Simple Drawing

1. Open the attached source file: Lab01.cpp

Drawing lines

·         Modify parameters in glVertex2i() and change the line width.

·         Add glVertex2i() to draw another 2 lines. Note that a line needs 2 vertices as the starting and ending positions.  Thus, the number of vertices within the glBegin() and glEnd() is of multiple of 2. 

·         Set different colors for the lines.

Drawing triangles

·         Change the GL_LINE_LOOP to GL_TRIANGLES. 

·         As a triangle will need 3 vertices, make sure the number of vertices within the glBegin() and glEnd() is of multiple of 3. 

·         Change the GL_TRIANGLES to GL_TRIANGLE_FAN.  What happen to your drawing?

Drawing rectangles

·         Change the GL_TRIANGLE_FAN to GL_QUADS.  

·         As a rectangle will need 4 vertices, make sure the number of vertices within the glBegin() and glEnd() is of multiple of 4.  What will happen if it is not?

·         Change the GL_QUADS to GL_QUAD_STRIP.  What happen to your drawing?

Drawing polygons

·         Change the GL_QUAD_STRIP to GL_POLYGONS.  Try drawing an irregular shape.

Screen Coordinate System:

·         Modify the parameters in the function calls to glutInitWindowSize() and glutInitWindowPosition() until you have a good understanding of what each parameter means.

·         Note that these values are in pixels.

Window Coordinate System:

·         Modify the parameters in the function call to gluOrtho2D(); until you have a good understanding of what each parameter means

·         Note that these values are in world units.

2. Create a program to draw the 2D objects as shown in the following diagram using GL_TRIANGLE, GL_QUAD and GL_POLYGON.

>>..................................................................................

#include <glut.h>  // Note: this automatically includes other

                      // required header files, including:

                      //    glu.h      (GL utility functions)

                      //    gl.h       (GL functions)

                      //    windows.h  (Microsoft windows stuff)

void init(void)

{

  glClearColor(1.0, 1.0, 1.0, 0.0); // set the background to white

  glMatrixMode(GL_PROJECTION);           // set projection parameters

  glLoadIdentity();

  gluOrtho2D(0.0,640.0,0.0,480.0);

}

void lineSegment(void)

{

  glClear( GL_COLOR_BUFFER_BIT ); // Clear display window

  glColor3f(1.0f, 0.0f, 0.0f);      // set the drawing color to red

  glBegin(GL_TRIANGLES);                        // start drawing in 'line' mode

    glVertex2i( 50, 200);                // specify the lines geometry

    glVertex2i( 150, 350);

       glVertex2i(250, 200);

                           // end drawing

  glEnd();

   glColor3f(0.0f, 1.0f, 0.0f);      // set the drawing color to red

  glBegin(GL_QUADS);                     // start drawing in 'line' mode

    glVertex2i( 300, 200);               // specify the lines geometry

    glVertex2i( 300, 350);

       glVertex2i(400, 350);

       glVertex2i(400, 200);

                           // end drawing

  glEnd();

  glColor3f(0.0f, 0.0f, 1.0f);      // set the drawing color to red

  glBegin(GL_POLYGON);                          // start drawing in 'line' mode

    glVertex2i(450, 200); 

       glVertex2i(550, 200);// specify the lines geometry

    glVertex2i(550, 300);

       glVertex2i(500, 400);

       glVertex2i(450, 300);

       glVertex2i(450, 200);

                           // end drawing

  glEnd();

  glFlush();                                    // send any buffered output to 

                                    // the OpenGL rendering process

}

//-------------------------------------------------------------------

void main( int argc, char **argv)

{

  glutInit(&argc, argv);                                      // Initialize GLUT

  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB ); // Set display mode

  glutInitWindowPosition(100,200);       // Set top-left display window position

  glutInitWindowSize(400,200);           // Set display window width and height

  glutCreateWindow("An Example");// Create display window

 

  init();                                       // Execute initialization procedure

  glutDisplayFunc(lineSegment);          // Send graphics to display window

  glutMainLoop();                               // Display everything and wait

}

  

Exercise

1.

Vertices specified, the interpretations are as follows:

GL_POINTS	Treats each vertex as a single point. Vertex n defines point n. N points are drawn.
GL_LINES	Treates each pair of vertices as an independent line segment. Vertices 2n-1 and 2n define line n. N/2 lines are drawn.
GL_LINE_STRIP	Draws a connected group of line segments from the first vertex to the last. Vertices n and n+1 define line n. N-1 lines drawn.
GL_LINE_LOOP	Draws a connected group of line segments from the first vertex to the last, then back to the first. Vertices n and n+1 define line n. The last line, however, is defined by vertices N and 1. N lines are drawn.
GL_TRIANGLES	Treates each triplet of vertices as an independent triangle. Vertices 3n-2, 3n-1, and 3n define triangle n. N/3 triangles are drawn.
GL_TRIANGLE_STRIP	Draws a connected group of triangles. One triangle is defined for each vertex presented after the first two vertices. For odd n, vertices n, n+1, and n+2 define triangle n. For even n, vertices n+1, n, and n+2 define triangle n. N-2 triangles are drawn.
GL_TRIANGLE_FAN	Draws a connected group of triangles. One triangle is defined for each vertex presented after the first two vertices. Vertices 1, n+1, and n+2 define triangle n. N-2 triangles are drawn.
GL_QUADS	Treats each group of four vertices as an independent quadrilateral. Vertices 4n-3, 4n-2, 4n-1, and 4n define quadrilateral n. N/4 quadrilaterals are drawn.
GL_QUAD_STRIP	Draws a connected group of quadrilaterals. One quadrilateral is defined for each pair of vertices presented after the first pair. Vertices 2n-1, 2n, 2n+2, and 2n+1 define quadrilateral n. N/2-1 quadrilaterals are drawn. Note that the order in which vertices are used to construct a quadrilateral from strip data is different from that used with independent data.
GL_POLYGON	Draws a single, convex polygon. Vertices 1 through N define this polygon.

Window Coordinate System

glutInitWindowPosition, glutInitWindowSize

  glutInitWindowPosition and   glutInitWindowSize set the initial window position and size respectively.
Usage

void glutInitWindowSize(int width, int height);

void glutInitWindowPosition(int x, int y);

width

Width in pixels.

height

Height in pixels.

x

Window X location in pixels.

y

Window Y location in pixels.

Description
Windows created by glutCreateWindow will be requested to be created with the current initial window position and size.
The initial value of the initial window position GLUT state is -1 and -1. If either the X or Y component to the initial window position is negative, the actual window position is left to the window system to determine. The initial value of the initial window size GLUT state is 300 by 300. The initial window size components must be greater than zero.
The intent of the initial window position and size values is to provide a suggestion to the window system for a window's initial size and position. The window system is not obligated to use this information. Therefore, GLUT programs should not assume the window was created at the specified size or position. A GLUT program should use the window's reshape callback to determine the true size of the window.

<<From http://www.opengl.org/resources/libraries/glut/spec3/spec3.html>>

World Coordinate System

gluOrtho2D

NAME

gluOrtho2D - define a 2-D orthographic projection matrix

C SPECIFICATION

void gluOrtho2D( GLdouble left, GLdouble right, GLdouble bottom, GLdouble top )

PARAMETERS

left, right		Specify the coordinates for the left and right vertical clipping planes.
bottom, top		Specify the coordinates for the bottom and top horizontal clipping planes.

DESCRIPTION

gluOrtho2D sets up a two-dimensional orthographic viewing region. This is equivalent to calling glOrtho with near = -1 and far = 1 .

2.