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Re: using BlobBuilder append method with generated binary data

From: Eric Uhrhane <ericu@google.com>
Date: Wed, 20 Oct 2010 08:17:18 -0700
Message-ID: <AANLkTikU_tfQvUgzV7B3MUn5NDMPa59LqGu-WoHBVPhW@mail.gmail.com>
To: Toni Ruottu <toni.ruottu@iki.fi>
Cc: public-webapps@w3.org

    BlobBuilder now has an append() method that takes an ArrayBuffer,
and FileReader has readAsArrayBuffer.  Do these together satisfy your


On Wed, Oct 20, 2010 at 3:39 AM, Toni Ruottu <toni.ruottu@iki.fi> wrote:
> I have discussed the topic before on some Chromium bug threads. I
> searched the archives of this mailing list for blobs and generated
> content, but was not too successful. There may be multiple different
> reasons why one would need to build blobs from generated data. The one
> that concerns me, is being able to do binary POSTs with XmlHTTPRequest
> in a standard manner. To send binary data over XHR one is required to
> send out a blob, but first the blob needs to be constructed with
> BlobBuilder. Let me fill you in on how I see the current situation.
> Maybe someone can spot an error in my line of thought.
> A new (empty) BlobBuilder is created by stating...
> var bb = new BlobBuilder();
> After creating a BlobBuilder, the user appends some data into the blob
> being generated by using the append function. The append function is
> overloaded and can be used to append multiple different types of data
> into the blob. Currently only existing blobs, and utf-8 text are
> supported.
> One can add an existing blob into the new one by doing...
> bb.append(existingBlob);
> One can add a text string in utf-8 format by doing...
> bb.append(aTextString);
> Once the data is in there, it is possible to construct the new blob with...
> var blob = bb.getBlob()
> A blob has a slice method which makes it possible to turn a large blob
> into smaller ones. It is thus possible to cut out single bytes from a
> blob and use them construct new blobs by appending copies of these
> sample bytes together with the BlobBuilder.
> As sample bytes can only be generated by writing text into the blob as
> utf-8, it is not possible to generate all 8-bit patterns. More
> specifically, the bit patterns that are illegal in any given utf-8
> string, are not possible to generate using the current API. This leads
> developers into using various hacks to overcome the issue.
> For example one could ask the user to provide a file with sample
> bytes. Methods for reading a file into a blob are available, so there
> is no need to generate the sample bytes. There is however no way to
> make sure that the user provided a file with correct bit-patterns as
> the blob interface is to a large degree a read-only interface. A
> demonstration of using a file to overcome the issue is available at
> http://www.cs.helsinki.fi/u/twruottu/testi/bpost.html
> To solve the problem a third append function which accepts a list of
> integers, and adds the corresponding bytes into the blob is needed.
> Once such method has been introduced it should be possible to write
> the 7-bit ascii string "foo" into a BlobBuilder by doing...
> bb.append([102,111,111]);
> It should also be possible to add bytes 0x00 and 0xff into the
> BlobBuilder by doing...
> bb.append([0,255]);
> Would such append method be a possible addition as I defined it? Is
> there some alternative way for achieving the same result. Which
> specification should such feature go into?
>  thank you for your time, --Toni Ruottu
Received on Wednesday, 20 October 2010 15:18:04 UTC

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