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Re: Issues with ROC's proposal

From: Robert O'Callahan <robert@ocallahan.org>
Date: Wed, 4 Sep 2013 09:54:47 +1200
Message-ID: <CAOp6jLbin+gveywmbjoQ7E0DRdYFcAbiNO0q5GfX5LA3Fu--vA@mail.gmail.com>
To: Marcus Geelnard <mage@opera.com>
Cc: WebAudio <public-audio@w3.org>
On Mon, Sep 2, 2013 at 9:56 PM, Marcus Geelnard <mage@opera.com> wrote:

> *The main things that bother me about the proposal are*:
>
>
> 1) The naming/functionality of getChannelData().
>
> If we disregard the case of the AudioProcessingEvent (see below), the main
> purpose (at least a very important function) of getChannelData() seems to
> be to *set* the channel data.
>
> We now have the copyChannelDataTo() method for getting a persistent
> version of the channel data, while the getChannelData() is used for getting
> a volatile (sometimes persistent!) version of the channel data that may
> later be transfered back to the AudioBuffer.
>
> IMO this is confusing (i.e. "get" ~= set/modify, "copy" == get), to say
> the least.
>

I see your point, but the idea of "getting a buffer" which you can then
read or write seems reasonably logical to me. Maybe we could change the
name to eliminate the illogical interpretation, but then you lose
compatibility. Doesn't seem worth it to me.

2) The getChannelData() method can return a persistent copy.
>
> If you call getChannelData() on an AudioBuffer that is in use, you will
> get a persistent copy of the AudioBuffer data (i.e. modifying the data does
> nothing to the AudioBuffer, and the array will never be neutered), which
> kind of goes against the purpose of getChannelData(). Again, I find this
> quite confusing.
>
> I think that a better solution would be to throw in that case (or possibly
> return an empty array).
>

I just disagree with this point. If you just want to read the buffer, then
having getChannelData always work instead of sometimes work is a clear win.
If you want to write to the buffer, the idea that modifications will affect
all future uses of the buffer and not any uses currently running makes
sense to me.

3) The AudioProcessingEvent still uses AudioBuffers.
>
> I realize that not all agree that this is a problem, but this complicates
> the semantics of the AudioBuffer (among other things). For instance, in an
> AudioProcessingEvent getChannelData() is used both for getting the input
> channel data and for setting the output channel data.
>
> IMO the better solution would be to pass pure Float32Arrays in the event
> handler instead (using neutering and ownership transfer as necessary).
>

Could be done. I doubt it's worth breaking compatibility however.


>  4) Data transfer from JS to an AudioBuffer is implicit.
>
> The data transfer from JS to an AudioBuffer is implicit by design, rather
> than explicit. This is confusing, and could lead to hard-to-find bugs.
>

I'm not sure what you mean here.

 In general it's also sub-optimal from a performance perspective, since
> it's easier to design a performance critical application if you can limit
> possible performance hits to explicit points in your code (e.g. let them
> happen during pre-processing/loading stages rather than during playback
> stages). Now, since the proposal relies heavily on neutering this might not
> be much of an issue, but I still think that it's a good idea to at least
> *consider* an implementation that does "acquire the contents" using a copy
> operation.
>

You mean you'd like "acquire the contents" to be an explicit operation that
could be executed before AudioBufferSourceNode.start(), right?

There are a few ways we can improve the situation here:
0) We could add an explicit API to pre-acquire-the-contents in
AudioBufferSourceNode. I don't like this option since authors would
probably not use it, since it would have practically no effect in most
implementations.
1) I had originally specified that assigning to
AudioBufferSourceNode.buffer would acquire the contents of the buffer. We
could go back to that, and it would help with the above issue since it
could be performed during preprocessing/loading. It may be slightly less
compatible but I suspect we'd be OK.
2) The UA can make AudioContext.decodeBufferData put the new AudioBuffer in
the arrays-neutered state (in the terminology of
https://wiki.mozilla.org/User:Roc/AudioBufferProposal#Implementation_Sketch).
So if it can't share memory, the buffer data will be in the right place as
long as the app doesn't call getChannelData on that AudioBuffer.
3) We could introduce a new constructor for AudioBuffer that takes a
sequence of Blobs/ArrayBuffers for the channels, neutering the ArrayBuffers
to steal their data. This constructor could put the new ArrayBuffer in the
arrays-neutered state similar to point 2.

We should probably do #3 anyway since it's more efficient than existing
APIs if you already have Blobs or ArrayBuffers containing samples (or if
you want to use Blobs to share read-only data between workers or domains).
If we do #3 and the UA does #2 then maybe #1 wouldn't matter.

Rob
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Received on Tuesday, 3 September 2013 21:55:16 UTC

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