CAP 4703 Computer Graphic Methods Prof. Roy Levow Lecture 3

Input and Interaction  User interaction is an important aspect of many graphic applications  Best supported through a standard API –Programmer avoids nasty implementaiton details  GLUT provides support for OpenGL

Input Devices  API supports logical devices –Defined by capabilities rather than physical design  Different physical devices may provide same capabilities –Mouse, Trackball, possibly Light Pen

Types of Input Devices  Pointing device –Mouse, Trackball –Data tablet reports position of stylus on tablet surfacd –Light pen reports position of pen on screen –Reports position in image  Keyboard device –Character stream

Types of Input Devices (cont)  Joystick –Reports x-y displacement –Usually treated as direction vector  Space ball –Fixed ball on stick –Senses x, y, x pressure as well as 3-axis rotational pressure

Logical Input Devices  String device –Provides character strings as input  Locator Device –Provides position in world coordinates  Pick device –Returns a reference to an object –Usually implemented with a locator

Logical Input Devices (cont)  Choice device –Selection from a discrete number of objects –Usually impemented with a window widget: menu, button, etc.  Dial device –Provides digitized analog input  Stroke device –Returns array of locations –Often the track of a locator device

Measure and Trigger  Measure of a device is the data it provides to the program  Trigger is the action that causes the data to be sent

Input Modes  Request mode –Program requests data from device –Requires trigger but input may not be accepted by program until a later time of the programmer’s choice  Sample mode –Input is immediate –No trigger is needed

Input Modes (cont)  Event mode –Supports asynchronous interaction –Events are queued for event handler

Clients and Servers  Servers perform tasks for clients –Client requests –Server responds  A windowed system is a server –Provides display and input services to a program using the display

Display Lists  Display list or display file –Instructions to display processor to generate image –Stored in memory  Immediate mode –Display server generates image and does not retain commands  Retained mode –Display list is stored on graphics server –Can be used to regenerate images

Definition & Execution of Display Lists  Generated like geometric objects  Bounded by –glNewList(item, GL_COMPILE) –glEndList()  Imaging commands are generated and sent to display server but not displayed  Displayed immedaitely if second parameter is GL_COMPILE_AND_EXECUTE

Executing Display Lists  Display list with –glCallList(item)  Each time list is displayed, current model-view and projection matrices are applied, possibly changing image  Display list is more efficient than regenerating image

Executing Display Lists (cont)  Attributes changed in a display list alter state and remain in effect after list is displayed –This may be a problem  Solution involves using OpenGL attribute and matrix stacks. Push current values before executing list and pop them after

Attribute and Matrix Stacks  Save with glPushAttrib(GL_ALL_ATTRIB_BITS);glPushMatrix();  Restore with glPopAttrib();glPopMatrix(); (skipping sectin 3.4.2)

Event-Driven Programming  Programmer provides functions to be executed when an event occurs  Functions are registered with system and called when event occurs –Implementation may be based on  functions with standard names as in Java  callback functions registered with system as in OpenGL  function has standard prototype to deliver event data

Mouse Event Callback  Function name may be freely chosen  Function prototype void mouse(int button, int state, int x, int y)  Declared constants for button and state like GLUT_LEFT_BUTTONGLUT_DOWN

Mouse Event Callback  Function registration glutMouseFunc(mouse)

Adding Events  Example program from section Example program Example program  main() has two added elements –glutReshapeFunc()  Called when window is reshaped –glutMouseFunc()  Called when mouse button is pressed or released

Program Functions  Mouse –Draw a square at the current location using default parameters when left button is pressed –Exit when middle button is pressed  What about windows?  Init –sets view –clear to black

Program Functions (cont)  drawSquare –Draws fixed size square in random color –Note inversion of y-coordinate  Translates window (mouse) to display  Reshape –A window event  parameters for GLsizei w, h –Resize view port and clipping to preserve aspect for square in window

Keyboard Events  Triggered when key is pressed only  Returns character and x, y mouse location when key is pressed

Display and Idle Callbacks  glutDisplayFunc(display) –Called whenever window needs to be redisplayed  glutPostRedisplay() –Redisplay only once each iteration of GLUT main loop –Avoids unnecessary redisplay

Menus  GLUT supports pop-up menus  Menu creation glutCreateMenu(demo_menu); glutAddMenuEntry("quit",1); glutAddMenuEntry("increase square size", 2); glutAddMenuEntry("decrease square size", 3); glutAttachMenu(GLUT_RIGHT_BUTTON);  Callback function has single int parameter for menu item

GLUT Menus (cont)  Also supports hierarchical menus sub_menu = glutCreateMenu(size_menu); glutAddMenuEntry("Increase square size", 2); glutAddMenuEntry("Decrease square size", 3); glutCreateMenu(top_menu); glutAddMenuEntry("Quit",1); glutAddSubMenu("Resize", sub_menu); glutAttachMenu(GLUT_RIGHT_BUTTON);

Picking  Selection –Adjust clipping region –Add items in region to hit list  Bounding rectangle or extent –Mark region –Add items in marked region to hit list  Use back buffer to display each item in different color –Identify selected items

OpenGL Selection Mode  Selection mode display allows picking –Selected item – hit –Stored on name stack  Key functions –Create array for stack void glSelectBuffer(GLsizei n, void glSelectBuffer(GLsizei n, // eg 512 GLunint *buff) –Initialize name stack void glInitNames()

OpenGL Selection Mode  Push name on stack void glPushName(GLunint name) void glPushName(GLunint name)  Pop top of stack void glPopName()  Replace top name void glLoadName(GLunint name)  Must initialize by pushing a useless value, say 0

Selection Mode  Values on stack at the time an item is drawn determine its name glLoadName(i);drawFigure(i);  If stack contains more than one value, name has multiple components – one for each value on the stack

Pick Matrix  Get current mouse location with glGetIntegerv (GL_VIEWPORT, viewport); // vp is GLint[4] viewport); // vp is GLint[4]  Set selection rectangle with gluPickMatrix(x, y, width, height, viewport) // x,y,w,h are GLdoubles

Example Pick Program ..\openglbk\picksquare.c..\openglbk\picksquare.c

Simple Paint Program  see newpaint.c newpaint.c

Animation  glutIdleFunction() specifies a callback function to use when the main loop is idle  The callback can draw an altered image each time it is called and call glutPostRedisplay() to display it  To disble the animation, register NULL as the Idle Function

Double Buffering  When redrawing an image, we want the new image to be displayed quickly enough that the redrawing operation will not be visible  Double Buffering creates the new image in a second buffer and displays it only on completion of drawing

Double Buffering (cont)  Enabled by setting –GLUT_DOUBLE IN glutInitDisplayMode()  Front buffer is displayed  Back buffer is the one into which we draw  Swap buffers –glutSwapBuffers()

Flicker  Clearing areas in an image can produce flicker  Double buffering can help with this  But we have to repeatedly draw things  Can set the drawing buffer with –glDrawBuffer(which)  GL_BACK, GL_FRONT_AND_BACK

Logic Operations  Drawing can be done in copy mode, also called replacement mode –pixels are set by drawing  or by performing one of 16 possible functions on 2 biits  in XOR mode –new pixel value is XORed with current pixel value

Logic Modes  OpenGL supports all 16 modes  Default is GL_COPY  Enable use of other modes with –glEnable(GL_COLOR_LOGIC_OP)  Set with –glLogicOP()  GL_COPY, GL_XOR, …  Redrawing in XOR mode erases  Can also change color, reverse with all 1’s