Ch 2 Graphics Programming page 1 CSC 367 Coordinate Systems (2.1.2) Device coordinates, or screen coordinates (pixels) put limitations on programmers and the code is not portable generally expressed in integers World coordinates an effort was made to hide the actual device coordinates from the programmer and allow ANY coordinate to be used generally expressed in floats programmer defines world space Mapping the mathematical transformation from one coordinate system to another involves relatively simple algebra Example Device coordinates of 640 x 480 World coordinates of 5000 x 4000 Map (2500, 2000) to ___________ Map (1000, 1000) to ___________

Ch 2 Graphics Programming page 2 CSC 367 OpenGL Originally SGIs GL modified to be more portable Collection of functions that interface to graphics hardware Works well on a network Protocol (format of instructions) is the same regardless of the computer. This makes the code portable or system independent It does NOT do windows input modeling It does do sophisticated rendering of 3D scenes

Ch 2 Graphics Programming page 3 CSC 367 uisGL (3D object library) The book explains that OpenGL is not OO This API helps to hide some ugly details #include Vertex uisVertex P1, P2, P3(10,10,0); P1.set(20,20,0); P2.set(40,40,0); P3 = P1 + P2; P3 = P1 / 2; P3 = P2 * 2.0; cout << P3;

Ch 2 Graphics Programming page 4 CSC 367 Typical API Functions (2.2.1) Primitives draw points, lines, polygons (maybe curves) Attributes colors, fill patterns, fonts Viewing position, orientation and lens for the virtual camera Input handle events from keyboard and mouse Control initialize programs, create windows Transformation rotate, translate, and scale objects

Ch 2 Graphics Programming page 5 CSC 367 Drawing Primitives (2.1.1) We will consider 2D to be a special case (x,y,0) Point or Vertex define geometric objects glVertex2f (x, y); glVertex3f (x,y,z); Array or Vector format float vertex[3]; glVertex3fv(vertex); Drawing individual points glBegin(GL_POINTS); glVertex3f (x,y,z); glEnd( ); <--- warning! Drawing a line glBegin(GL_LINES); glVertex3f(x,y,z); glEnd( );

Ch 2 Graphics Programming page 6 CSC 367 Polygons (2.3.1) a series of connected lines simple polygon edges do not cross the interior can be filled with a color convex polygons can be tested by connecting all vertices with each other. If all of the lines remain inside the polygon then it is covex. concave polygons are not convex Drawing a polygon float P1[3] = {10, 10, 0}; glBegin(GL_POLYGON); glVertex3fv (P1); glVertex3fv (P2); glVertex3fv (P3); glVertex3fv (P4); glEnd(); Interior can be filled with a color Edges can be turned on or off

Ch 2 Graphics Programming page 7 CSC 367 Attributes (2.3.5) the name for any property that determines how an object appears a solid red line is different that a dashed green line color, line thickness, fill pattern The general approach is to set current attributes and then display the object. Each geometric type has a set of attributes. A line has color, width and style. glPointSize (2.0) // 2 pixels wide

Ch 2 Graphics Programming page 8 CSC 367 Color (2.4) Ignore color theory and mathematics Three colored spot lights analogy Different than mixing paints RGB color or true color C = T 1 R + T 2 G + T 3 B T is the intensity that ranges from glColor3f (0, 0, 0); // balck glColor3f (1, 0, 0); // red glClearColor (1, 1, 1, 0); // white background glClear (); // clear the screen The number of colors that can be displayed are determined by the number of bits available for each pixel true color requires 24 bits - 8 bits for each

Ch 2 Graphics Programming page 9 CSC 367 Indexed Color not often needed any longer when the number of bits per pixel is limited rather than limit the possible range of colors we limit the possible number of colors painter example that can mix an unlimited number of paints but can only fit so many on the cardboard plate Color-lookup Table the 8-bit representation is used as an index into the color lookup table contains a limited number of entries but each entry can represent any color set current color by specifying an index common configuration is 256 indices

Ch 2 Graphics Programming page 10 CSC 367 Viewing 2.5 Viewing rectangle or window items within the window will be seen and others will be clipped 3D Viewing Volume glOrtho(left,right,bottom,top,near,far) Be aware of right handed coordinate system positive Z comes out from screen Viewport a rectangular region within the window glViewport(x,y,width,height) Aspect Ratio the ratio of the viewing rectangle should be the same as the viewport

Ch 2 Graphics Programming page 11 CSC 367 Matrix Mode (2.5.3) Primitives are multiplied by matrices before being displayed. OpenGL has two important matrices MODELVIEW - for viewing parameters PROJECTION - for display parameters obj’ = obj x MV obj’’ = obj’ x P Typical initialization glMatrixMode(GL_PROJECTION); glLoadIdentity(); glOrtho(0,10,0,10,0,10); glMatrixMode(GL_MODELVIEW); Leave in MODELVIEW mode

Ch 2 Graphics Programming page 12 CSC 367 GLUT (GL Untility Toolkit) 2.6 Windowing system independent API for OpenGL applications Allows one to window management simple menus event processing loop input We will use an X11 implementation of GLUT. There could be a Windows version or Macintosh version. X11 is a protocal that allows device independent display of graphics over a network. Using real X11 commands would be a pain. Somewhat equivalent to programming in assembler. Client Server model. The client is the application and the computer that does the display is the server.

Ch 2 Graphics Programming page 13 CSC 367 Event Processing 2.6 What stops the image from disappearing after it has been drawn? Event Processing Loop glutMainLoop() this causes an infinite loop until closing the window by hand Callback Mechanism a programmer defined function that glutMainLoop will call when GLUT determines an event has occurred. this is called “registering a call back” Display function glutDisplayFunc(*func(void)) Additional Events mouse button presses keyboard window movement

Ch 2 Graphics Programming page 14 CSC 367 Sample Code look a source discuss Makefile

Ch 2 Graphics Programming page 15 CSC 367 Sierpinski Gasket interesting example from the book Choose a random point within the triangle Loop Find midpoint between current point and a random vertex Make a mark Update current point Extended to 3D requires a tetrahedron