12. Selection. - The objects selected are specified by hit records in the selection buffer. E.R. Bachmann & P.L. McDowell MV 4202 Page 1 of 13

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1 12. Selection Picking is a method of capturing mouse clicks at some window position and determining what objects are beneath it. The basic idea is to enter the selection rendering mode with a small viewing volume under the mouse pointer. Any object which would be rendered inside this small viewing volume is a hit. - The number of hits is given by glrendermode as the selection rendering mode is exited - The objects selected are specified by hit records in the selection buffer E.R. Bachmann & P.L. McDowell MV 4202 Page 1 of 13

2 Basic Steps 1. Specify the array to be used for the returned hit records with glselectbuffer( ) 2. Enter the selection mode using glrendermode() 3. Initialize the name stack using glinitnames() and glpushname() 4. Define the viewing volume to use in selection 5. Alternately issue primitive drawing commands and commands to manipulate the name stack so that each primitive of interest has an appropriate name assigned 6. Exit the selection mode and process the hit records E.R. Bachmann & P.L. McDowell MV 4202 Page 2 of 13

3 The Selection Buffer The selection buffer is filled with hit records during the selection mode rendering process - A hit record is generated whenever a named primitive is rendered that would have been contained in the viewing volume. - The selection buffer is an array of unsigned integers - Each hit record occupies at least four elements of the array First record element contains the number of names on the name stack when the hit occurred Second and third elements give the min and max z coordinates of the hit objects coordinates are normalized to the range [ ] * The remaining elements give the names of the hit objects The bottom stack element is given first E.R. Bachmann & P.L. McDowell MV 4202 Page 3 of 13

4 The selection buffer is specified using void glselectbuffer( GLsizei size, GLuint *buffer ); where - buffer is an array of unsigned integers that act as the selection buffer - size gives the maximum number of values in the array Rendering Modes Selection is one of three OpenGL rendering modes: Render - Default mode - All objects are drawn in the frame buffer Selection - Objects are not drawn in the frame buffer - Primitives that are drawn inside the viewing volume produce hit records Feedback - Objects are not drawn in the frame buffer - Information is written to the feedback buffer about how the scene would have been rendered E.R. Bachmann & P.L. McDowell MV 4202 Page 4 of 13

5 The rendering mode is set using where GLint glrendermode( GLenum mode ); - mode may be GL_RENDER (default), GL_SELECT, or GL_FEEDBACK - The rendering mode will remain at the set value until glrendermode is called with a different argument - The return value is the number of selection hits when leaving the selection mode and entering rendering mode - A selection buffer must be specified before entering the selection mode The Name Stack Objects or groups of primitives can be named - Object names are unsigned integers A names list is maintained on the name stack - Names can be pushed onto the stack or can replace the current top name - When a hit occurs during selection, all names on the stack are copied into the selection buffer E.R. Bachmann & P.L. McDowell MV 4202 Page 5 of 13

6 The name stack is managed using void glinitnames( void ); - Clears the name stack void glpushname( GLuint name ); - Pushes name onto the stack - The stack has a max depth of at least 64 void glpopname( void ); - Pops one name off the top of the stack void glloadname( GLuint name ); - Replaces the name on top of the stack with name - Generates an error if the stack is empty (Call glpushname( 0 ); following stack initialization to avoid this) Calls to glpushname(), glpopname(), and glloadname() are ignored when not in the selection mode E.R. Bachmann & P.L. McDowell MV 4202 Page 6 of 13

7 Example 12.1 Naming the planets and moons. #define EARTH 1 #define MARS 2 #define MOON1 3 #define MOON2 4 // Called to draw scene void COpenGLView::OnDraw(CDC* pdc) { // Clear the window with current clearing color glclear( GL_COLOR_BUFFER_BIT GL_DEPTH_BUFFER_BIT ); // Save the matrix state and do the rotations glmatrixmode( GL_MODELVIEW ); glloadidentity(); // Translate the whole scene out and into view gltranslatef( 0.0f, 0.0f, f ); // Initialize the names stack glinitnames(); glpushname( 0 ); // to avoid stack empty error // Draw the Earth glpushmatrix(); glcolor3f( 0.0f, 0.0f, 1.0f ); gltranslatef( f, 0.0f, 0.0f ); glloadname( EARTH ); auxsolidsphere( 30.0 ); // Draw the Moon gltranslatef( 45.0f, 0.0f, 0.0f ); glcolor3f( 0.86f, 0.86f, 0.86f ); glpushname( MOON1 ); auxsolidsphere( 5.0 ); E.R. Bachmann & P.L. McDowell MV 4202 Page 7 of 13

8 glpopname(); glpopmatrix(); // Draw Mars glcolor3f( 1.0f, 0.0f, 0.0f ); glpushmatrix(); gltranslatef( 100.0f, 0.0f, 0.0f ); glloadname( MARS ); auxsolidsphere( 20.0 ); // Draw Moon1 gltranslatef( -40.0f, 40.0f, 0.0f ); glcolor3f(.75f,.75f,.75f ); glpushname( MOON1 ); auxsolidsphere( 5.0 ); glpopname(); // Draw Moon2 gltranslatef( 0.0f, -80.0f, 0.0f ); glpushname( MOON2 ); auxsolidsphere( 5.0 ); glpopname(); glpopmatrix(); // Restore the matrix state glpopmatrix(); // Modelview matrix // Flush drawing commands glflush(); } SwapBuffers( m_pdc->getsafehdc() ); E.R. Bachmann & P.L. McDowell MV 4202 Page 8 of 13

9 Picking Picking occurs when the mouse position is used to create a modified viewing volume during selection - Usually triggered by a mouse click in the window - A special picking matrix is used in conjunction with the projection matrix to restrict the drawing to a small viewing volume - The position of the viewing volume is determined using the mouse position - Objects drawn inside this viewing volume cause hits The picking matrix is created using void glupickmatrix( GLdouble x, GLdouble y, GLdouble width, GLdouble height, GLint viewport[4] ); where - x and y specify the center of the picking region in window coordinates - width and height define the size of the picking region in pixels - viewport indicates the current viewport boundaries The current viewport can be obtained with the call glgetintegerv(gl_viewport, GLint *viewport ); E.R. Bachmann & P.L. McDowell MV 4202 Page 9 of 13

10 glupickmatrix() should be called prior to multiplying a standard projection matrix using gluperspective(), glfrustum() or glortho() - The current projection matrix is normally saved first using glpushmatrix() Example 12.2 Processing a mouse selection. #define BUFFER_LENGTH 64 void ProcessSelection( int xpos, int ypos ) { // Space for selection buffer GLuint selectbuff[ BUFFER_LENGTH ]; // Hit counter and viewport storeage GLint hits, viewport[4]; // Setup selection buffer glselectbuffer( BUFFER_LENGTH, selectbuff ); // Get the viewport glgetintegerv( GL_VIEWPORT, viewport ); // Switch to projection and save the matrix glmatrixmode( GL_PROJECTION ); glpushmatrix(); // Change render mode glrendermode( GL_SELECT ); // Establish new clipping volume // around mouse cursor point // (xpos, ypos) extending two pixels // in the vertical and horizontal directions glloadidentity(); glupickmatrix( xpos, (viewport[3] - ypos), 20, 20, viewport ); E.R. Bachmann & P.L. McDowell MV 4202 Page 10 of 13

11 // Apply perspective matrix gluperspective( 45.0f, faspect, 1.0, ); // Draw the scene glmatrixmode( GL_MODELVIEW ); glloadidentity(); setviewpoint(); drawscene(); // Collect the hits hits = glrendermode( GL_RENDER ); // If a single hit occurred, display the info if ( hits >= 1 ) { ProcessPlanet( selectbuff ); } // Restore the projection matrix glmatrixmode( GL_PROJECTION ); glpopmatrix(); // Go back to modelview for normal rendering glmatrixmode( GL_MODELVIEW ); } // end ProcessSelection E.R. Bachmann & P.L. McDowell MV 4202 Page 11 of 13

12 Processing the hits void ProcessPlanet( GLuint *pselectbuff ) { int id, count; // How many names on the name stack count = pselectbuff[0]; // Bottom of the name stack id = pselectbuff[3]; // Select on Earth or Mars, whichever was picked switch ( id ) { case EARTH: ConsoleOut( You clicked Earth.\n ); // If there is another name on the name // stack, then it must be the moon that was // selected -- this is what was actually // clicked on if ( count == 2 ) ConsoleOut( Specifically the moon.\n ); break; case MARS: ConsoleOut("You clicked Mars.\n"); // The name stack is only two deep. // The precise moon that was selected will // be here. if ( count == 2 ){ if ( pselectbuff[4] == MOON1 ){ ConsoleOut( Specifically Moon #1.\n ); } else { ConsoleOut( Specifically Moon #2.\n ); } } E.R. Bachmann & P.L. McDowell MV 4202 Page 12 of 13

13 break; default: ConsoleOut( Nothing was clicked on!\n ); break; } } // end ProcessPlanet E.R. Bachmann & P.L. McDowell MV 4202 Page 13 of 13

Better Interactive Programs

Better Interactive Programs Objectives Learn to build more sophisticated interactive programs using Picking Select objects from the display Rubberbanding Interactive drawing of lines and rectangles Display