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ICS 415 Computer Graphics OpenGL

ICS 415 Computer Graphics OpenGL. Dr. Muhammed Al-Mulhem March 1, 2009. Intro to OpenGL. OpenGL operates as an infinite loop Put things in the scene (points, colored lines, textured polys) Describe the camera (location, orientation, field of view) Listen for keyboard events

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ICS 415 Computer Graphics OpenGL

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  1. ICS 415Computer GraphicsOpenGL Dr. Muhammed Al-Mulhem March 1, 2009 Dr. Muhammed Al-Mulhem

  2. Intro to OpenGL • OpenGL operates as an infinite loop • Put things in the scene (points, colored lines, textured polys) • Describe the camera (location, orientation, field of view) • Listen for keyboard events • Render – draw the scene Dr. Muhammed Al-Mulhem

  3. Intro to OpenGL • OpenGL uses matrices • Matrix describes camera type • Matrix describes current configuration of the 3D space Dr. Muhammed Al-Mulhem

  4. Intro to OpenGL • OpenGL coordinate system • Right-handed • Hold out your right hand and hold your thumb, index, and middle fingers orthogonal to one another. • Call your thumb the x-axis, index = y-axis, and middle z-axis • This is the OpenGL coordinate system Dr. Muhammed Al-Mulhem

  5. Basic OpenGL Syntax • Functions are prefixed with gl, and each component word has its first letter capitalized • glBegin • glCopyPixel • Constants begin with uppercase letter GL. • Component word within constants are written in capital letters, and the underscore (_) is used as a separator between all component words • GL_RGB, • GL_AMBIENT_AND_DIFFUSE Dr. Muhammed Al-Mulhem

  6. Data Types • Data types use special built-in data type names. Each name begins with GL and the remainder of the name is a standard data type name, written in small letters. • GLbyte • GLint • GLfloat • GLdouble • GLboolean Dr. Muhammed Al-Mulhem

  7. Related Libraries • In addition to the OpenGl basic (core) library, there are a number of associated libraries for handling special operations, such as GLU. • GLU provides routines for setting up viewing and projection matrices, describing complex objects with lines and polygon approximations, etc. • OpenGL implementation include GLU library and all GLU function names start with the prefix glu. Dr. Muhammed Al-Mulhem

  8. Heder Files • For all graphics programs, we need to include header files for the following libraries: • OpenGL core library # include <gl.h> • GLU library # include <glu.h> • Windows library # include <windows.h> Dr. Muhammed Al-Mulhem

  9. Heder Files • If we use GLUT to handle the window – managing operation, we do not need the above header files, we need only the glut.h file. # include <glut.h> Dr. Muhammed Al-Mulhem

  10. Heder Files • Also, we will often need to include the following header files that are required by the C++ code. # include <stdio.h> # include <stdlib.h> # include <math.h> Dr. Muhammed Al-Mulhem

  11. Display Window Management using GLUT • First you need to initialize GLUT glutInit (&argc, argv); • Next, we can state that a display window is to be created on the screen with a given caption for the title bar. glutCreateWindow (“An Example OpenGL Program”); Dr. Muhammed Al-Mulhem

  12. Display Window Management using GLUT • Then we need to specify what the display window is to contain. • For this, we create a picture using OpenGL functions and pass the picture definition to the GLUT routine glutDisplayFunc, which assigns our picture to the display window. • For example, an OpenGL code for describing a line segment in a procedure called lineSegment. glutDisplayFunc (lineSegment); Dr. Muhammed Al-Mulhem

  13. Display Window Management using GLUT • Next, we need one more GLUT function to complete the window processing operation.. • After execution of the following statement, all display windows that we have created, including their grphic content, are now activated glutMainLoop (); Dr. Muhammed Al-Mulhem

  14. Display Window Management using GLUT • The glutMainLoop (); do the following: • Displays the initial graphics • Puts the program into an infinite loop that checks for input from devices such as a mouse or keyboard. Dr. Muhammed Al-Mulhem

  15. Display Window Management using GLUT • We use a GLUT function to give the an initial location for the top left corner of the display window. • For example, the following statement specifies that the top-left corner of the display window is 50 pixels to the right of the left edge and 100 pixels from the top edge of the screen. glutInitWindowPosition (50, 100); Dr. Muhammed Al-Mulhem

  16. Display Window Management using GLUT • Similarly, the GLUT size function is used to set the initial pixel width and height of the display window. • For example, A window with width of 400 pixels and height of 300 pixels glutInitDisplaySize (400, 300); • After the display window on the screen, we can reposion and resize it. Dr. Muhammed Al-Mulhem

  17. Dr. Muhammed Al-Mulhem

  18. Modeling Transformations • glTranslate (x, y, z) • Multiplies the current matrix by a matrix that moves (translates) an object by the given x, y, and z values. Dr. Muhammed Al-Mulhem

  19. Modeling Transformations • glRotate (angle, x, y, z) • Multiplies the current matrix by a matrix that rotates an object in a counterclockwise direction about the ray from the origin through the point (x, y, z). • The angle parameter specifies the angle of rotation in degrees. For example: glRotatef(45.0, 0.0, 0.0, 1.0). Dr. Muhammed Al-Mulhem

  20. Modeling Transformations • glScale (x, y, z) • Multiplies the current matrix by a matrix that stretches, shrinks, or reflects an object along the axes. • Each x, y, and z coordinate of every point in the object is multiplied by the corresponding argument x, y, or z. • For example: glScalef(2.0, -0.5, 1.0). Dr. Muhammed Al-Mulhem

  21. OpenGL: Conventions • Functions in OpenGL start with gl • Most functions just gl (e.g., glColor()) • Functions starting with glu are utility functions (e.g., gluLookAt()) • Functions starting with glx are for interfacing with the X Windows system (e.g., in gfx.c) • Constants are written in CAPITAL letters • Example: GLUT_SINGLE, GLUT_RGB Dr. Muhammed Al-Mulhem

  22. OpenGL: Conventions • Function names indicate argument type and number • Functions ending with f take floats • Functions ending with i take ints • Functions ending with b take bytes • Functions ending with ub take unsigned bytes • Functions that end with v take an array. • Examples • glColor3f() takes 3 floats • glColor4fv() takes an array of 4 floats Dr. Muhammed Al-Mulhem

  23. OpenGL: Simple Use • Open a window and attach OpenGL to it • Set projection parameters (e.g., field of view) • Setup lighting, if any • Main rendering loop • Set camera pose with gluLookAt() • Camera position specified in world coordinates • Render polygons of model • Simplest case: vertices of polygons in world coordinates Dr. Muhammed Al-Mulhem

  24. OpenGL: Simple Use • Open a window and attach OpenGL to it • glutCreateWindow() or FLTK window method Dr. Muhammed Al-Mulhem

  25. OpenGL: Perspective Projection • Set projection parameters (e.g., field of view) • Typically, we use a perspective projection • Distant objects appear smaller than near objects • Vanishing point at center of screen • Defined by a view frustum (draw it) • Other projections: orthographic, isometric Dr. Muhammed Al-Mulhem

  26. Setting up Camera • glMatrixMode(GL_MODELVIEW); • glLoadIdentity(); • gluLookAt( eyeX, eyeY, eyeZ, lookX, lookY, lookZ, upX, upY, upZ); • eye[XYZ]: camera position in world coordinates • look[XYZ]: a point centered in camera’s view • up[XYZ]: a vector defining the camera’s vertical • Creates a matrix that transforms points in world coordinates to camera coordinates • Camera at origin • Looking down -Z axis • Up vector aligned with Y axis Dr. Muhammed Al-Mulhem

  27. OpenGL: Perspective Projection • In OpenGL: • Projections implemented by projection matrix • gluPerspective() creates a perspective projection matrix: glSetMatrix(GL_PROJECTION); glLoadIdentity(); //load an identity matrix gluPerspective(vfov, aspect, near, far); • Parameters to gluPerspective(): • vfov: vertical field of view • aspect: window width/height • near, far: distance to near & far clipping planes Dr. Muhammed Al-Mulhem

  28. OpenGL: Lighting • Setup lighting, if any • Simplest option: change the current color between polygons or vertices • glColor() sets the current color • Or OpenGL provides a simple lighting model: • Set parameters for light(s) • Intensity, position, direction & falloff (if applicable) • Set material parameters to describe how light reflects from the surface • Won’t go into details now; check the red book if interested Dr. Muhammed Al-Mulhem

  29. OpenGL: Specifying Geometry • Geometry in OpenGL consists of a list of vertices in between calls to glBegin() and glEnd() • A simple example: telling GL to render a triangle glBegin(GL_POLYGON); glVertex3f(x1, y1, z1); glVertex3f(x2, y2, z2); glVertex3f(x3, y3, z3); glEnd(); • Usage: glBegin(geomtype) where geomtype is: • Points, lines, polygons, triangles, quadrilaterals, etc... Dr. Muhammed Al-Mulhem

  30. Primitive Types • GL_POINTS • GL_LINE • {S | _STRIP | _LOOP} • GL_TRIANGLE • {S | _STRIP | _FAN} • GL_QUAD • {S | _STRIP} • GL_POLYGON Dr. Muhammed Al-Mulhem

  31. GL_POLYGON • List of vertices defines polygon edges • Polygon must be convex Dr. Muhammed Al-Mulhem

  32. Non-planar Polygons • Imagine polygon with non-planar vertices • Some perspectives will be rendered as concave polygons • These concave polygons may not rasterize correctly Dr. Muhammed Al-Mulhem

  33. OpenGL: More Examples • Example: GL supports quadrilaterals: glBegin(GL_QUADS); glVertex3f(-1, 1, 0); glVertex3f(-1, -1, 0); glVertex3f(1, -1, 0); glVertex3f(1, 1, 0); glEnd(); • This type of operation is called immediate-mode rendering; each command happens immediately Dr. Muhammed Al-Mulhem

  34. OpenGL: Drawing Triangles • You can draw multiple triangles between glBegin(GL_TRIANGLES) and glEnd(): float v1[3], v2[3], v3[3], v4[3]; ... glBegin(GL_TRIANGLES); glVertex3fv(v1); glVertex3fv(v2); glVertex3fv(v3); glVertex3fv(v1); glVertex3fv(v3); glVertex3fv(v4); glEnd(); • Each set of 3 vertices forms a triangle • What do the triangles drawn above look like? • How much redundant computation is happening? Dr. Muhammed Al-Mulhem

  35. OpenGL: Triangle Strips • An OpenGL triangle strip primitive reduces this redundancy by sharing vertices: • glBegin(GL_TRIANGLE_STRIP); • glVertex3fv(v0); • glVertex3fv(v1); • glVertex3fv(v2); • glVertex3fv(v3); • glVertex3fv(v4); • glVertex3fv(v5); • glEnd(); • triangle 0 is v0, v1, v2 • triangle 1 is v2, v1, v3 (why not v1, v2, v3?) • triangle 2 is v2, v3, v4 • triangle 3 is v4, v3, v5 (again, not v3, v4, v5) v2 v4 v0 v5 v1 v3 Dr. Muhammed Al-Mulhem

  36. OpenGL: More Examples • Example: GL supports quadrilaterals: glBegin(GL_QUADS); glVertex3f(-1, 1, 0); glVertex3f(-1, -1, 0); glVertex3f(1, -1, 0); glVertex3f(1, 1, 0); glEnd(); • This type of operation is called immediate-mode rendering; each command happens immediately Dr. Muhammed Al-Mulhem

  37. OpenGL: Front/Back Rendering • Each polygon has two sides, front and back • OpenGL can render the two differently • The ordering of vertices in the list determines which is the front side: • When looking at the front side, the vertices go counterclockwise • This is basically the right-hand rule • Note that this still holds after perspective projection Dr. Muhammed Al-Mulhem

  38. OpenGL: Drawing Triangles • You can draw multiple triangles between glBegin(GL_TRIANGLES) and glEnd(): float v1[3], v2[3], v3[3], v4[3]; ... glBegin(GL_TRIANGLES); glVertex3fv(v1); glVertex3fv(v2); glVertex3fv(v3); glVertex3fv(v1); glVertex3fv(v3); glVertex3fv(v4); glEnd(); • Each set of 3 vertices forms a triangle • What do the triangles drawn above look like? • How much redundant computation is happening? Dr. Muhammed Al-Mulhem

  39. OpenGL: Triangle Strips • An OpenGL triangle strip primitive reduces this redundancy by sharing vertices: • glBegin(GL_TRIANGLE_STRIP); • glVertex3fv(v0); • glVertex3fv(v1); • glVertex3fv(v2); • glVertex3fv(v3); • glVertex3fv(v4); • glVertex3fv(v5); • glEnd(); • triangle 0 is v0, v1, v2 • triangle 1 is v2, v1, v3 (why not v1, v2, v3?) • triangle 2 is v2, v3, v4 • triangle 3 is v4, v3, v5 (again, not v3, v4, v5) v2 v4 v0 v5 v1 v3 Dr. Muhammed Al-Mulhem

  40. Double Buffering • Avoids displaying partially rendered frame buffer • OpenGL generates one raster image while another raster image is displayed on monitor • glxSwapBuffers (Display *dpy, Window, w) • glutSwapBuffers (void) Dr. Muhammed Al-Mulhem

  41. Learn OpenGL by example • robot.c from the OpenGL Programming Guide Dr. Muhammed Al-Mulhem

  42. Learn OpenGL by example • Two bodies • Upper arm • Lower arm • Major tasks • Position • Orientation Dr. Muhammed Al-Mulhem

  43. Learn OpenGL by example • Both bodies originally at origin Dr. Muhammed Al-Mulhem

  44. Learn OpenGL by example • Headers • #include <GL/gl.h> • #include <GL/glu.h> • #include <GL/glut.h> Dr. Muhammed Al-Mulhem

  45. Learn OpenGL by example • int main(int argc, char** argv) { • glutInit(&argc, argv); • glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB); • glutInitWindowSize (500, 500); • glutInitWindowPosition (100, 100); • glutCreateWindow (argv[0]); • init (); • glutDisplayFunc(display); • glutReshapeFunc(reshape); • glutKeyboardFunc(keyboard); • glutMainLoop(); • return 0; } Dr. Muhammed Al-Mulhem

  46. Learn OpenGL by example • void init(void) { • glClearColor (0.0, 0.0, 0.0, 0.0); • glShadeModel (GL_FLAT); • } Dr. Muhammed Al-Mulhem

  47. Learn OpenGL by example • void display(void){ • glClear (GL_COLOR_BUFFER_BIT); • glPushMatrix(); • glTranslatef (-1.0, 0.0, 0.0); • glRotatef ((GLfloat) shoulder, 0.0, 0.0, 1.0); • glTranslatef (1.0, 0.0, 0.0); • glPushMatrix(); • glScalef (2.0, 0.4, 1.0); • glutWireCube (1.0); • glPopMatrix(); • Continued… Dr. Muhammed Al-Mulhem

  48. Learn OpenGL by example • glTranslatef (1.0, 0.0, 0.0); • glRotatef ((GLfloat) elbow, 0.0, 0.0, 1.0); • glTranslatef (1.0, 0.0, 0.0); • glPushMatrix(); • glScalef (2.0, 0.4, 1.0); • glutWireCube (1.0); • glPopMatrix(); • glPopMatrix(); • glutSwapBuffers(); • } Dr. Muhammed Al-Mulhem

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