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Jason McKesson committed d5bf9bd

Added a section on GL versions and implementations.

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Documents/Basics/Tutorial 00.xml

                 you call a rendering function.</para>
             <para>The specification is written by the OpenGL <glossterm>Architectural Review
                     Board</glossterm> (<acronym>ARB</acronym>), a group of representatives from
-                companies like Apple, NVIDIA, and AMD (the ATi part), among others. The ARB is part
+                companies like Apple, NVIDIA, and AMD (the ATI part), among others. The ARB is part
                 of the <link xlink:href="http://www.khronos.org/">Khronos Group</link>.</para>
             <para>The specification is a very complicated and technical document. I do not suggest
                 that the novice graphics programmer read it. If you do however, the most important
                 actually does. What it means is that the user should not be able to tell the
                 difference. If a piece of hardware can provide the same behavior in a different way,
                 then the specification allows this, so long as the user can never tell.</para>
+            <formalpara>
+                <title>OpenGL Implementations</title>
+                <para>While the OpenGL ARB does control the specification, it does not control
+                    OpenGL's code. OpenGL is not something you download from a centralized location.
+                    For any particular piece of hardware, it is up to the developers of that
+                    hardware to write an <glossterm>OpenGL Implementation</glossterm> for that
+                    hardware. Implementations, as the name suggests, implement the OpenGL
+                    specification, exposing the OpenGL API as defined in the spec.</para>
+            </formalpara>
+            <para>Who controls the OpenGL implementation is different for different operating
+                systems. On Windows, OpenGL implementations are controlled virtually entirely by the
+                hardware makers themselves. On Mac OSX, OpenGL implementations are controlled by
+                Apple; they decide what version of OpenGL is exposed and what additional
+                functionality can be provided to the user. Apple writes much of the OpenGL
+                implementation on Mac OSX, which the hardware developers writing to an Apple-created
+                internal driver API. On Linux, things are... complicated.</para>
+            <para>The long and short of this is that if you are writing a program and it seems to be
+                exhibiting off-spec behavior, that is the fault of the maker of your OpenGL
+                implementation. On Windows, the various graphics hardware makers put their OpenGL
+                implementations in their regular drivers. So if you suspect a bug in their
+                implementation, the first thing you should do is make sure your graphics drivers are
+                up-to-date; the bug may have been corrected since the last time you updated your
+                drivers.</para>
+            <formalpara>
+                <title>OpenGL Versions</title>
+                <para>There are many versions of the OpenGL Specification. OpenGL versions are not
+                    like most Direct3D versions, which typically change most of the API. Code that
+                    works on one version of OpenGL will almost always work on later versions of
+                    OpenGL.</para>
+            </formalpara>
+            <para>The only exception to this deals with OpenGL 3.0 and above, relative to previous
+                versions. v3.0 deprecated a number of older functions, and v3.1 removed those
+                functions from the API<footnote>
+                    <para>Deprecation only means marking those functions as to be removed in later
+                        functions. They are still available for use in 3.0.</para>
+                </footnote>. This also divided the specification into 2 variations (called
+                profiles): core and compatibility. The compatibility profile has all of the
+                functions removed in 3.1, while the core profile does not. Theoretically, OpenGL
+                implementations could implement just the core profile; this would leave software
+                that relies on the compatibility profile non-functional on that
+                implementation.</para>
+            <para>As a practical matter, none of this matters at all. No OpenGL driver developer is
+                going to ship drivers that only implement the core profile. So in effect, this means
+                nothing at all; all OpenGL versions are all effectively backwards compatible.</para>
         </section>
     </section>
     <section xml:id="Intro_Glossary">
                         perform a computation.</para>
                 </glossdef>
             </glossentry>
+            <glossentry>
+                <glossterm>OpenGL</glossterm>
+                <glossdef>
+                    <para>A specification that defines the effective behavior of a
+                        rasterization-based rendering system.</para>
+                </glossdef>
+            </glossentry>
+            <glossentry>
+                <glossterm>Architectural Review Board</glossterm>
+                <glossdef>
+                    <para>The body of the Khronos Group that governs the OpenGL
+                        specification.</para>
+                </glossdef>
+            </glossentry>
+            <glossentry>
+                <glossterm>OpenGL Implementation</glossterm>
+                <glossdef>
+                    <para>The software that implements the OpenGL specification for a particular
+                        system.</para>
+                </glossdef>
+            </glossentry>
         </glosslist>
     </section>
 </article>
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