL_TEXTURE0_ARB, )
GL_MODULATE
1
glMultiTexCoord2f( GL_TEXTURE1_ARB, )
Lookup filter
GL_DECAL
2
Post-texturing color
glMultiTexCoord2f( GL_TEXTURE2_ARB, )
Lookup filter
GL_BLEND
35
Extending the OpenGLTexture Environment

• ARB_multitexture is just the start
• Standard OpenGL has just GL_REPLACE, GL_MODULATE,
  GL_DECAL, and GL_BLEND
• ENV_texture_env_add introduces GL_ADD
• ENV_texture_env_combine
• Powerful (A?B C) ? scale bias
• A, B, C variables use various color inputs
• Future texture environments even more powerful

36
Examples of More PowerfulTexture Environments

• NVIDIA GPU OpenGL extensions
• TNT and later GPUs support NV_texture_env_combine4
  
• A B C D computation
• multiple texture inputs available in a single
  stage
• enables single-pass emboss bump mapping
• GeForce and later GPUs support NV_register_combine
  rs
• dot products, signed math, much more
• register processing model for non-sequential data
  flows

37
Cube Map Texturing

• Direct Use of Cube Maps
• Latest hardware supports this
• DirectX 7 support
• OpenGL extension support
• Hardware directly performs cube map accesses
• Requires hardware support for good performance
• Cube map construction much easier

38
Using Cube Mapsin OpenGL

• Very easy to use ARB extension
• New texture target GL_TEXTURE_CUBE_MAP_ARB
• Used for manipulating cube map as a whole
• Six cube map face targets
• Used for manipulating particular cube map faces
• GL_TEXTURE_CUBE_MAP_dir_axis _ARB
• dir is POSITIVE, NEGATIVE
• axis is X, Y, Z
• Use 2D texture image API (glTexImage2D, etc)

39
Cube Map Support

• Official OpenGL standard now
• OpenGL cube map support was introduced by NVIDIA
• GeForce drivers have EXT_texture_cube_map
  extension
• OpenGL Architectural Review Board (ARB) recently
  accepted the extension as an ARB standard
• renamed ARB_texture_cube_map
• but same identical functionality
• semantics enumerant values same as the EXT
  version
• Other vendors such as ATI planning cube map
  support

40
OpenGL Cube MapSetup Usage

• Binding to a cube map texture
• glEnable(GL_TEXTURE_CUBE_MAP_ARB)
• glBindTexture(GL_TEXTURE_CUBE_MAP_ARB, cmap)
• Loading cube map faces
• glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X_ARB,
  level, format, width, height, border, format,
  type, positiveXpixels)
• glTexImage2D(GL_TEXTURE_CUBE_MAP_NEGATIVE_X_ARB,
  level, format, width, height, border, format,
  type, negativeXpixels)
• glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_Y_ARB,
  level, format, width, height, border, format,
  type, negativeYpixels)
• . . . similarly load other three faces . . .

41
OpenGL Cube Map Rendering Usage

• Enabling a cube map texture
• glEnable(GL_TEXTURE_CUBE_MAP_ARB)
• Explicit cube map coordinates
• glTexCoord3f(vx, vy, vz) // (vx,vy,vz) is
  unnormalized direction vector
• Generated cube map coordinates (recommended)
• glTexGeni(GL_S, GL_TEXTURE_GEN_MODE,
  GL_REFLECTION_MAP_ARB)glTexGeni(GL_T,
  GL_TEXTURE_GEN_MODE, GL_REFLECTION_MAP_ARB)glTex
  Geni(GL_R, GL_TEXTURE_GEN_MODE,
  GL_REFLECTION_MAP_ARB)glEnable(GL_TEXTURE_GEN_S)
  glEnable(GL_TEXTURE_GEN_T)glEnable(GL_TEXTURE_
  GEN_R)glEnable(GL_NORMALIZE)

42
Advantages of Cube Maps

• Compared to Other Approaches
• View-independent
• Unlike sphere mapping
• Uses a single texture unit
• Unlike dual-paraboloid mapping
• Entire texture resolution is sampled
• Unlike sphere or dual-paraboloid approaches
• Improved environment sampling

43
Getting Back to Object Space

• No magic here. Just undo the transformation.

Gldouble objX, objY, objZ // get the
modelview, project, and viewport glGetDoublev(
GL_MODELVIEW_MATRIX, mv_mat ) glGetDoublev(
GL_PROJECTION_MATRIX, proj_mat )
glGetIntegerv( GL_VIEWPORT, viewport ) // get
the current depth buffer g_depth_view new
GLfloatwinWidth winHeight glReadPixels( 0,
0, winWidth, winHeight, GL_DEPTH_COMPONENT,
GL_FLOAT, depth_view ) for( y 0 y lt
winHeight y ) // go
through every pixel in frame buffer
for( x 0 x lt winWidth x )
if ( depth_view lt 1.00 )
gluUnProject(x, y, depth_view, mv_mat, proj_mat,
viewport, objX, objY, objZ)
depth_view