Chapter 13. Selection and Feedback

1
Chapter 13.Selection and Feedback

• OpenGL Programming Guide Third Edition
• Presented by Jung Lee

2
Chapter Objectives

• Select a region of the screen
• Pick an object drawn on the screen
• Use the OpenGL feedback mode
• To obtain the results of rendering calculations

3
Interactive Applications

• Identify, modify, delete, and manipulate
• Difficult to know which object is selected
• Since objects on the screen typically undergo
• Multiple rotations, translations, and perspective
  transformations

4
Selection and Feedback Modes

• In both selection and feedback modes
• Drawing information is returned
• Rather than being sent to the framebuffer
• The contents the other buffers are not affected
• Section overview
• Selection / Picking
• How to use selection mode and related routines
• Feedback
• How to obtain drawing information
• How that information is formatted

5
Selection
6
Overview

• Process
• Draw your scene into the framebuffer
• Enter selection mode
• Redraw the scene
• In selection mode
• The contents of the framebuffer dont change
• When exiting selection mode
• Returns a list of the primitives that intersect
  the viewing volume

7
Hit

• Each primitive that intersects the viewing volume
• Causes a selection hit
• The list of primitives is returned as
• An array of integer-valued names and
• Related data the hit records
• Correspond to the current contents of the name
  stack

8
The Basic Steps (1/2)

• Specify the array to be used for the returned hit
  records
• glSelectBuffer()
• Enter selection mode
• glRenderMode(GL_SELECT)
• Initialize the name stack
• glInitNames() and glPushName()

9
The Basic Steps (2/2)

• Define the viewing volume
• Usually different from the viewing volume to draw
  the scene
• Save and then restore the current transformation
  state
• glPushMatrix() and glPopMatrix()
• Primitive drawing commands and commands to
  manipulate the name stack
• Each primitive of interest is assigned an
  appropriate name
• Exit selection mode
• Process the returned selection data
• Hit records

10
void glSelectBuffer(GLsizei size, GLuint
buffer)

• Specify the array to be used for the returned
  selection data
• buffer
• A pointer to an array of unsigned integers
• size
• Maximum number of values that can be stored

11
GLint glRenderMode(GLenum mode)

• Controls whether the application is in
• Rendering, selection, or feedback mode
• mode ? glGetIntegerv(GL_RENDER_MODE)
• GL_RENDER(default), GL_SELECT, GL_FEEDBACK
• Before selection/feedback mode is entered
• glSelectBuffer() or glFeedbackBuffer()
• Return value when either mode is exited
• The number of selection hits or
• The number of values in the feedback array
• Negative value the array is overflowed

12
Creating the Name Stack

• Initialize with glInitNames()
• Add integer names
• While issuing corresponding drawing commands
• Commands to manipulate the name stack
• glPushName(), glPopName(), glLoadName()
• Calls are ignored if not in selection mode
• Same drawing code to simplify your code
• For both selection and rendering modes

13
Example 13-1

• glInitNames()
• glPushName()
• glPushMatrix() / save the current
  transformation state /
• / create your desired viewing volume here /
• glLoadName(1)
• drawSomeObject()
• glLoadName(2)
• drawAnotherObject()
• glLoadName(3)
• drawYetAnotherObject()
• drawJustOneMoreObject()
• glPopMatrix() / restore the previous
  transformation state /

14
void glInitNames(void)

• Clears the name stack

15
void glPushName(GLuint name)

• Pushes name onto the name stack
• Pushing a name beyond the capacity of the stack
  generates error
• GL_STACK_OVERFLOW
• The name stacks depth
• Vary among different OpenGL implementations
• At least 64 names
• glGetIntegerv(GL_NAME_STACK_DEPTH)

16
void glPopName(void)

• Pops a name off the top of the name stack
• Popping an empty stack
• Generates the error, GL_STACK_UNDERFLOW

17
void glLoadName(GLuint name)

• Replace the value at the top of the name stack
  with name
• If the stack is empty
• Generates the error, GL_INVALID_OPERATION

18
The Hit Record

• Whenever a name stack manipulation command is
  executed or
• glRenderMode() is called
• Written into the selection array
• If there has been a hit since the last time
• Valid coordinates produced by glRasterPos()
• Can cause a selection hit
• If the polygon would have been culled
• No hit occurs

19
Contents of the Hit Record

• The number of names on the name stack
• Both the minimum and maximum z-values
• Of all vertices of the primitives
• Lie in the range 0, 1
• Multiplied by 232-1(0x7fffffff)
• The contents of the name stack at the hit
• With the bottommost element first

20
Example 13-2. select
21
Picking
22
Overview

• Use a special picking matrix
• In conjunction with the projection matrix
• To restrict drawing to a small region of the
  viewport
• Differences from regular selection mode
• Usually triggered by an input device
• gluPickMatrix()
• Multiply the current projection matrix by a
  picking matrix
• Called prior to multiplying a standard projection
  matrix
• gluPerspective() or glOrtho()
• Save the projection matrix first

23
Simple Example

• glMatrixMode(GL_PROJECTION)
• glPushMatrix()
• glLoadIdentity()
• gluPickMatrix(...)
• gluPerspective, glOrtho, gluOrtho2D, or
  glFrustum
• / draw scene for picking perform picking /
• glPopMatrix()

24
gluPickMatrix(GLdouble x, GLdouble y, GLdouble
width, GLdouble height, GLint viewport4)

• Creates a projection matrix
• Restricts drawing to a small region of the
  viewport
• Be multiplied onto the current matrix stack
• Parameters
• (x, y) the center of the picking region
• In window coordinates ex) cursor location
• width and height the size of the picking region
• In screen coordinates
• viewport current viewport boundaries
• glGetIntegerv(GL_VIEWPORT, GLint viewport)

25
Window Coordinates
26
Miscellaneous

• Another way to perform picking
• Described in chapter 14
• Use color values
• To identify different components of an object
• Hints for writing a program with selection
• For 2D drawing programs
• Easier to do your own picking calculations
• ex) Bounding boxes for 2D objects
• Extremely fast and simple

27
Example 13-3. picksquare
28
Example 13-6. pickdepth
29
pickdepth2
30
Feedback
31
Characteristics

• Similar to selection
• The information that is sent back
• In selection mode
• Assigned names
• To an array of integer values
• In feedback mode
• Drawing information about transformed primitives
• To an array of floating-point values
• Type of primitive, vertex, color, or other data
• Transformed by lighting and viewing operations
• glRenderMode(GL_FEEDBACK)

32
How You Enter and Exit Feedback Mode

• Call glFeedbackBuffer()
• Specify the array to hold the feedback
  information
• Call glRenderMode(GL_FEEDBACK)
• Draw the primitives
• Can make several calls to glPassThrough()
• Insert markers into the returned feedback data
• Exit feedback mode
• By calling glRenderMode(GL_RENDER)
• The returned integer value
• The number of values stored in the feedback array

33
void glFeedbackBuffer(GLsizei size, GLenum type,
GLfloat buffer)

• Establishes a buffer for the feedback data
• Must be called before feedback mode is entered
• buffer
• Pointer to an array
• size
• Maximum number of values
• type
• Information fed back for each vertex

34
glFeedbackBuffer() type Values
ltTable 13-1gt
35
The Feedback Array

• Begins with a code indicating the primitive type
• Followed by the values
• The primitives vertices
• Associated data
• Pass-through markers can be returned

36
Feedback Array Syntax
ltTable 13-2gt
37
Using Markers in Feedback Mode

• Feedback occurs after
• Transformations, lighting, polygon culling, and
  interpretation of polygons
• By glPolygonMode()
• Polygons with more than three edges
• Broken up into triangles
• Call glPassThrough()
• Cause GL_PASS_THROUGH_TOKEN to be written into
  the feedback array
• To help parse the feedback data
• To separate the feedback values

38
void glPassThrough(GLfloat token)

• Inserts a marker into the stream of values in
  feedback array
• The marker consists of
• The code GL_PASS_THROUGH_TOKEN
• Followed by a single floating-point value, token
• No effect when called outside of feedback mode
• Calling between glBegin() and glEnd()
• Generates a GL_INVALID_OPERATION error

39
Example 13-7. feedback
40
glGetIntegerv()

• GL_MAX_CLIP_PLANES 6
• GL_MAX_LIGHTS 8
• GL_MAX_MODELVIEW_STACK_DEPTH 32
• GL_MAX_NAME_STACK_DEPTH 128
• GL_MAX_PROJECTION_STACK_DEPTH 10
• GL_POINT_SIZE_RANGE 164