GRAPHICS PROGRAMMING

Transcription

1 GRAPHICS PROGRAMMING

2 Content The Sierpinski Gasket The Opengl Application Programming Interface - Graphics Functions - Graphics pipeline - Coordinates System -The OpenGL Interface Primitives and attributes Text Colors

3 The Sierpinski Gasket The Sierpinski gasket is an object that can be defined as attractive fixed set with the overall shape of an equilateral triangle, subdivided recursively into smaller equilateral triangles. Suppose that we start with three points in space. As long as the points are not collinear, they are the vertices of a unique triangle and also define a unique plane.we assume that this plane is the plane z = 0 and that these points, as specified in some convenient coordinate system, are (x1, y1, 0), (x2, y2, 0), and (x3, y3, 0).

4 The Sierpinski Gasket con.. The construction proceeds as follows: 1. Pick an initial point p = (x, y, 0) at random inside the triangle. 2. Select one of the three vertices at random. 3. Find the point q halfway between p and the randomly selected vertex. 4. Display q by putting some sort of marker, such as a small circle, at the corresponding location on the display. 5. Replace p with q. 6. Return to step 2

5 The Graphics execution Approaches Immediate Mode: Primitives (vectors, points) flow through the system and produce images. These data are lost. New images are created by re-executing the display function and regenerating the primitives. Retained Mode: The primitives are stored in a display list (in compiler form). Images can be recreated by executing the display list even without a network between the server and client,display lists should be more efficient then repeated executions of the display function.

6 The Opengl Application Programming Interface OpenGL s structure is similar to that of most modern APIs, such as DirectX. Hence, any effort that you put into learning OpenGL will carry over to other software systems. Although OpenGL is easy to learn, compared with other APIs, it is nevertheless powerful. Our prime goal is to study computer graphics; we are using an API to help us attain that goal

7 Graphics System as a Black Box We can think of the graphics system as a black box whose inputs are function calls from an application program; measurements from input devices, such as the mouse and keyboard; and possibly other input, such as messages from The outputs are primarily the graphics sent to our output devices.

8 Graphics Functions A graphics system performs multiple tasks to produce output and handle user input. An API for interfacing with this system can contain hundreds of individual functions. It will be helpful to divide these functions into seven major groups: 1. Primitive functions 2. Attribute functions 3. Viewing functions 4. Transformation functions 5. Input functions 6. Control functions 7. Query functions

9 Primitive functions define the low-level objects or atomic entities that our system can display. Depending on the API, the primitives can include points, line segments, polygons, pixels, text, and various types of curves and surfaces. OpenGL supports a very limited set of primitives directly, only points, line segments, and triangles. For the most important objects such as Regular polyhedra, quadrics, and Bezier curves and surfaces that are not directly supported by OpenGL, there are libraries that provide the necessary code.

10 Attribute functions If primitives are the what of an API then attributes are the how. The attributes govern the way that a primitive appears on the display. Attribute functions allow us to perform operations ranging from choosing the color with which we display a line segment, to picking a pattern with which to fill the inside of a polygon, to selecting a typeface for the titles on a graph.

11 Viewing functions Allow us to specify various views, although APIs differ in the degree of flexibility they provide in choosing a view. OpenGL does not provide any viewing functions but relies on the use of transformations in the shaders to provide the desired view.

12 Transformation functions One of the characteristics of a good API is that it provides the user with a set of transformation functions that allows her to carry out transformations of objects, such as rotation, translation, and scaling.

13 Input functions For interactive applications, an API must provide a set of input functions to allow us to deal with the diverse forms of input that characterize modern graphics systems. We need functions to deal with devices such as keyboards, mice, and data tablets

14 Control functions In any real application, we also have to worry about handling the complexities of working in a multiprocessing, multi window environment usually an environment where we are connected to a network and there are other users. The control functions enable us to communicate with the window system, to initialize our programs, and to deal with any errors that take place during the execution of our programs.

15 Query functions within our applications we can often use other information within the API, including camera parameters or values in the frame buffer. A good API provides this information through a set of query functions.

16 Coordinates Systems The object coordinate system The World coordinate system The Screen coordinate system

17 The OpenGL Interface OpenGL functions are in a single library named GL (or OpenGL in Windows). Function names begin with the letters gl. We use two available libraries, the OpenGL Extension Wrangler (GLEW) and the OpenGL Utility Toolkit (GLUT). GLEW removes operating system dependencies. GLUT provides the minimum functionality that should be expected in any modern windowing system.

18 Library organization

19 Control Function (GLUT) OpenGL Utility Toolkit(GLUT) Is a library of functions that provides a simple interface between the systems. provides the facilities for interaction that OpenGL lacks. It provides functions for managing windows on the display screen, and handling input events from the mouse and keyboard. The application programs that we produce using GLUT should run under multiple window systems. All GLUT function names start with glut.

20 Include Files For all OpenGL applications, you mustto include the gl.h header file in every file. Almost all OpenGL applications use GLU, the aforementioned OpenGL Utility Library, which also requires inclusion of the glu.h header file. So almost every OpenGL source file begins with: #include <GL/gl.h> #include <GL/glu.h> If you are using the OpenGL Utility Toolkit (GLUT) for managing your window manager tasks, you should include: #include <GL/glut.h> Note that glut.h guarantees that gl.h and glu.h are properly included for you so including these three files is redundant. To make your GLUT programs portable, include glut.h and do not include gl.h or glu.h explicitly.

21 Setting Up Compilers Windows Using MS Visual C++ Most of the following files (ie. OpenGL and GLU) will already be present if you have installed MS Visual C++ v5.0 or later. The following GLUT files will need to be copied into the specified directories.

22 Install libraries libraries (place in the lib\ subdirectory of Visual C++) opengl32.lib glu32.lib glut32.lib include files (place in the include\gl\ subdirectory of Visual C++) gl.h glu.h glut.h dynamically-linked libraries (place in the \Windows\System subdirectory opengl32.dll glu32.dll glut32.dll

23 Install libraries Compiling OpenGL/GLUT Programs Create a new project: choose File New from the File Menu select the Projects tab choose Win32 Console Application fill in your Project name

24 Primitives and attributes We can separate primitives into two classes: geometric primitives and image, or raster, primitives. Geometric primitives are specified in the problem domain and include points, line segments, polygons, curves, and surfaces. These primitives pass through a geometric pipeline, as shown in Figure where they are subject to a series of geometric operations that determine whether a primitive is visible, where on the display it appears if it is visible, and the rasterization of the primitive into pixels in the frame buffer.

25 Opengl Pipeline

26 Primitives and attributes con.. The basic OpenGL geometric primitives are specified by sets of vertices. When we want to display some geometry, we execute functions whose parameters specify how the vertices are to be interpreted.

27 OpenGL geometric primitives All OpenGL geometric primitives are variants of points, line segments, and triangular polygons. A point can be displayed as a single pixel or a small group of pixels. Finite sections of lines between two vertices, called line segments.

28 Points and Line segment type If we wish to display points or line segments, we have a few choices in OpenGL The primitives and their type specifications include the following:

29 Polygon in Opengl The only OpenGL polygons that OpenGL supports are triangles. Triangle types

30 Polygon in Opengl Triangle strip and triangle fan

31 Text Graphical output in applications such as data analysis and display requires annotation, such as labels on graphs In nongraphical programs textual output is the norm, text in computer graphics is problematic. In computer graphics we often wish to display text in a multitude of fashions by controlling type styles, sizes, colors, and other parameters. There are two forms of text: stroke and raster.

32 Stroke Text Is constructed as are other geometric objects. We use vertices to specify line segments or curves that outline each character. In systems that support stroke text as a primitive, there is a variety of attributes they include the direction of the text string,the height and width of the characters, the font, and the style (bold, italic, underlined)

33 Raster Text Is simple and fast. Characters are defined as rectangles of bits called bit blocks. Each block defines a single character by the pattern of 0 and1 bits in the block.

34 Colors Color is one of the most interesting aspects of both human perception and computer graphics. Additive color is the primary colors add together to give the perceived color, examples projectors and slide (positive) film. Subtractive color is more appropriate. We start with a white surface, such as a sheet of paper. Colored pigments remove color components from light that is striking the surface