32. Introduction to OpenGL
Dated: 03-07-2025
As a software interface for graphics hardware, OpenGL renders multidimensional objects into a frame buffer.1 OpenGL is industry-standard graphics software with which programmers can create high-quality still and animated three-dimensional color images.
Where Applicable
It is built for compatibility across hardware and operating systems, making it easy to port across different operating systems (i.e. Linux and Windows). It is designed for use by C and C++ programmers.
It is introduced since \(1992\).
Developer Driven Advantages
Industry Standard
An independent consortium, the OpenGL Architecture Review Board, guides the OpenGL specification. With broad industry support, OpenGL is the only truly open, vendor-neutral, multiplatform graphics standard.
Stable
Backward compatibility requirements ensure that existing applications do not become obsolete.
Reliable and Portable
All OpenGL applications produce consistent visual display results on any OpenGL API-compliant hardware, regardless of operating system or windowing system.
Evolving
Because of its thorough and forward-looking design, OpenGL allows new hardware innovations to be accessible through the API via the OpenGL extension mechanism. In this way, innovations appear in the API in a timely fashion, letting application developers and hardware vendors incorporate new features into their normal product release cycles.
Scalable
OpenGL API-based applications can run on systems ranging from consumer electronics to PCs, workstations, and supercomputers. As a result, applications can scale to any class of machine that the developer chooses to target.
Easy to Use
OpenGL is well structured with an intuitive design and logical commands. Efficient OpenGL routines typically result in applications with fewer lines of code than those that make up programs generated using other graphics libraries or packages. In addition, OpenGL drivers encapsulate information about the underlying hardware, freeing the application developer from having to design for specific hardware features.
Well Documented
Numerous books have been published about OpenGL, and a great deal of sample code is readily available, making information about OpenGL inexpensive and easy to obtain.
Simplifies Software Development, Speeds Time-to-market
From graphics primitives like point and lines to complex lighting and texture mapped curved surfaces, OpenGL provides functions with different standard abiding bindings for languages like
CC++JavaFortranAda
This increases the developer productivity, decreasing the time to markets.
Available Everywhere
OpenGL runs on every major operating system including
Mac OSOS/2UNIXWindows 95Windows 98Windows 2000Windows NTLinuxOPENStepBeOS
Architected for Flexibility and Differentiation
Using the OpenGL extension mechanism, hardware developers can differentiate their products by developing extensions that allow software developers to access additional performance and technological innovations.
The following topics present a global view of how OpenGL works:
Primitives and Commands
Discusses
- Points
- Line segments
- Polygons
- Processing of commands
A vertex defines a point which is the endpoint of a line or a corner of polygon where two edges meet. Each vertex also has some data associated with it such as
vertexcoordinatescolorsnormals2texture coordinatesedge flags
Commands are always processed in the order in which they are received.
OpenGL Graphic Control
Provides some graphics operations such as
Transformation matrices3- Lighting equation coefficients
- Antialiasing methods
Pixel4 update operators
However, it doesn't provide you means to describe or model a complex geometric object. OpenGL is procedural rather than descriptive.
Execution Model
The client application can issue commands which are interpreted by OpenGL (the server) which could possibly run on a different hardware. This makes OpenGL network transparent.
A server maintains several OpenGL contexts each of which is an encapsulated OpenGL state. A client can connect to any of these contexts.
The window system:
- Determines which portions of the
frame buffer1OpenGLmay access at any given time. - Communicates to
OpenGLhow those portions are structured.
Frame buffer1 configuration is done outside of OpenGL in conjunction with the window system. OpenGL initialization takes place when the window system allocates a window for OpenGL rendering.
Basic OpenGL Operation
Gives a high-level description of how OpenGL processes data to produce a corresponding image in the frame buffer.1
OpenGL processing of data
source: learn.microsoft.com
Display List
Rather than having all commands proceed immediately through the pipeline, you can choose to accumulate some of them in a display list for processing later.
Evaluator
The evaluator stage of processing provides an efficient way to approximate curve and surface geometry by evaluating polynomial commands of input values.
Per-vertex Operations and Primitive Assembly
OpenGL processes geometric primitives such as
pointsline segmentspolygons
all of which are described by vertices.
Vertices are transformed and lit, and primitives are clipped to the viewport in preparation for rasterization.
Rasterization
The rasterization stage produces a series of frame buffer addresses and associated values using a two-dimensional description of a
pointline segmentpolygon
Each fragment so produced is fed into the last stage, per fragment operations.
Per Fragment Operations
These are the final operations performed on the data before it is stored as pixels4 in the framebuffer.1
Following pixel operations, the pixel data is either:
- Stored as
texture memory, for use in therasterizationstage. - Rasterized, with the resulting fragments merged into the
framebuffer1 just as if they were generated from geometric data.
OpenGL Processing Pipeline
source: learn.microsoft.com
References
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Read more about vector normals. ↩
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Read more about transformation matrices. ↩