Coordinate System

glOrtho? glOrtho(0.0, 500.0, 0.0, 500.0, -1.0,1.0); Set the viewing volume Anything outside this volume will be clipped

Window Size glutInitWindowSize(320,240); Set the display window Can be any values However, distortion can occur due to different aspect ratio (w/h)

Different window size 320, 240 240, 320

Coordinate system the basic coordinate system of the screen window: coordinates that are essentially in pixels, extending from 0 to some value screenWidth -1 in x, and from 0 to some value screenHeight -1 in y. Clearly we want to make a separation between the values we use in a program to describe the geometrical objects and the size and position of the pictures of them on the display.

World window and viewport world coordinates: It is the usual Cartesian xy-coordinate system used in mathematics, based on whatever units are convenient. We define a rectangular world window in these world coordinates. The world window specifies which part of the “world” should be drawn. The understanding is that whatever lies inside the window should be drawn; whatever lies outside should be clipped away and not drawn. viewport is a rectangular in the screen window on the screen.

Viewport Transformation MyWindow w Clipping Window h x y void glViewport(Glint x, GLint y, GLsizei w, Glsizei h); Default viewport corresponds to entire window drawable area.

Doing it in OpenGL. For 2D drawing the world window is set by the function gluOrtho2D(), and the viewport is set by the function glViewport(). void gluOrtho2D(GLdouble left, GLdouble right, GLdouble bottom, Gldouble top); void glViewport(GLint x, GLint y, GLint width, GLint height); By default the viewport is the entire screen window: if W and H are the width and height of the screen window, respectively, the default viewport has lower left corner at (0, 0) and upper right corner at (W, H).

Example: Drawing a house Assume the screen Window size is 200x200 glOrtho(0.0, 10.0, 0.0, 15.0, -1.0, 1.0); //world window Five vertices 0 0 10 0 10 10 5 15 0 10 glViewport(0,0,100,100) Not specifying viewport

A distorted house glViewport(0,0, 200,100) Remember window size is 200x200 Distorted with respect to the original picture that were shown in the full viewing window (check the aspect ratio)

Proportionality forces the mappings to have a linear form: sx = A * x + C sy = B * y + D for some constants A, B, C and D. The constants A and B scale the x and y coordinates, and C and D shift (or translate) them.

Window to ViewPort Transformation

Because OpenGL uses matrices to set up all its transformations, gluOrtho2D() must be preceded by two functions glMatrixMode(GL_PROJECTION) and glLoadIdentity(). //Setwindow routine l, r, b, t Void setWindow(float, l, float r, float b, float t){ glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluOrtho2D(l, r, b, t); // sets the window }

Tiling the screen window with the dinosaur motif.

Tiling the display with copies of the dinosaur. // set a fixed window (0, 640.0, 0, 480.0) for(int i = 0; i < 5; i++) // for each column for(int j = 0; j < 5; j++) // for each row { glViewport(i * 64, j * 44, 64, 44); // set the next viewport drawPolylineFile(“dino.dat”); // draw it again } if((i+j)%2) ==0) //set a fixed window (0, 640.0, 0, 480.0) using the set window routine else // set a fixed window (0, 640.0, 480.0, 0) //upside-down window

Drawing Dot with Mouse void myMouse(int button, int state, int x, int y) { if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) drawDot(x, screenHeight -y); else if(button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN) exit(-1); } drawDot rountine? Void drawDot(GLint x Glint y){ glBegin(GL_POINTS): glVertex2i(x,y); glEnd;

Specifying a rectangle with the mouse. void myMouse(int button, int state, int x, int y){ static GLintPoint corner[2]; // a struct with GLint x and GLint y static int numCorners = 0; // initial value is 0 if(button == GLUT_LEFT_BUTTON && state == GLUT_DOWN){ corner[numCorners].x = x; corner[numCorners].y = screenHeight - y; // flip y coordinate numCorners++; // have another point if(numCorners == 2){ glRecti(corner[0].x, corner[0].y, corner[1].x, corner[1].y); //write this //procedure yourself numCorners = 0; // back to 0 corners}} else if(button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN) glClear(GL_COLOR_BUFFER_BIT); // clear the window glFlush(); }

A polyline drawer based on mouse clicks. void myMouse(int button, int state, int x, int y){ #define NUM 20 static GLintPoint List[NUM]; static int last = -1; // last index used so far // test for mouse button as well as for a full array if(button == GLUT_LEFT_BUTTON && state == GLUT_DOWN && last < NUM -1){ List[++last].x = x; // add new point to list List[ last].y = screenHeight - y; // window height is 480 glClear(GL_COLOR_BUFFER_BIT); // clear the screen glBegin(GL_LINE_STRIP); // redraw the polyline for(int i = 0; i <= last; i++) glVertex2i(List[i].x, List[i].y); glEnd(); glFlush();} else if(button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN) last = -1; // reset the list to empty }

Freehand drawing with a fat brush. void myMovedMouse(int mouseX, int mouseY) { GLint x = mouseX; //grab the mouse position GLint y = screenHeight -mouseY; // flip it as usual GLint brushSize = 20; glRecti(x,y, x + brushSize, y + brushSize); glFlush(); }

An example of the keyboard callback function void myKeyboard(unsigned char theKey, int mouseX, int mouseY) { GLint x = mouseX; GLint y = screenHeight - mouseY; // flip the y value as always switch(theKey) case ‘p’: drawDot(x, y); // draw a dot at the mouse position break; case ‘E’: exit(-1); //terminate the program default: break; // do nothing }}