3D Coordinate Systems in Modeling

OpenGL Rendering Process

• OpenGL is an Application Programming Interface (API) used to create 2D and 3D graphics.
• It generates high-quality color images using geometric and image primitives.
• Several windowing toolkits are designed for use with OpenGL.
• OpenGL is based on the GL graphics package developed by Silicon Graphics.
• GL was specific to Silicon Graphics systems, meaning code written using GL runs only on Silicon Graphics hardware.
• OpenGL was developed in the early 1992 as a free platform-independent version of GL.
• OpenGL is the core library containing around 130 graphics drawing and operation functions.
• It has several associated libraries that are also helpful.
• OpenGL is a cross-platform, hardware-accelerated, language-independent API for producing 3D graphics, including 2D.
• Modern computers have dedicated Graphics Processing Units (GPUs) with their own memory to accelerate graphics rendering.
• OpenGL provides a cross-language, cross-platform application programming interface (API) for rendering 2D and 3D vector graphics.

OpenGL Libraries and Development Environments

• GLUT (GL Utilities Toolkit) provides extra routines for drawing 3D objects and other primitives.
• Using GLUT with OpenGL allows for the creation of windows-system independent code (independent of the operating system).
• GLU (OpenGL Utilities) offers extra routines for complex 3D object projections and rendering.
• GLUI (OpenGL User Interface) has additional routines for creating user interfaces, including buttons, checkboxes, and radio buttons for OpenGL applications.
• Dev-C++ is a free C++ development environment that supports OpenGL and GLUT.
• Microsoft Visual C++ also supports OpenGL but not GLUT.
• The GLUT library is downloadable from http://www.xmission.com/~nate/glut.html

OpenGL Function Syntax

• Each routine in OpenGL or its associated libraries follows a specific syntax.
• The function name prefix is either gl, glu, or glut depending on which library the routine is from.
• The main part of the function name indicates its purpose.
• Function suffixes indicate the number and type of arguments expected, e.g., 3f indicates 3 floating-point arguments.
• Predefined symbolic constants (always in capital letters) are used as arguments in some functions. They adhere to the same prefix convention as function names (e.g., GL_RGB, GL_POLYGON, GLUT_SINGLE).

OpenGL Data Types

• OpenGL has built-in data types, often similar to C++ data types but prefixed with GL (e.g., GLshort, GLint, GLfloat, GLdouble).
• Using these built-in types makes OpenGL code more platform-independent.

OpenGL Header Files

• Including appropriate header files is crucial for using OpenGL libraries.
• The needed header files depend on the libraries being used (OpenGL core, GL Utilities, GLUI user-interface library, GLUT).
• If GLUT is used, other header files are automatically included in a program.

OpenGL Initialization

• Initialization is typically done in the myInit() function.
• The glutInit() function must be called at the beginning of the OpenGL program.
• glutInitDisplayMode(), sets the display mode (e.g., RGB, double-buffered).
• glutInitWindowSize() sets the window size (pixels).
• glutInitWindowPosition() specifies the window's position.
• glutCreateWindow() creates the window.

OpenGL Event Loop

• An OpenGL program is event-driven.
• Events (like mouse clicks and keyboard presses) trigger callback functions.
• A display function (glutDisplayFunc()) renders the image whenever needed (e.g., redrawing).
• glutMainLoop() starts the event loop in an application.

OpenGL Initialization Routines

• OpenGL operates with a state system. State variables control the rendering process.
• Drawing attributes like color, point size, background color are set before drawing primitives.
• glClearColor(); sets the background color.
• glColor3f() specifies the current color.
• glPointSize() sets the point size.

OpenGL Coordinate System

• 2D coordinate systems typically have X positive to the right, Y positive upward (often bottom-to-top coordinate systems).
• Adding Z as a third coordinate offers a choice -- Z points in or out of the screen.
• OpenGL generally uses a right-hand coordinate system.
• Model Coordinate System (MCS): Defines the reference space where the model's geometrical data is stored.
• World Coordinate System (WCS): Relates the individual MCSs within a scene.
• Viewpoint coordinate system (camera coordinates): Based on the viewer's perspective, used for defining the region of the world to be seen.
• Viewport coordinate system: The specific portion of the screen where the object's view is displayed.

OpenGL Display Function

• glutDisplayFunc() specifies the callback function invoked to draw primitives on the screen.
• glClear() cleans the screen.
• glBegin(GL_PRIMITIVE_TYPE), glVertex() pairs define the primitives and their vertices.
• glEnd() finishes drawing the shape.
• glFlush() forces the immediate display of the primitives.

OpenGL Callbacks

• Callback functions are procedures called by the system in response to particular events (like mouse clicks and keyboard presses).
• glutMouseFunc() handles mouse events.
• glutKeyboardFunc() handles keyboard events.
• glutReshapeFunc() handles window resizing events.
• glutteDisplayFunc(): The routine executed whenever OpenGL decides on redrawing.

Output Primitives and Attributes

• Graphics primitives are the basic building blocks of an image (points, lines, circles, polygons).
• Attributes are properties controlling how primitives are rendered (color, thickness, style).

OpenGL Rendering Primitives (Point, Line, Polygons)

• OpenGL uses glBegin()...glEnd()pairs to draw primitives.
• glVertex2/3/4f()specifies vertices in 3D space.
• GL_POINTS, GL_LINES, GL_TRIANGLES, GL_QUAD: Define graphic styles.

OpenGL Menus

• OpenGL provides pop-up menus.
• Menus are linked to specific mouse buttons (often right click).
• Callback functions associated to each menu entry define the action performed when an item from the menu is selected.

OpenGL Keyboard Events

• OpenGL uses a keyboard callback function (glutKeyboardFunc()).
• The callback function is triggered when a key is pressed, passing the key's ASCII value and position of the mouse cursor as input.

OpenGL Reshape Functions

• OpenGL uses a reshape function (glutReshapeFunc()) to manage handling window resizing events.
• This function manages changes needed to handle the reshape window and adjusts the view setup of the displayed window.

Viewing Using a Synthetic Camera

• A synthetic camera attempts to emulate the visual characteristics of a real camera or the human eye (e.g., depth of field, distortions).
• It differs from standard computer graphics by aiming to reproduce real-world imperfections.

Image Formats

• Various image formats exist (JPEG, PNG, GIF, TIFF, BMP, RAW).
• These formats differ in their compression methods (lossless vs. lossy).
• Lossless compression maintains image quality, while lossy compression results in smaller files but may slightly reduce quality.
• Common uses for specific formats differ (e.g., web graphics, professional printing , archiving).
• Programs exist for viewing and handling various image types.