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Reprinted with Permission.<br> +<br> + +<table border="0" cellspacing="0" style="border-collapse: collapse" width="100%" cellpadding="0" id="AutoNumber1" height="12" bgcolor="#666699"> + <tr> + <td width="47%" height="12" valign="middle"><p><b><font color="#FFFFFF">OpenAL + Tutorials</font></b></p></td> + <td width="53%" height="12" align="right" valign="middle"><p align="right"><a href="http://devmaster.net/"><font color="#66FF99">DevMaster.net</font></a></p></td> + </tr> + </table> + +<p class="ArticleTitle"><font size="5">A Closer Look at ALC</font></p> +<p class="ArticleTitle"><font size="4"><strong>Lesson 4</strong></font></p> +<p align="right" class="ArticleAuthor">Author: <a href="mailto:[email protected]">Jesse + Maurais<br> + </a>Adapted for Java by: <a href="mailto:[email protected]">Athomas + Goldberg </a></p> +<p align="justify">Up until now we have been letting Alut do all the real tricky + stuff for us. For example handling the audio devices. It's really nice that + the Alut library is there to provide this functionality, but any smart coder + will want to know exactly what their doing. We may want to, at some point, use + the Alc directly. In this tutorial we will expose the Alc layer and take a look + at how to handle the devices on our own.</p> +<pre class=code> +ALFactory.initialize(); +ALC alc = ALFactory.getALC(); + + +String deviceName = "DirectSound3D"; + +ALC.Device device = alc.alcOpenDevice(deviceName); +</pre> +<p align="justify">So what is an Alc device? Try to think of it in terms of a + resource. OpenAL grabs a handle to the hardware being used, which must in turn + be shared with the entire system. A device can be of a specific implementation + as well, as in this case where we are using DirectSound as the audio device. + This code grabs a handle to the hardware device and readies it to be used by + the application. Eventually we should see more devices made for specific soundcards.</p> +<p>Passing <font color="#0000FF" face="Courier New, Courier, mono">null</font> + to 'alcOpenDevice' is a perfectly valid argument. It forces the Alc to use a + default device.</p> +<pre> +ALC.Context context = alc.alcCreateContext(device, <font color="#0000FF">null</font>); +alc.alcMakeContextCurrent(context); +</pre> +<p align="justify">What is an Alc context? OpenGL coders will recall that there + was rendering contexts used by OpenGL that controlled the state management across + different windows. An 'HGLRC' as they are called could be created several times + to enable multiple rendering windows. And different rendering states for each + context could be achieved. An Alc context works on the same principal. First + we tell it which device to use (which we have already created), then we make + that context current. In theory you could create multiple rendering contexts + for different windows, and set the state variables differently and have it work + just fine. Although the term "rendering context" usually applies to + a visual rendering, this is the term preferred in the sdk docs and should be + the term used.</p> +<p align="justify">You may notice too that the second parameter in 'alcCreateContext' + has been set to <font face="Courier New, Courier, mono">null</font>. The OpenAL + sdk from Creative Labs defines the following variables which are optional flags + to that parameter.</p> +<ul> + <li>ALC_FREQUENCY</li> + <li>ALC_REFRESH</li> + <li>ALC_SYNC</li> +</ul> +<p align="justify">If you were to create multiple contexts you could make them + interchangeable by making a call to 'alcMakeContextCurrent'. Sending NULL to + 'alcMakeContextCurrent' is also a perfectly valid argument. It will prevent + processing of any audio data. Be aware that even if you have multiple rendering + contexts, you can only have one current at a time, and when your application + needs to use two contexts interchangeably you must be the one to make sure the + appropriate context is current. And if you do decide to do this, then there + may be times when you want to know exactly which context is current without + going through a big check.</p> +<pre class=code>ALC.Context curContext = alc.alcGetCurrentContext(); +</pre> +<p>Once you have your context you can also obtain the device in use by that context.</p> +<pre class=code>ALC.Device curDevice = alc.alcGetContextsDevice(curContext); +</pre> +<p align="justify">Above we used the context we retrieved to find out which device + it was using. There is also one other cool feature that was built into Alc for + handling contexts.</p> +<pre class=code>alc.alcSuspendContext(context); +<span class=codeComment><font color="#006600">// Processing has been suspended to context.</font></span> + +alc.alcProcessContext(context); +<span class=codeComment><font color="#006600">// Processing has been re-enabled to context.</font></span> +</pre> +<p align="justify">What we have done above was stop, and then resume processing + of audio data to the context. When processing has been suspended, no sound will + be generated from data sent through that context. A further note on the rendering + context: the OpenAL 1.0 spec does imply, but does not explicitly say, that sources + and buffers may be used across contexts. The "lifetime" of a source + or buffer during the application, is said to be valid as long as the source + and buffer id is valid (i.e. they have not been deleted).</p> +<pre class=code>alc.alcMakeContextCurrent(<font color="#0000FF">null</font>); +alc.alcDestroyContext(context); +alc.alcCloseDevice(device); +</pre> +<p align="justify">And that is how we clean up. The current context is defaulted + to <font face="Courier New, Courier, mono">null</font>, the context we created + is released, and the handle to the device is given back to the system resources. + There is but a few more Alc functions we have not yet covered.</p> +<pre class=code><font color="#0000FF">public int</font> alcGetError(); + +<font color="#0000FF">public</font> <font color="#0000FF">boolean</font> alcIsExtensionPresent(ALC.Device device, String extName); + +<font color="#0000FF">public</font> <font color="#0000FF">int</font> alcGetEnumValue(ALC.Device device, String enumName); + +<font color="#0000FF">public </font>String alcGetString(ALC.Device device, <font color="#0000FF">int</font> token); + +<font color="#0000FF">public</font> <font color="#0000FF">void</font> alcGetIntegerv(ALC.Device device, <font color="#0000FF">int</font> token, <font color="#0000FF">int</font> size, <font color="#0000FF">int</font>[] dest); +</pre> +<p align="justify">It may be pretty obvious to you what these do, but lets humour + ourselves and have a closer look. First we have 'alcGetError' which is just + like 'alGetError' but will return Alc errors. The next two functions are for + querying Alc extensions. This was just the creators planning ahead, as there + are no Alc extensions either. The last function, 'alcGetInteger', will return + the Alc version when passed 'ALC_MAJOR_VERSION' or 'ALC_MINOR_VERSION'.</p> +<p>The function 'alcGetString' is pretty cool. It can take any of the following + three parameters to 'token':</p> +<ul> + <li>ALC_DEFAULT_DEVICE_SPECIFIER</li> + <li>ALC_DEVICE_SPECIFIER</li> + <li>ALC_EXTENSIONS</li> +</ul> +<p align="justify">The first will return the device string which your OpenAL implementation + will prefer you to use. In current OpenAL this should be "DirectSound3D", + like we used above. The second token will return a list of specifiers, but in + current OpenAL will only return "DirectSound" (without the "3D" + for some reason). The last will return a list of Alc extensions, of which none + exist yet.</p> +<p align="justify">Well that's most of Alc for you. I hope it gave you a better + understanding of how OpenAL interacts with the operation system. You might try + writing your own initialization routines so you can cast off Alut altogether. + Either way have fun with it.</p> +<table border="0" cellspacing="1" style="border-collapse: collapse" width="100%" id="AutoNumber2" bgcolor="#666699"> + <tr> + <td width="40%"> <p dir="ltr"><font color="#FFFFFF" size="2">� 2003 DevMaster.net. + All rights reserved.</font></td> + <td width="60%"> <p align="right" dir="ltr"><font size="2"><a href="mailto:[email protected]"> + <font color="#FFFFFF">Contact us</font></a><font color="#FFFFFF"> if you + want to write for us or for any comments, suggestions, or feedback.</font></font></td> + </tr> +</table> +<p align="justify"> </p> +</body> +</html>
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