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Chapter 13. Selection and Feedback

Chapter 13. Selection and Feedback. OpenGL Programming Guide Third Edition Presented by Jung Lee. 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. Interactive Applications.

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Chapter 13. Selection and Feedback

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  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 don’t 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 stack’s 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 viewport[4]); • 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 <Table 13-1>

  35. The Feedback Array • Begins with a code indicating the primitive type • Followed by the values • The primitive’s vertices • Associated data • Pass-through markers can be returned

  36. Feedback Array Syntax <Table 13-2>

  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: 1~64

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