JSR 231 Specification Overview




JSR 231 Specification Overview


This specification, an optional set of packages, describes the Java(TM) bindings to the native 3D graphics library, OpenGL(R).

Package Structure

The packages defined by this specification include:

API Binding Conventions

The Java language bindings to the pre-existing C APIs in these packages have been created using a consistent set of rules. Vendor-defined extensions should make use of the same rules in order to provide a consistent developer experience.

The rules for creating the Java language binding are described in the following sections. These rules should be followed as closely as possible for all future APIs that share the javax.media.opengl namespace.

Function Naming

Functions are named in the same way as in the C binding. That is, an OpenGL API function glClear is bound to Java method GL.glClear. Although it would be possible to drop the gl prefix (since it is redundant with the interface name GL), the resulting code was deemed to look too foreign to experienced OpenGL developers. For the same reason, we have also carried over all type suffixes like 3f and 3fv from methods such as glColor3f and glColor3fv, respectively.

Extension suffixes, such as EXT, ARB, and vendor-specific suffixes, are retained so as to match C conventions.

Mapping of Constants

Constants are named in the same way as in the C binding. For instance, the OpenGL constant GL_RGB is bound to Java constant GL.GL_RGB.

Mapping of Primitive Types

All 8-bit integral types become byte, all 16-bit integral types become short, and all 32-bit integral types become int. All 32-bit floating-point types become float and all 64-bit floating-point types become double.

Integer return values that can only be GL_TRUE or GL_FALSE are mapped to boolean.

Mapping of Pointer Arguments

OpenGL functions that take pointer arguments fall into several categories:
Functions that take an untyped (void*) pointer argument for immediate use are given a single binding that takes a New I/O (NIO) Buffer object. The Buffer may be of any type allowable by the function (and compatible with the other arguments to the function) and may be direct or indirect. An example of an OpenGL API in this category is glTexImage2D.

Functions that take a typed pointer (e.g., GLfloat *) argument for immediate use are given two bindings. The first takes a Java primitive array with a type that matches the C pointer type (i.e., GLfloat* maps to float[]). The second takes a typed Buffer object (i.e., GLfloat* maps to FloatBuffer). An example of an OpenGL API in this category is glColor3fv.

Functions that take an untyped (void*) pointer argument for deferred use are given a single binding that takes a Buffer object. The Buffer may be of any type allowable by the function (and compatible with the other arguments to the function), but must be direct. That is, it may not have been created from a Java primitive array using the wrap method. The functions that fall into this category generally have names ending with the suffix "pointer." An example of an OpenGL API in this category is glVertexPointer. Because these functions do not consume the data located at the given pointer immediately, but only at some unspecified later time, it is not possible to use a Java primitive array whose memory location may change.

Functions that take a typed (e.g., GLfloat*) pointer argument for deferred use are given a single binding that takes a typed Buffer object (i.e., GLfloat* maps to FloatBuffer). The Buffer must be direct. That is, it may not have been created from a Java primitive array using the wrap method. An example of an OpenGL API in this category is glFeedbackBuffer.

Methods that read or write a specific number of values from an array or Buffer argument do not read or write any subsequent elements of the array or Buffer.

An outgoing C char* pointer, if representing a null-terminated, read-only C string, maps to a Java String. An outgoing C char** pointer, if similarly representing an array of read-only C strings, maps to a Java String[] (array of String objects). All other char* pointers, including those representing mutable C strings as used in some Get methods, are mapped to byte[] and ByteBuffer.

Index Parameter for Arrays

Each C method argument that is mapped to a primitive array in Java is actually mapped to two separate parameters: the appropriate primitive array type in Java and an integer offset parameter. The value of the integer offset is the index which the method will start reading from within the array. Earlier indices will be ignored. This mapping provides more congruity with existing Java APIs and allows reuse of a single array across multiple Java method calls by changing the index in much the same way that C pointers permit for C arrays.

Reduction of Method Explosions

Since there are two ways to expand a given C method pointer parameter, it would be possible for C methods with multiple pointer arguments to expand to many Java methods if one was to consider every possible combination of mappings (the C method would expand to the number of pointer parameters to the power of 2). In order to avoid an API explosion, we restrict a given Java method to like kind mappings only. In other words, a given C method with N typed pointer parameters for immediate use, where N >= 1, will map to exactly two Java methods: One with all primitive arrays and one with all Buffer types.

Also, methods that accept multiple Buffer arguments require all direct or all non-direct Buffers. Direct and non-direct buffers should never be mixed within an API call by an application.

Byte ordering of Buffers

When allocating a New I/O Buffer (in particular, a direct ByteBuffer) to be passed to the APIs in these packages, it is essential to set the byte ordering of the newly-allocated ByteBuffer to the native byte ordering of the platform: e.g. ByteBuffer.allocateDirect(...).order(ByteOrder.nativeOrder());. The byte order of the ByteBuffer indicates how multi-byte values such as int and float are stored in the Buffer either using methods like putInt and putFloat or views such as IntBuffer or FloatBuffer. The Java bindings perform no conversion or byte swapping on the outgoing data to OpenGL, and the native OpenGL implementation expects data in the host CPU's byte order, so it is essential to always match the byte order of the underlying platform when filling Buffers with data.

Auto-slicing of Buffers

When a Buffer object is passed to an OpenGL function binding, the actual pointer argument that is passed down to the OpenGL C implementation is equal to the starting pointer of the Buffer data, plus an offset given by the Buffer.position() function, multiplied by the data type size in bytes (1 for a ByteBuffer, 2 for a ShortBuffer, 4 for a IntBuffer or FloatBuffer, and 8 for DoubleBuffer). The array offset given by Buffer<type>.arrayOffset() is also added in the offset for wrapped arrays.

This feature is known as "auto-slicing," as it mimics the effect of calling slice() on the Buffer object without the overhead of explicit object creation.

Errors and Exceptions

For performance reasons, OpenGL functions do not return error values directly. Instead, applications must query for errors using functions such as glGetError. This behavior is largely preserved in the Java language bindings, as described below.

In the interest of efficiency, the Java API does not generally throw exceptions. However, running an application with the DebugGL composable pipeline, which is part of the API, will force an exception to be thrown at the point of failure.

Many errors are defined by OpenGL merely to set the error code, rather than throwing an exception. For example, passing a bad enumerated parameter value may result in the error flag being set to GL.GL_INVALID_VALUE. Attempting to check for such errors in the binding layer would require either replicating the error-checking logic of the underlying engine, or querying the error state after every function. This would greatly impact performance by inhibiting the ability of the hardware to pipeline work.


Exception behavior is defined in cases that could otherwise lead to illegal memory accesses in the underlying OpenGL engine. Implementations should take necessary steps to prevent the GL from accessing or overwriting memory except for properly allocated Buffers and array method arguments.

An implementation should take care to validate arguments correctly before invoking native methods that could potentially access memory illegally. In particular, methods that validate the contents of an array (such as a list of GL attributes) or a Buffer should take precautions against exploits in which a separate thread attempts to alter the contents of the argument during the time interval following validation but preceding passage of the argument to the underlying native engine.

Sharing of Server-Side OpenGL Objects between GLContexts

Sharing of server-side OpenGL objects such as textures and display lists among OpenGL contexts is supported in this specification. However, the specification does not guarantee that a request to share objects between two contexts will succeed. Certain OpenGL implementations restrict sharing in certain ways. Applications should assume that sharing is a request more than an assertion. Implementations of this specification should, but are not required to, provide timely notification of a failure of a request to share objects between two GLContexts, perhaps in the form of a GLException. The situation in which sharing is most likely to work on all platforms is between two GLContexts associated with the same GLDrawable.

Criteria Used for Inclusion of APIs into the Java Bindings

OpenGL API Inclusion Criteria

OpenGL functions and OpenGL extensions have been included in the Java bindings according the following rules:

OpenGL GLU API Inclusion Criteria

Bindings for all core GLU APIs have been included with the exception of the GLU NURBS APIs.  These APIs may be included in a future maintenance release of the Java bindings.

OpenGL Extensions

Creating New Extensions

While the Java APIs for OpenGL extensions are unconditionally exposed, the underlying functions may not be present. A program can query whether a potentially unavailable function is actually available at runtime by using the method GL.isFunctionAvailable.

Bindings for OpenGL extensions not covered in this specification may be supplied by individual vendors or groups. Such bindings may be considered for inclusion in a future version of the JSR. In order to avoid fragmentation, vendors creating extension bindings should expose new extensions using the method GL.getExtension. This method is intended to provide a mechanism for vendors who wish to provide access to new OpenGL extensions without changing the public API of the core package.

Names for added extension methods and extension-defined constants and Java bindings for C parameters should follow the guidelines set forth in this specfication.

Accessing Platform-Specific Extensions

Platform-specific extensions such as those that begin with WGL, GLX, CGL, etc. are not included in the API.  Each implementation can choose to export all, some, or none of these APIs via the GL.getPlatformGLExtensions API which returns an Object whose underlying data type is specific to a given implementation.
Therefore, any usage of these APIs is both platform and implementation specific.

OpenGL Version on Runtime System

Even though OpenGL extensions whose functionality was included into core OpenGL by version 1.3, inclusive, are not included in the bindings, it should be noted that OpenGL version 1.3 is not an absolute requirement on the runtime system.  This is because a user could query whether any particular function is available before calling certain core APIs that might not be present.  However, in general, it is reasonable to expect at least OpenGL 1.3 to be installed on the runtime system and an implementor of the API is free to require the presence of at least OpenGL 1.3 on the target system.
In future revisions of the API, this minimum standard may be raised.

Runtime Version Information

Any Java Bindings for OpenGL implementation should include version information in its jar manifest file.  This information can then easily be accessed at runtime via the java.lang.Package API.  At least the following information is included in the Reference Implementation jar file manifest: Specification Title, Specification Vendor, Specification Version, Implementation Vendor, and Implementation Version.

Future Maintenance Updates

New core APIs found in future versions of OpenGL, as well as new OpenGL extensions, are expected to be added to the bindings and included into the javax.media.opengl namespace via future maintenance updates to the API.

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Revision History

Copyright 2005 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms.