vk955: переводы

→What Is OpenGL?

OpenGL is an application programming interface---‘‘API’’ for short---which is merely a software library for accessing features in graphics hardware.

Version 4.3 of the OpenGL library (which this text covers) contains over 500 distinct commands that you use to specify the objects, images, and operations needed to produce interactive three-dimensional computer-graphics applications.

OpenGL is designed as a streamlined, hardware-independent interface that can be implemented on many different types of graphics hardware systems, or entirely in software (if no graphics hardware is present in the system) independent of a computer’s operating or windowing system. As such, OpenGL doesn’t include functions for performing windowing tasks or processing user input; instead, your application will need to use the facilities provided by the windowing system where the application will execute. Similarly, OpenGL doesn’t provide any functionality for describing models of three-dimensional objects, or operations for reading image files (like JPEG files, for example). Instead, you must construct your three-dimensional objects from a small set of geometric primitives---points, lines, triangles, and patches.

Since OpenGL has been around a while---it was first developed at Silicon Graphics Computer Systems with Version 1.0 released in July of 1994--- there are both many versions of OpenGL, as well as many software libraries built on OpenGL for simplifying application development, whether you’re writing a video game, creating a visualization for scientific or medical purposes, or just showing images. However, the more modern version of OpenGL differs from the original in significant ways. In this book, we describe how to use the most recent versions of OpenGL to create those applications.

The following list briefly describes the major operations that an OpenGL application would perform to render an image. (See ‘‘OpenGL’s Rendering Pipeline’’ for detailed information on these operations.)

• Specify the data for constructing shapes from OpenGL’s geometric primitives.
• Execute various shaders to perform calculations on the input primitives to determine their position, color, and other rendering attributes.
• Convert the mathematical description of the input primitives into their fragments associated with locations on the screen. This process is called rasterization.
• Finally, execute a fragment shader for each of the fragments generated by rasterization, which will determine the fragment’s final color and position.
• Possibly perform additional per-fragment operations such as determining if the object that the fragment was generated from is visible, or blending the fragment’s color with the current color in that screen location.

OpenGL is implemented as a client-server system, with the application you write being considered the client, and the OpenGL implementation provided by the manufacturer of your computer graphics hardware being the server. In some implementations of OpenGL (such as those associated with the X Window System), the client and server will execute on different machines that are connected by a network. In such cases, the client will issue the OpenGL commands, which will be converted into a window-system specific protocol that is transmitted to the server via their shared network, where they are executed to produce the final image.

→This chapter introduces OpenGL. It has the following major sections:
• ‘‘What Is OpenGL?’’ explains what OpenGL is, what it does and doesn’t do, and how it works.
• ‘‘Your First Look at an OpenGL Program’’ provides a first look at what an OpenGL program looks like.
• ‘‘OpenGL Syntax’’ describes the format of the command names that OpenGL uses.
•"OpenGL's Rendering Pipeline’’ discusses the processing pipeline that OpenGL uses in creating images.
• ‘‘Our First Program: A Detailed Discussion’’ dissects the first program presented and provides more detail on the activities of each section of the program.

→Chapter Objectives
After reading this chapter, you’ll be able to do the following:
• Describe the purpose of OpenGL, what it can and cannot do in
creating computer-generated images.
• Identify the common structure of an OpenGL application.
• Enumerate the shading stages that compose the OpenGL rendering
pipeline.