JSR 231 Draft Public Review Specification

JSR 231 Specification Overview

Preface

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:

The javax.media.opengl package

This package contains Java bindings for all core OpenGL methods through version 2.0, inclusive, as well as most OpenGL extensions defined at the time of this specification. Early OpenGL extensions whose functionality was incorporated into core OpenGL by version 1.3, inclusive, are specifically excluded.
The javax.media.opengl.glu package

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 pointer argument for immediate use
Functions that take a typed pointer argument for immediate use
Functions that take an untyped pointer argument for deferred use
Functions that take a typed pointer argument for deferred use

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.

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.

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.

Security

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.

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.

Revision History

Early Draft Review, October/November 2005

Public Review, December/January 2005