Introduction to OpenGL Part 4

1
Introduction to OpenGL(Part 4)

• Ref OpenGL Programming Guide (The Red Book)

2
Topics

• Part 1
• Introduction
• Geometry
• Viewing
• Light Material
• Display List

• Part 2
• Alpha Channel
• Polygon Offset
• Part 3
• Image
• Texture Mapping
• Part 4
• Framebuffers
• Selection Feedback

3
OpenGL

• Framebuffers

4
Types of Buffers

• color (front/back, left/right, aux)
• Front/back animation
• Left/right stereo vision
• Aux not available on PC (use framebuffer object
  FBO instead)

• depth buffer depth test
• VC 24 bits available
• Value 0.0,1.0
• stencil buffer restrict drawing to certain
  portions of the screen
• VC 8 bits available
• accumulation buffer accumulating composite image

5
Using Framebuffers

• clearing buffers
• clearing individual buffer is expensive
• Use glClear with bitwise-ORed masks to clear
  multiple buffers
• selecting color buffers for writing/clearing
• glDrawBuffer useful in FBO (framebuffer object)

6
Masking Buffers

• Before OpenGL writes data into the enabled color,
  depth, or stencil buffers, a masking operation is
  applied to the data, as specified with one of the
  following commands.
• A bitwise logical AND is performed with each mask
  and the corresponding data to be written

7
Masking Buffers (cont)

• void glColorMask(GLboolean red, GLboolean green,
  GLboolean blue, GLboolean alpha)
• void glDepthMask(GLboolean flag)
• void glStencilMask(GLuint mask)
• If a 1 appears in mask, the corresponding bit in
  the stencil buffer is written where a 0 appears,
  the bit is not written.
• The default values of all the GLboolean masks are
  GL_TRUE, and the default values for the two
  GLuint masks are all 1's

8
Only Green Mask TRUE
9
Per-fragment Operation Sequence

• Fragments the pieces after rasterization, sent
  to framebuffers if they survive these tests
• glScissor
• restrict drawing to part of the window
• a simpler (and faster) version of stencil test
  (that does not need stencil buffer)
• application gui panel

Dithering and logical op not covered here
10
Scissor Test

• While the scissor test is enabled, only pixels
  that lie within the scissor box can be modified
  by drawing commands.
• Including glClear (shown right)
• Note validity of current raster position is
  determined by frustum (world coordinate) while
  scissor test affects the window coordinate
• Single-pixel window glScissor (x,y,1,1)

See also stencil_close
11
Fragment Operations (cont)

• Alpha test
• accept/reject a fragment based on its alpha value
• implement transparency
• use this test to filter opaque objects
• see-through decal (billboarding) reject the
  transparent fragments (from ruining the depth
  buffer)

• Stencil Test
• require stencil buffer
• glStencilFunc, glStencilOp

12
Stenciling
13
Basic Applications

• Restrict rendering to limited portions of the
  screen
• Stenciling
• Decal
• Shadow and lightmap

14
Mimicking Stencil

• Compose stencil template
• Control template then render
• Multi-pass rendering

silhouette
15
Decal Using Stencil Buffers
Stencil to resolve z-fighting
16
Shadow and Lightmap
Shadow and light blended with the textured
polygon underneath
17
Technical Details

• glStencilFunc (fun, ref, mask)
• func Specifies the test function. Eight tokens
  are valid GL_NEVER, GL_LESS, GL_LEQUAL,
  GL_GREATER, GL_GEQUAL, GL_EQUAL, GL_NOTEQUAL, and
  GL_ALWAYS.
• If it's GL_LESS, for example, then the fragment
  passes if reference value is less than the value
  in stencil buffer
• ref Specifies the reference value for the stencil
  test. ref is clamped to the range 0, 2n - 1,
  where n is the number of bitplanes in the stencil
  buffer.
• VC n 8
• mask Specifies a mask that is ANDed with both the
  reference value and the stored stencil value when
  the test is done

18
Technical Details (cont)

• glStencilOp (fail, zfail, zpass)
• fail Specifies the action to take when the
  stencil test fails.
• zfail Specifies stencil action when the stencil
  test passes, but the depth test fails.
• zpass Specifies stencil action when both the
  stencil test and the depth test pass, or when the
  stencil test passes and either there is no depth
  buffer or depth testing is not enabled.
• Six symbolic constants are accepted GL_KEEP,
  GL_ZERO, GL_REPLACE, GL_INCR, GL_DECR, and
  GL_INVERT.

Set value to ref
Bitwise invert
19
Stenciling

• Steps to draw 2 coplanar rectangles
• Make the stencil for yellow one first (by drawing
  the green polygon)
• Draw the yellow one with the stencil
• Draw the green one

20
Stenciling (cont)
Stencil buffer
Color buffer
21
Decaling
0 Base (-5,0)?(5,6) 1 R
(-3,1)?(2,4) 2 G (1,2)?(3,5) 3 B
(0,0)?(4,3)
Foreground blocker

• Draw everything else (set up the depth buffer)
• Draw base set the blocker id to be 8 rest 0
• Arrange the decal id in increasing order
• The decals can be drawn in any order

Test the program
22
StencilOp(fail, zfail, zpass)
Color buffer
Stencil buffer
23
StencilOp(fail, zfail, zpass)
Color buffer
Stencil buffer
Drawn in different order!
24
Generating Silhouette
Stencil Buffer
Debug with GLIntercept
25
See also

• Different line style
• Hidden line removal
• Haloed lines

• Cheap silhouette

26
Stencil Planar Shadow

• Shadow matrix to project model onto a flat shadow
  receiver
• Blending stencil buffer techniques involved

27
CSG (constructive solid geometry)
Cone INTERSECT Sphere
Cone SUBTRACT Sphere
Sphere UNION Cone
28
Reflection
Dinosaur is reflected by the planar floor.
Easy hack, draw dino twice, second time
hasglScalef(1,-1,1) to reflect through the floor
Notice right images reflection falls off the
floor!
29
Stencil Reflection
Clear stencil to zero.Draw floor polygon with
stencil set to one.Only draw reflection where
stencil is one.
Reflection and shadow
30
Shadow Volume Using Stencil Buffers
Triangle blockinglight source.
Use stencil to tag whetherpixel is inside or
outsideof the shadow volume. Two passes light
pixels outsidevolume no lighting forpixels
inside the volume.
Shadow volumeprojected by trianglefrom the
light source.
31
More Complicated Case
32
Hybrid Shadows
Shadow volumes
Planar projected shadows
33
Accumulation Buffer

• series of images generated on standard color
  buffers accumulated one at a time into the
  accumulation buffers result copied back into a
  color buffer for viewing

34
Accumulation Buffer (cont)

• scene antialiasing, motion blur, photographic
  depth of field
• softshadow, jittering
• analogy multiple exposures
• takes longer, but higher quality

• accum.buffer may have higher precision
• 16 bits for each RGBA channels
• scene antialiasing by spatially jittering the
  image

35
Accumulation Buffer APIs

• glClearAccum (0,0,0,0)
• glGetIntegerv (GL_ACCUM_RED_BITS, value)
• 16 for each RGBA channels
• glAccum(GL_ACCUM, 1.f/counts)
• glAccum(GL_RETURN, 1.f)

36
Accumulation Buffer
scene antialiasing
motion blur
depth of field
37
Ref
38
FSAA with Multi-Sampling(ref)

• glMultiSample

39
Motion Blur w/o Accum.Buffer
Details scene dynamically render to texture
modulate with a polygon (1,1,1,a)
40
Scene Anti-aliasing

• Can be done in real-time w/o accumulation buffer
• By outlining the silhouette edges
• See Tom Hall

FPS 10.8
FPS 9.6
41
OpenGL

• Selection and Feedback

42
How to do Picking?

• Revisit unproject.c
• A line segment between (x1, y1, zNear) and (x2,
  y2, zFar)
• Check which object got hit
• Complexity problem?!
• Can do better?

Simple tracking on XZ plane
43
Selection (Basic Idea)

• draw scene into framebuffer
• enter selection mode and redraw the scene
• the content of framebuffer unchanged until you
  exit the selection mode
• when exit, returns a list of primitives that
  intersected the viewing volume
• as an array of names (integer) and hit records
• Picking a variation of selection
• triggered by mouse mouse defines v.volume

44
Selection (Steps)

• glSelectBuffer (maxselects, selectbuffer)
• glRenderMode (GL_SELECT)
• glInitNames, glPushName (0)
• Something needs to be pushed on the empty stack
  (otherwise, it cannot be loaded)
• define viewing volume
• Need to save current projection matrix for
  further rendering
• issue drawing commands
• Load a name then draw something if the thing
  drawn is in v.volume, it generates a hit (with
  the name)
• glRenderMode (GL_RENDER)
• Return the number of hits
• process select buffer for hits

45
Name Stack

• name GLuint
• glInitNames
• causes the name stack to be initialized to its
  default empty state
• glPushName/glPopName
• as the name implies
• glLoadName
• replace the top of the stack
• need to push something right after InitNames for
  first load
• commands ignored unless in selection mode

46
Hit Record

• Primitive intersecting v.vol causes a hit
• stack pointer points to the beginning of
  selection array and updates when hit occurs
• culled polygons generate no hit
• composite objects (w/ single name) generate one
  hit
• hit records arent updated until glRenderMode is
  called

• each hit record
• number of names on the name stack
• min and max window-coord z values
• 0,1
• contents of name stack w/ bottommost element first

47
void selectObjects(void) GLuint
selectBuf512 GLint hits glSelectBuffer
(512, selectBuf) (void) glRenderMode
(GL_SELECT) glInitNames()
glPushName(0) glPushMatrix ()
glMatrixMode (GL_PROJECTION) glLoadIdentity
() glOrtho (0.0, 5.0, 0.0, 5.0, 0.0, 10.0)
glMatrixMode (GL_MODELVIEW) glLoadIdentity
() glLoadName(1) drawTriangle (2.0, 2.0,
3.0, 2.0, 2.5, 3.0, -5.0) glLoadName(2)
drawTriangle (2.0, 7.0, 3.0, 7.0, 2.5, 8.0,
-5.0) glLoadName(3) drawTriangle (2.0,
2.0, 3.0, 2.0, 2.5, 3.0, 0.0) drawTriangle
(2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -10.0)
glPopMatrix () hits glRenderMode
(GL_RENDER) processHits (hits, selectBuf)
void display(void) glClearColor (0.0, 0.0,
0.0, 0.0) glClear(GL_COLOR_BUFFER_BIT
GL_DEPTH_BUFFER_BIT) drawScene ()
selectObjects () glFlush()
void drawScene (void) glMatrixMode
(GL_PROJECTION) glLoadIdentity ()
gluPerspective (40.0, 4.0/3.0, 1.0, 100.0)
glMatrixMode (GL_MODELVIEW) glLoadIdentity
() gluLookAt (7.5, 7.5, 12.5, 2.5, 2.5, -5.0,
0.0, 1.0, 0.0) glColor3f (0.0, 1.0, 0.0) /
green / drawTriangle (2.0, 2.0, 3.0, 2.0,
2.5, 3.0, -5.0) glColor3f (1.0, 0.0, 0.0) /
red / drawTriangle (2.0, 7.0, 3.0, 7.0,
2.5, 8.0, -5.0) glColor3f (1.0, 1.0, 0.0) /
yellow / drawTriangle (2.0, 2.0, 3.0, 2.0,
2.5, 3.0, 0.0) drawTriangle (2.0, 2.0, 3.0,
2.0, 2.5, 3.0, -10.0) drawViewVolume (0.0,
5.0, 0.0, 5.0, 0.0, 10.0) void processHits
(GLint hits, GLuint buffer) unsigned int
i, j GLuint names, ptr printf ("hits
d\n", hits) ptr (GLuint ) buffer for
(i 0 i lt hits i) / for each hit /
names ptr printf (" number of names
for hit d\n", names) ptr printf("
z1 is g", (float) ptr/0x7fffffff) ptr
printf(" z2 is g\n", (float) ptr/0x7fffffff)
ptr printf (" the name is ")
for (j 0 j lt names j) / for each name
/ printf ("d ", ptr) ptr
printf ("\n")
48
Process Hits

• When returning to RENDER mode,
• All hit records got recorded in a GLuint array as
  follows
• of names, zmin, zmax, names
• Names for an object need not be a single integer
• object type, object number, object part,

49
Picking

• restrict drawing to a small region of the
  viewport (near the cursor)
• mouse click to initiate the selection mode
• Issues
• interpreting mouse (x,y)!
• hierarchical pick name stack manipulation
• 3D pick (picking and depth values)

50
gluPickMatrix

• creates a projection matrix that can be used to
  restrict drawing to a small region of the viewport

Often called from mouse callback need to convert
glut to OpenGL mouse coordinates
51
void drawSquares(GLenum mode) GLuint i, j
for (i 0 i lt 3 i) if (mode
GL_SELECT) glLoadName (i) for (j
0 j lt 3 j ) if (mode
GL_SELECT) glPushName (j)
glColor3f ((GLfloat) i/3.0, (GLfloat) j/3.0,
(GLfloat) boardij/3.0)
glRecti (i, j, i1, j1) if (mode
GL_SELECT) glPopName ()
void processHits (GLint hits, GLuint
buffer) unsigned int i, j GLuint ii,
jj, names, ptr printf ("hits d\n",
hits) ptr (GLuint ) buffer for (i 0
i lt hits i) / for each hit / names
ptr printf (" number of names for this
hit d\n", names) ptr printf(" z1 is
g", (float) ptr/0x7fffffff) ptr
printf(" z2 is g\n", (float) ptr/0x7fffffff)
ptr printf (" names are ") for
(j 0 j lt names j) / for each name /
printf ("d ", ptr) if (j 0)
/ set row and column / ii
ptr else if (j 1) jj
ptr ptr
printf ("\n") boardiijj
(boardiijj 1) 3 void
pickSquares(int button, int state, int x, int
y) GLuint selectBuf512 GLint hits
GLint viewport4 if (button !
GLUT_LEFT_BUTTON state ! GLUT_DOWN)
return glGetIntegerv (GL_VIEWPORT,
viewport) glSelectBuffer (512, selectBuf)
(void) glRenderMode (GL_SELECT)
glInitNames() glPushName(0) glMatrixMode
(GL_PROJECTION) glPushMatrix ()
glLoadIdentity () gluPickMatrix ((GLdouble)
x, (GLdouble) (viewport3 - y),
5.0, 5.0, viewport) gluOrtho2D (0.0, 3.0,
0.0, 3.0) drawSquares (GL_SELECT)
glMatrixMode (GL_PROJECTION) glPopMatrix ()
glFlush () hits glRenderMode
(GL_RENDER) processHits (hits, selectBuf)
glutPostRedisplay()
52
Example picking spheres and cubes
53
Hints on Using Selection

• 2D application
• handle your own picking
• 3D
• organize your program and data structure so that
  it is easy to draw list of object in either
  selection or rendering mode
• Use name creatively

54
Feedback

• key difference what information is sent back
• transformed primitives is sent back to an array
  of floating point values (window coordinate)
• tokens of primitive type and other data for that
  primitive

• steps
• glFeedbackBuffer
• glRenderMode (GL_FEEDBACK)
• draw primitives
• glRenderMode (GL_RENDER)
• parse the feedback array

55
void drawGeometry (GLenum mode) glBegin
(GL_LINE_STRIP) glNormal3f (0.0, 0.0, 1.0)
glVertex3f (30.0, 30.0, 0.0) glVertex3f
(50.0, 60.0, 0.0) glVertex3f (70.0, 40.0,
0.0) glEnd () if (mode GL_FEEDBACK)
glPassThrough (1.0) glBegin (GL_POINTS)
glVertex3f (-100.0, -100.0, -100.0) /
will be clipped / glEnd () if (mode
GL_FEEDBACK) glPassThrough (2.0)
glBegin (GL_POINTS) glNormal3f (0.0, 0.0,
1.0) glVertex3f (50.0, 50.0, 0.0) glEnd
() / Write contents of one vertex to stdout
/ void print3DcolorVertex (GLint size, GLint
count, Lfloat buffer) int i printf ("
") for (i 0 i lt 7 i) printf
("4.2f ", buffersize-(count)) count
count - 1 printf ("\n")
/ Write contents of entire buffer. (Parse
tokens!) / void printBuffer(GLint size, GLfloat
buffer) GLint count GLfloat token
count size while (count) token
buffersize-count count-- if (token
GL_PASS_THROUGH_TOKEN) printf
("GL_PASS_THROUGH_TOKEN\n") printf ("
4.2f\n", buffersize-count) count--
else if (token GL_POINT_TOKEN)
printf ("GL_POINT_TOKEN\n")
print3DcolorVertex (size, count, buffer)
else if (token GL_LINE_TOKEN)
printf ("GL_LINE_TOKEN\n")
print3DcolorVertex (size, count, buffer)
print3DcolorVertex (size, count, buffer)
else if (token GL_LINE_RESET_TOKEN)
printf ("GL_LINE_RESET_TOKEN\n")
print3DcolorVertex (size, count, buffer)
print3DcolorVertex (size, count, buffer)

void display(void) GLfloat
feedBuffer1024 GLint size glMatrixMode
(GL_PROJECTION) glLoadIdentity () glOrtho
(0.0, 100.0, 0, 100.0, 0.0, 1.0)
glClearColor (0.0, 0.0, 0.0, 0.0)
glClear(GL_COLOR_BUFFER_BIT) drawGeometry
(GL_RENDER) glFeedbackBuffer (1024,
GL_3D_COLOR, feedBuffer) (void) glRenderMode
(GL_FEEDBACK) drawGeometry (GL_FEEDBACK)
size glRenderMode (GL_RENDER) printBuffer
(size, feedBuffer)
56
Feedback (cont)

• feedback picking
• more refined picking (not just pick object, but
  some point on the object)
• feedback buffer type
• 2D/3D/4D COLOR TEXTURE
• glPassThrough
• tokens that helps parsing the feedback data

• feedback array syntax
• (table 12-2)
• an application
• rendereps.c
• use feedback buffer info to write EPS

57
End of Part 4
58
GLIntercept

• An OpenGL debugging facility that intercepts
  OpenGL calls to create informative logs
• Comes with some built-in logging scripts
• Rename the one you choose to gliConfig.ini
• The one particular useful for stencil buffer
  debugging is gliConfig_XMLFrame.ini
• Ctrl-shift-key to create frame log
• View the resulting XML in IE

59
Copy OpenGL32.dll and the chosen/renamed
gliConfig.ini to the directory where EXE resides
Each ctrl-shift-F creates a Frame log
Use IE to open the XML
60
gliConfig_XMLFrame.ini
By default, it only does color buffer logging If
you need to see depth and/or stencil
buffers, modify these two lines.
61
Example (Stencil Silhouette)
(pre,post,diff)
Red means no difference Green means same pixels
Click to enlarge
62
Example (cont)
BACK