Breaking Eggs And Making Omelettes

A blog dealing with technical multimedia matters, binary reverse engineering, and the occasional video game hacking.

http://multimedia.cx/eggs/

Les articles publiés sur le site

  • Things I Have Learned About Emscripten

    1er septembre 2015, par Multimedia Mike — Cirrus Retro

    3 years ago, I released my Game Music Appreciation project, a website with a ludicrously uninspired title which allowed users a relatively frictionless method to experience a range of specialized music files related to old video games. However, the site required use of a special Chrome plugin. Ever since that initial release, my #1 most requested feature has been for a pure JavaScript version of the music player.

    “Impossible!” I exclaimed. “There’s no way JS could ever run fast enough to run these CPU emulators and audio synthesizers in real time, and allow for the visualization that I demand!” Well, I’m pleased to report that I have proved me wrong. I recently quietly launched a new site with what I hope is a catchier title, meant to evoke a cloud-based retro-music-as-a-service product: Cirrus Retro. Right now, it’s basically the same as the old site, but without the wonky Chrome-specific technology.

    Along the way, I’ve learned a few things about using Emscripten that I thought might be useful to share with other people who wish to embark on a similar journey. This is geared more towards someone who has a stronger low-level background (such as C/C++) vs. high-level (like JavaScript).

    General Goals
    Do you want to cross-compile an entire desktop application, one that relies on an extensive GUI toolkit? That might be difficult (though I believe there is a path for porting qt code directly with Emscripten). Your better wager might be to abstract out the core logic and processes of the program and then create a new web UI to access them.

    Do you want to compile a game that basically just paints stuff to a 2D canvas? You’re in luck! Emscripten has a porting path for SDL. Make a version of your C/C++ software that targets SDL (generally not a tall order) and then compile that with Emscripten.

    Do you just want to cross-compile some functionality that lives in a library? That’s what I’ve done with the Cirrus Retro project. For this, plan to compile the library into a JS file that exports some public functions that other, higher-level, native JS (i.e., JS written by a human and not a computer) will invoke.

    Memory Levels
    When porting C/C++ software to JavaScript using Emscripten, you have to think on 2 different levels. Or perhaps you need to force JavaScript into a low level C lens, especially if you want to write native JS code that will interact with Emscripten-compiled code. This often means somehow allocating chunks of memory via JS and passing them to the Emscripten-compiled functions. And you wouldn’t believe the type of gymnastics you need to execute to get native JS and Emscripten-compiled JS to cooperate.

    “Emscripten: Pointers and Pointers” is the best (and, really, ONLY) explanation I could find for understanding the basic mechanics of this process, at least when I started this journey. However, there’s a mistake in the explanation that left me confused for a little while, and I’m at a loss to contact the author (doesn’t anyone post a simple email address anymore?).

    Per the best of my understanding, Emscripten allocates a large JS array and calls that the memory space that the compiled C/C++ code is allowed to operate in. A pointer in C/C++ code will just be an index into that mighty array. Really, that’s not too far off from how a low-level program process is supposed to view memory– as a flat array.

    Eventually, I just learned to cargo-cult my way through the memory allocation process. Here’s the JS code for allocating an Emscripten-compatible byte buffer, taken from my test harness (more on that later):

    var musicBuffer = fs.readFileSync(testSpec['filename']);
var musicBufferBytes = new Uint8Array(musicBuffer);
var bytesMalloc = player._malloc(musicBufferBytes.length);
var bytes = new Uint8Array(player.HEAPU8.buffer, bytesMalloc, musicBufferBytes.length);
bytes.set(new Uint8Array(musicBufferBytes.buffer));

    So, read the array of bytes from some input source, create a Uint8Array from the bytes, use the Emscripten _malloc() function to allocate enough bytes from the Emscripten memory array for the input bytes, then create a new array… then copy the bytes…

    You know what? It’s late and I can’t remember how it works exactly, but it does. It has been a few months since I touched that code (been fighting with front-end website tech since then). You write that memory allocation code enough times and it begins to make sense, and then you hope you don’t have to write it too many more times.

    Multithreading
    You can’t port multithreaded code to JS via Emscripten. JavaScript has no notion of threads! If you don’t understand the computer science behind this limitation, a more thorough explanation is beyond the scope of this post. But trust me, I’ve thought about it a lot. In fact, the official Emscripten literature states that you should be able to port most any C/C++ code as long as 1) none of the code is proprietary (i.e., all the raw source is available); and 2) there are no threads.

    Yes, I read about the experimental pthreads support added to Emscripten recently. Don’t get too excited; that won’t be ready and widespread for a long time to come as it relies on a new browser API. In the meantime, figure out how to make your multithreaded C/C++ code run in a single thread if you want it to run in a browser.

    Printing Facility
    Eventually, getting software to work boils down to debugging, and the most primitive tool in many a programmer’s toolbox is the humble print statement. A print statement allows you to inspect a piece of a program’s state at key junctures. Eventually, when you try to cross-compile C/C++ code to JS using Emscripten, something is not going to work correctly in the generated JS “object code” and you need to understand what. You’ll be pleading for a method of just inspecting one variable deep in the original C/C++ code.

    I came up with this simple printf-workalike called emprintf():

    #ifndef EMPRINTF_H
#define EMPRINTF_H

#include <stdio .h>
#include <stdarg .h>
#include <emscripten .h>

#define MAX_MSG_LEN 1000

/* NOTE: Don't pass format strings that contain single quote (') or newline
* characters. */
static void emprintf(const char *format, ...)
{
    char msg[MAX_MSG_LEN];
    char consoleMsg[MAX_MSG_LEN + 16];
    va_list args;

    /* create the string */
    va_start(args, format);
    vsnprintf(msg, MAX_MSG_LEN, format, args);
    va_end(args);

    /* wrap the string in a console.log('') statement */
    snprintf(consoleMsg, MAX_MSG_LEN + 16, "console.log('%s')", msg);

    /* send the final string to the JavaScript console */
    emscripten_run_script(consoleMsg);
}

#endif  /* EMPRINTF_H */

    Put it in a file called “emprint.h”. Include it into any C/C++ file where you need debugging visibility, use emprintf() as a replacement for printf() and the output will magically show up on the browser’s JavaScript debug console. Heed the comments and don’t put any single quotes or newlines in strings, and keep it under 1000 characters. I didn’t say it was perfect, but it has helped me a lot in my Emscripten adventures.

    Optimization Levels
    Remember to turn on optimization when compiling. I have empirically found that optimizing for size (-Os) leads to the best performance all around, in addition to having the smallest size. Just be sure to specify some optimization level. If you don’t, the default is -O0 which offers horrible performance when running in JS.

    Static Compression For HTTP Delivery
    JavaScript code compresses pretty efficiently, even after it has been optimized for size using -Os. I routinely see compression ratios between 3.5:1 and 5:1 using gzip.

    Web servers in this day and age are supposed to be smart enough to detect when a requesting web browser can accept gzip-compressed data and do the compression on the fly. They’re even supposed to be smart enough to cache compressed output so the same content is not recompressed for each request. I would have to set up a series of tests to establish whether either of the foregoing assertions are correct and I can’t be bothered. Instead, I took it into my own hands. The trick is to pre-compress the JS files and then instruct the webserver to serve these files with a ‘Content-Type’ of ‘application/javascript’ and a ‘Content-Encoding’ of ‘gzip’.

    1. Compress your large Emscripten-build JS files with ‘gzip’: ‘gzip compiled-code.js’
    2. Rename them from extension .js.gz to .jsgz
    3. Tell the webserver to deliver .jsgz files with the correct Content-Type and Content-Encoding headers

    To do that last step with Apache, specify these lines:

    AddType application/javascript jsgz
AddEncoding gzip jsgz

    They belong in either a directory’s .htaccess file or in the sitewide configuration (/etc/apache2/mods-available/mime.conf works on my setup).

    Build System and Build Time Optimization
    Oh goodie, build systems! I had a very specific manner in which I wanted to build my JS modules using Emscripten. Can I possibly coerce any of the many popular build systems to do this? It has been a few months since I worked on this problem specifically but I seem to recall that the build systems I tried to used would freak out at the prospect of compiling stuff to a final binary target of .js.

    I had high hopes for Bazel, which Google released while I was developing Cirrus Retro. Surely, this is software that has been battle-tested in the harshest conditions of one of the most prominent software-developing companies in the world, needing to take into account the most bizarre corner cases and still build efficiently and correctly every time. And I have little doubt that it fulfills the order. Similarly, I’m confident that Google also has a team of no fewer than 100 or so people dedicated to developing and supporting the project within the organization. When you only have, at best, 1-2 hours per night to work on projects like this, you prefer not to fight with such cutting edge technology and after losing 2 or 3 nights trying to make a go of Bazel, I eventually put it aside.

    I also tried to use Autotools. It failed horribly for me, mostly for my own carelessness and lack of early-project source control.

    After that, it was strictly vanilla makefiles with no real dependency management. But you know what helps in these cases? ccache! Or at least, it would if it didn’t fail with Emscripten.

    Quick tip: ccache has trouble with LLVM unless you set the CCACHE_CPP2 environment variable (e.g.: “export CCACHE_CPP2=1”). I don’t remember the specifics, but it magically fixes things. Then, the lazy build process becomes “make clean && make”.

    Testing
    If you have never used Node.js, testing Emscripten-compiled JS code might be a good opportunity to start. I was able to use Node.js to great effect for testing the individually-compiled music player modules, wiring up a series of invocations using Python for a broader test suite (wouldn’t want to go too deep down the JS rabbit hole, after all).

    Be advised that Node.js doesn’t enjoy the same kind of JIT optimizations that the browser engines leverage. Thus, in the case of time critical code like, say, an audio synthesis library, the code might not run in real time. But as long as it produces the correct bitwise waveform, that’s good enough for continuous integration.

    Also, if you have largely been a low-level programmer for your whole career and are generally unfamiliar with the world of single-threaded, event-driven, callback-oriented programming, you might be in for a bit of a shock. When I wanted to learn how to read the contents of a file in Node.js, this is the first tutorial I found on the matter. I thought the code presented was a parody of bad coding style:

    var fs = require("fs");
var fileName = "foo.txt";

fs.exists(fileName, function(exists) {
  if (exists) {
    fs.stat(fileName, function(error, stats) {
      fs.open(fileName, "r", function(error, fd) {
        var buffer = new Buffer(stats.size);

        fs.read(fd, buffer, 0, buffer.length, null, function(error, bytesRead, buffer) {
          var data = buffer.toString("utf8", 0, buffer.length);

          console.log(data);
          fs.close(fd);
        });
      });
    });
  }});

    Apparently, this kind of thing doesn’t raise an eyebrow in the JS world.

    Now, I understand and respect the JS programming model. But this was seriously frustrating when I first encountered it because a simple script like the one I was trying to write just has an ordered list of tasks to complete. When it asks for bytes from a file, it really has nothing better to do than to wait for the answer.

    Thankfully, it turns out that Node’s fs module includes synchronous versions of the various file access functions. So it’s all good.

    Conclusion
    I’m sure I missed or underexplained some things. But if other brave souls are interested in dipping their toes in the waters of Emscripten, I hope these tips will come in handy.

  • Things I Have Learned About Emscripten

    1er septembre 2015, par Multimedia Mike — Cirrus Retro

    3 years ago, I released my Game Music Appreciation project, a website with a ludicrously uninspired title which allowed users a relatively frictionless method to experience a range of specialized music files related to old video games. However, the site required use of a special Chrome plugin. Ever since that initial release, my #1 most requested feature has been for a pure JavaScript version of the music player.

    “Impossible!” I exclaimed. “There’s no way JS could ever run fast enough to run these CPU emulators and audio synthesizers in real time, and allow for the visualization that I demand!” Well, I’m pleased to report that I have proved me wrong. I recently quietly launched a new site with what I hope is a catchier title, meant to evoke a cloud-based retro-music-as-a-service product: Cirrus Retro. Right now, it’s basically the same as the old site, but without the wonky Chrome-specific technology.

    Along the way, I’ve learned a few things about using Emscripten that I thought might be useful to share with other people who wish to embark on a similar journey. This is geared more towards someone who has a stronger low-level background (such as C/C++) vs. high-level (like JavaScript).

    General Goals
    Do you want to cross-compile an entire desktop application, one that relies on an extensive GUI toolkit? That might be difficult (though I believe there is a path for porting qt code directly with Emscripten). Your better wager might be to abstract out the core logic and processes of the program and then create a new web UI to access them.

    Do you want to compile a game that basically just paints stuff to a 2D canvas? You’re in luck! Emscripten has a porting path for SDL. Make a version of your C/C++ software that targets SDL (generally not a tall order) and then compile that with Emscripten.

    Do you just want to cross-compile some functionality that lives in a library? That’s what I’ve done with the Cirrus Retro project. For this, plan to compile the library into a JS file that exports some public functions that other, higher-level, native JS (i.e., JS written by a human and not a computer) will invoke.

    Memory Levels
    When porting C/C++ software to JavaScript using Emscripten, you have to think on 2 different levels. Or perhaps you need to force JavaScript into a low level C lens, especially if you want to write native JS code that will interact with Emscripten-compiled code. This often means somehow allocating chunks of memory via JS and passing them to the Emscripten-compiled functions. And you wouldn’t believe the type of gymnastics you need to execute to get native JS and Emscripten-compiled JS to cooperate.

    “Emscripten: Pointers and Pointers” is the best (and, really, ONLY) explanation I could find for understanding the basic mechanics of this process, at least when I started this journey. However, there’s a mistake in the explanation that left me confused for a little while, and I’m at a loss to contact the author (doesn’t anyone post a simple email address anymore?).

    Per the best of my understanding, Emscripten allocates a large JS array and calls that the memory space that the compiled C/C++ code is allowed to operate in. A pointer in C/C++ code will just be an index into that mighty array. Really, that’s not too far off from how a low-level program process is supposed to view memory– as a flat array.

    Eventually, I just learned to cargo-cult my way through the memory allocation process. Here’s the JS code for allocating an Emscripten-compatible byte buffer, taken from my test harness (more on that later):

    var musicBuffer = fs.readFileSync(testSpec['filename']);
var musicBufferBytes = new Uint8Array(musicBuffer);
var bytesMalloc = player._malloc(musicBufferBytes.length);
var bytes = new Uint8Array(player.HEAPU8.buffer, bytesMalloc, musicBufferBytes.length);
bytes.set(new Uint8Array(musicBufferBytes.buffer));

    So, read the array of bytes from some input source, create a Uint8Array from the bytes, use the Emscripten _malloc() function to allocate enough bytes from the Emscripten memory array for the input bytes, then create a new array… then copy the bytes…

    You know what? It’s late and I can’t remember how it works exactly, but it does. It has been a few months since I touched that code (been fighting with front-end website tech since then). You write that memory allocation code enough times and it begins to make sense, and then you hope you don’t have to write it too many more times.

    Multithreading
    You can’t port multithreaded code to JS via Emscripten. JavaScript has no notion of threads! If you don’t understand the computer science behind this limitation, a more thorough explanation is beyond the scope of this post. But trust me, I’ve thought about it a lot. In fact, the official Emscripten literature states that you should be able to port most any C/C++ code as long as 1) none of the code is proprietary (i.e., all the raw source is available); and 2) there are no threads.

    Yes, I read about the experimental pthreads support added to Emscripten recently. Don’t get too excited; that won’t be ready and widespread for a long time to come as it relies on a new browser API. In the meantime, figure out how to make your multithreaded C/C++ code run in a single thread if you want it to run in a browser.

    Printing Facility
    Eventually, getting software to work boils down to debugging, and the most primitive tool in many a programmer’s toolbox is the humble print statement. A print statement allows you to inspect a piece of a program’s state at key junctures. Eventually, when you try to cross-compile C/C++ code to JS using Emscripten, something is not going to work correctly in the generated JS “object code” and you need to understand what. You’ll be pleading for a method of just inspecting one variable deep in the original C/C++ code.

    I came up with this simple printf-workalike called emprintf():

    #ifndef EMPRINTF_H
#define EMPRINTF_H

#include <stdio .h>
#include <stdarg .h>
#include <emscripten .h>

#define MAX_MSG_LEN 1000

/* NOTE: Don't pass format strings that contain single quote (') or newline
* characters. */
static void emprintf(const char *format, ...)
{
    char msg[MAX_MSG_LEN];
    char consoleMsg[MAX_MSG_LEN + 16];
    va_list args;

    /* create the string */
    va_start(args, format);
    vsnprintf(msg, MAX_MSG_LEN, format, args);
    va_end(args);

    /* wrap the string in a console.log('') statement */
    snprintf(consoleMsg, MAX_MSG_LEN + 16, "console.log('%s')", msg);

    /* send the final string to the JavaScript console */
    emscripten_run_script(consoleMsg);
}

#endif  /* EMPRINTF_H */

    Put it in a file called “emprint.h”. Include it into any C/C++ file where you need debugging visibility, use emprintf() as a replacement for printf() and the output will magically show up on the browser’s JavaScript debug console. Heed the comments and don’t put any single quotes or newlines in strings, and keep it under 1000 characters. I didn’t say it was perfect, but it has helped me a lot in my Emscripten adventures.

    Optimization Levels
    Remember to turn on optimization when compiling. I have empirically found that optimizing for size (-Os) leads to the best performance all around, in addition to having the smallest size. Just be sure to specify some optimization level. If you don’t, the default is -O0 which offers horrible performance when running in JS.

    Static Compression For HTTP Delivery
    JavaScript code compresses pretty efficiently, even after it has been optimized for size using -Os. I routinely see compression ratios between 3.5:1 and 5:1 using gzip.

    Web servers in this day and age are supposed to be smart enough to detect when a requesting web browser can accept gzip-compressed data and do the compression on the fly. They’re even supposed to be smart enough to cache compressed output so the same content is not recompressed for each request. I would have to set up a series of tests to establish whether either of the foregoing assertions are correct and I can’t be bothered. Instead, I took it into my own hands. The trick is to pre-compress the JS files and then instruct the webserver to serve these files with a ‘Content-Type’ of ‘application/javascript’ and a ‘Content-Encoding’ of ‘gzip’.

    1. Compress your large Emscripten-build JS files with ‘gzip’: ‘gzip compiled-code.js’
    2. Rename them from extension .js.gz to .jsgz
    3. Tell the webserver to deliver .jsgz files with the correct Content-Type and Content-Encoding headers

    To do that last step with Apache, specify these lines:

    AddType application/javascript jsgz
AddEncoding gzip jsgz

    They belong in either a directory’s .htaccess file or in the sitewide configuration (/etc/apache2/mods-available/mime.conf works on my setup).

    Build System and Build Time Optimization
    Oh goodie, build systems! I had a very specific manner in which I wanted to build my JS modules using Emscripten. Can I possibly coerce any of the many popular build systems to do this? It has been a few months since I worked on this problem specifically but I seem to recall that the build systems I tried to used would freak out at the prospect of compiling stuff to a final binary target of .js.

    I had high hopes for Bazel, which Google released while I was developing Cirrus Retro. Surely, this is software that has been battle-tested in the harshest conditions of one of the most prominent software-developing companies in the world, needing to take into account the most bizarre corner cases and still build efficiently and correctly every time. And I have little doubt that it fulfills the order. Similarly, I’m confident that Google also has a team of no fewer than 100 or so people dedicated to developing and supporting the project within the organization. When you only have, at best, 1-2 hours per night to work on projects like this, you prefer not to fight with such cutting edge technology and after losing 2 or 3 nights trying to make a go of Bazel, I eventually put it aside.

    I also tried to use Autotools. It failed horribly for me, mostly for my own carelessness and lack of early-project source control.

    After that, it was strictly vanilla makefiles with no real dependency management. But you know what helps in these cases? ccache! Or at least, it would if it didn’t fail with Emscripten.

    Quick tip: ccache has trouble with LLVM unless you set the CCACHE_CPP2 environment variable (e.g.: “export CCACHE_CPP2=1”). I don’t remember the specifics, but it magically fixes things. Then, the lazy build process becomes “make clean && make”.

    Testing
    If you have never used Node.js, testing Emscripten-compiled JS code might be a good opportunity to start. I was able to use Node.js to great effect for testing the individually-compiled music player modules, wiring up a series of invocations using Python for a broader test suite (wouldn’t want to go too deep down the JS rabbit hole, after all).

    Be advised that Node.js doesn’t enjoy the same kind of JIT optimizations that the browser engines leverage. Thus, in the case of time critical code like, say, an audio synthesis library, the code might not run in real time. But as long as it produces the correct bitwise waveform, that’s good enough for continuous integration.

    Also, if you have largely been a low-level programmer for your whole career and are generally unfamiliar with the world of single-threaded, event-driven, callback-oriented programming, you might be in for a bit of a shock. When I wanted to learn how to read the contents of a file in Node.js, this is the first tutorial I found on the matter. I thought the code presented was a parody of bad coding style:

    var fs = require("fs");
var fileName = "foo.txt";

fs.exists(fileName, function(exists) {
  if (exists) {
    fs.stat(fileName, function(error, stats) {
      fs.open(fileName, "r", function(error, fd) {
        var buffer = new Buffer(stats.size);

        fs.read(fd, buffer, 0, buffer.length, null, function(error, bytesRead, buffer) {
          var data = buffer.toString("utf8", 0, buffer.length);

          console.log(data);
          fs.close(fd);
        });
      });
    });
  }});

    Apparently, this kind of thing doesn’t raise an eyebrow in the JS world.

    Now, I understand and respect the JS programming model. But this was seriously frustrating when I first encountered it because a simple script like the one I was trying to write just has an ordered list of tasks to complete. When it asks for bytes from a file, it really has nothing better to do than to wait for the answer.

    Thankfully, it turns out that Node’s fs module includes synchronous versions of the various file access functions. So it’s all good.

    Conclusion
    I’m sure I missed or underexplained some things. But if other brave souls are interested in dipping their toes in the waters of Emscripten, I hope these tips will come in handy.

    The post Things I Have Learned About Emscripten first appeared on Breaking Eggs And Making Omelettes.

  • Emscripten and Web Audio API

    29 avril 2015, par Multimedia Mike — HTML5

    Ha! They said it couldn’t be done! Well, to be fair, I said it couldn’t be done. Or maybe that I just didn’t have any plans to do it. But I did it– I used Emscripten to cross-compile a CPU-intensive C/C++ codebase (Game Music Emu) to JavaScript. Then I leveraged the Web Audio API to output audio and visualize the audio using an HTML5 canvas.

    Want to see it in action? Here’s a demonstration. Perhaps I will be able to expand the reach of my Game Music site when I can drop the odd Native Client plugin. This JS-based player works great on Chrome, Firefox, and Safari across desktop operating systems.

    But this endeavor was not without its challenges.

    Programmatically Generating Audio
    First, I needed to figure out the proper method for procedurally generating audio and making it available to output. Generally, there are 2 approaches for audio output:

    1. Sit in a loop and generate audio, writing it out via a blocking audio call
    2. Implement a callback that the audio system can invoke in order to generate more audio when needed

    Option #1 is not a good idea for an event-driven language like JavaScript. So I hunted through the rather flexible Web Audio API for a method that allowed something like approach #2. Callbacks are everywhere, after all.

    I eventually found what I was looking for with the ScriptProcessorNode. It seems to be intended to apply post-processing effects to audio streams. A program registers a callback which is passed configurable chunks of audio for processing. I subverted this by simply overwriting the input buffers with the audio generated by the Emscripten-compiled library.

    The ScriptProcessorNode interface is fairly well documented and works across multiple browsers. However, it is already marked as deprecated:

    Note: As of the August 29 2014 Web Audio API spec publication, this feature has been marked as deprecated, and is soon to be replaced by Audio Workers.

    Despite being marked as deprecated for 8 months as of this writing, there exists no appreciable amount of documentation for the successor API, these so-called Audio Workers.

    Vive la web standards!

    Visualize This
    The next problem was visualization. The Web Audio API provides the AnalyzerNode API for accessing both time and frequency domain data from a running audio stream (and fetching the data as both unsigned bytes or floating-point numbers, depending on what the application needs). This is a pretty neat idea. I just wish I could make the API work. The simple demos I could find worked well enough. But when I wired up a prototype to fetch and visualize the time-domain wave, all I got were center-point samples (an array of values that were all 128).

    Even if the API did work, I’m not sure if it would have been that useful. Per my reading of the AnalyserNode API, it only returns data as a single channel. Why would I want that? My application supports audio with 2 channels. I want 2 channels of data for visualization.

    How To Synchronize
    So I rolled my own visualization solution by maintaining a circular buffer of audio when samples were being generated. Then, requestAnimationFrame() provided the rendering callbacks. The next problem was audio-visual sync. But that certainly is not unique to this situation– maintaining proper A/V sync is a perennial puzzle in real-time multimedia programming. I was able to glean enough timing information from the environment to achieve reasonable A/V sync (verify for yourself).

    Pause/Resume
    The next problem I encountered with the Web Audio API was pause/resume facilities, or the lack thereof. For all its bells and whistles, the API’s omission of such facilities seems most unusual, as if the design philosophy was, “Once the user starts playing audio, they will never, ever have cause to pause the audio.”

    Then again, I must understand that mine is not a use case that the design committee considered and I’m subverting the API in ways the designers didn’t intend. Typical use cases for this API seem to include such workloads as:

    • Downloading, decoding, and playing back a compressed audio stream via the network, applying effects, and visualizing the result
    • Accessing microphone input, applying effects, visualizing, encoding and sending the data across the network
    • Firing sound effects in a gaming application
    • MIDI playback via JavaScript (this honestly amazes me)

    What they did not seem to have in mind was what I am trying to do– synthesize audio in real time.

    I implemented pause/resume in a sub-par manner: pausing has the effect of generating 0 values when the ScriptProcessorNode callback is invoked, while also canceling any animation callbacks. Thus, audio output is technically still occurring, it’s just that the audio is pure silence. It’s not a great solution because CPU is still being used.

    Future Work
    I have a lot more player libraries to port to this new system. But I think I have a good framework set up.

  • Emscripten and Web Audio API

    29 avril 2015, par Multimedia Mike — HTML5

    Ha! They said it couldn’t be done! Well, to be fair, I said it couldn’t be done. Or maybe that I just didn’t have any plans to do it. But I did it– I used Emscripten to cross-compile a CPU-intensive C/C++ codebase (Game Music Emu) to JavaScript. Then I leveraged the Web Audio API to output audio and visualize the audio using an HTML5 canvas.

    Want to see it in action? Here’s a demonstration. Perhaps I will be able to expand the reach of my Game Music site when I can drop the odd Native Client plugin. This JS-based player works great on Chrome, Firefox, and Safari across desktop operating systems.

    But this endeavor was not without its challenges.

    Programmatically Generating Audio
    First, I needed to figure out the proper method for procedurally generating audio and making it available to output. Generally, there are 2 approaches for audio output:

    1. Sit in a loop and generate audio, writing it out via a blocking audio call
    2. Implement a callback that the audio system can invoke in order to generate more audio when needed

    Option #1 is not a good idea for an event-driven language like JavaScript. So I hunted through the rather flexible Web Audio API for a method that allowed something like approach #2. Callbacks are everywhere, after all.

    I eventually found what I was looking for with the ScriptProcessorNode. It seems to be intended to apply post-processing effects to audio streams. A program registers a callback which is passed configurable chunks of audio for processing. I subverted this by simply overwriting the input buffers with the audio generated by the Emscripten-compiled library.

    The ScriptProcessorNode interface is fairly well documented and works across multiple browsers. However, it is already marked as deprecated:

    Note: As of the August 29 2014 Web Audio API spec publication, this feature has been marked as deprecated, and is soon to be replaced by Audio Workers.

    Despite being marked as deprecated for 8 months as of this writing, there exists no appreciable amount of documentation for the successor API, these so-called Audio Workers.

    Vive la web standards!

    Visualize This
    The next problem was visualization. The Web Audio API provides the AnalyzerNode API for accessing both time and frequency domain data from a running audio stream (and fetching the data as both unsigned bytes or floating-point numbers, depending on what the application needs). This is a pretty neat idea. I just wish I could make the API work. The simple demos I could find worked well enough. But when I wired up a prototype to fetch and visualize the time-domain wave, all I got were center-point samples (an array of values that were all 128).

    Even if the API did work, I’m not sure if it would have been that useful. Per my reading of the AnalyserNode API, it only returns data as a single channel. Why would I want that? My application supports audio with 2 channels. I want 2 channels of data for visualization.

    How To Synchronize
    So I rolled my own visualization solution by maintaining a circular buffer of audio when samples were being generated. Then, requestAnimationFrame() provided the rendering callbacks. The next problem was audio-visual sync. But that certainly is not unique to this situation– maintaining proper A/V sync is a perennial puzzle in real-time multimedia programming. I was able to glean enough timing information from the environment to achieve reasonable A/V sync (verify for yourself).

    Pause/Resume
    The next problem I encountered with the Web Audio API was pause/resume facilities, or the lack thereof. For all its bells and whistles, the API’s omission of such facilities seems most unusual, as if the design philosophy was, “Once the user starts playing audio, they will never, ever have cause to pause the audio.”

    Then again, I must understand that mine is not a use case that the design committee considered and I’m subverting the API in ways the designers didn’t intend. Typical use cases for this API seem to include such workloads as:

    • Downloading, decoding, and playing back a compressed audio stream via the network, applying effects, and visualizing the result
    • Accessing microphone input, applying effects, visualizing, encoding and sending the data across the network
    • Firing sound effects in a gaming application
    • MIDI playback via JavaScript (this honestly amazes me)

    What they did not seem to have in mind was what I am trying to do– synthesize audio in real time.

    I implemented pause/resume in a sub-par manner: pausing has the effect of generating 0 values when the ScriptProcessorNode callback is invoked, while also canceling any animation callbacks. Thus, audio output is technically still occurring, it’s just that the audio is pure silence. It’s not a great solution because CPU is still being used.

    Future Work
    I have a lot more player libraries to port to this new system. But I think I have a good framework set up.

  • Dreamcast Track Sizes

    1er mars 2015, par Multimedia Mike — Sega Dreamcast

    I’ve been playing around with Sega Dreamcast discs lately. Not playing the games on the DC discs, of course, just studying their structure. To review, the Sega Dreamcast game console used special optical discs named GD-ROMs, where the GD stands for “gigadisc”. They are capable of holding about 1 gigabyte of data.

    You know what’s weird about these discs? Each one manages to actually store a gigabyte of data. Each disc has a CD portion and a GD portion. The CD portion occupies the first 45000 sectors and can be read in any standard CD drive. This area is divided between a brief data track and a brief (usually) audio track.

    The GD region starts at sector 45000. Sometimes, it’s just one humongous data track that consumes the entire GD region. More often, however, the data track is split between the first track and the last track in the region and there are 1 or more audio tracks in between. But the weird thing is, the GD region is always full. I made a study of it (click for a larger, interactive graph):


    Dreamcast Track Sizes

    Some discs put special data or audio bonuses in the CD region for players to discover. But every disc manages to fill out the GD region. I checked up on a lot of those audio tracks that divide the GD data and they’re legitimate music tracks. So what’s the motivation? Why would the data track be split in 2 pieces like that?

    I eventually realized that I probably answered this question in this blog post from 4 years ago. The read speed from the outside of an optical disc is higher than the inside of the same disc. When I inspect the outer data tracks of some of these discs, sure enough, there seem to be timing-sensitive multimedia FMV files living on the outer stretches.

    One day, I’ll write a utility to take apart the split ISO-9660 filesystem offset from a weird sector.

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