Simple Python struct issue
I saw an article on O'Reilly about using NumPy and Dislin to analyze
and visualize WAV files. It's a really fantastic article but was a
little out of date. I updated the script to work with the newer
modules &etc but am still having trouble getting it working.
The line
temp[i,:] = array(struct.unpack("%dB"%(fft_length), tempb),Float) -
128.0
always returns the same error: "Traceback (most recent call last):
File "pysono.py", line 31, in
temp[i,:] = array(struct.unpack("%dB"%(fft_length),tempb),float) -
128.0
struct.error: unpack requires a string argument of length 256" when I
do python pysono.py test.wav 256
I'm sure it's probably something simple but I just can't see what it
is!
Here's the original code: http://onlamp.com/python/2001/01/31/graphics/pysono.py
Thanks!
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Re: Simple Python struct issue
On Oct 2, 12:15 pm, Simon Forman wrote:
> On Fri, Oct 2, 2009 at 12:07 PM, Carlo DiCelico
>
>
>
>
>
> wrote:
> > I saw an article on O'Reilly about using NumPy and Dislin to analyze
> > and visualize WAV files. It's a really fantastic article but was a
> > little out of date. I updated the script to work with the newer
> > modules &etc but am still having trouble getting it working.
>
> > The line
>
> > temp[i,:] = array(struct.unpack("%dB"%(fft_length), tempb),Float) -
> > 128.0
>
> > always returns the same error: "Traceback (most recent call last):
> > File "pysono.py", line 31, in
> > temp[i,:] = array(struct.unpack("%dB"%(fft_length),tempb),float) -
> > 128.0
> > struct.error: unpack requires a string argument of length 256" when I
> > do python pysono.py test.wav 256
>
> > I'm sure it's probably something simple but I just can't see what it
> > is!
>
> > Here's the original
> > code:http://onlamp.com/python/2001/01/31/graphics/pysono.py
>
> > Thanks!
>
> In:
>
> struct.unpack("%dB" % (fft_length), tempb)
>
> tempb is not length 256.
>
> Also, note that (foo) is the same as just foo. To create a tuple of
> length 1 you must say (foo,)
>
> HTH,
> ~Simon
I'm sorry, I'm not sure what you're referring to when you say "Also,
note that (foo) is the same as just foo. To create a tuple of length
1 you must say (foo,)". The 256 is passed into the script as an
argument and then assigned to the variable fft_length, which you can
see in that line. So, whatever value I pass in comes out in the error.
What you're saying is that the error happens because tempb isn't the
length of any of those values (i.e., 128, 256, 512, etc)? (1) How
could I determine the length of tempb? and (2) It's doing this ->
tempb = fp.readframes(fft_length); right before doing the struct.unpack
(); could the splitting of the frames into frames of length fft_length
be causing this error?
Thanks for the help!
Carlo
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Re: Simple Python struct issue
On Oct 2, 3:17 pm, Simon Forman wrote:
> On Fri, Oct 2, 2009 at 12:35 PM, Carlo DiCelico
>
>
>
>
>
> wrote:
> > On Oct 2, 12:15 pm, Simon Forman wrote:
> >> On Fri, Oct 2, 2009 at 12:07 PM, Carlo DiCelico
> >> wrote:
> >> > I saw an article on O'Reilly about using NumPy and Dislin to analyze
> >> > and visualize WAV files. It's a really fantastic article but was a
> >> > little out of date. I updated the script to work with the newer
> >> > modules &etc but am still having trouble getting it working.
>
> >> > The line
>
> >> > temp[i,:] = array(struct.unpack("%dB"%(fft_length), tempb),Float) -
> >> > 128.0
>
> >> > always returns the same error: "Traceback (most recent call last):
> >> > File "pysono.py", line 31, in
> >> > temp[i,:] = array(struct.unpack("%dB"%(fft_length),tempb),float) -
> >> > 128.0
> >> > struct.error: unpack requires a string argument of length 256" when I
> >> > do python pysono.py test.wav 256
>
> >> > I'm sure it's probably something simple but I just can't see what it
> >> > is!
>
> >> > Here's the original
> >> > code:http://onlamp.com/python/2001/01/31/graphics/pysono.py
>
> >> > Thanks!
>
> >> In:
>
> >> struct.unpack("%dB" % (fft_length), tempb)
>
> >> tempb is not length 256.
>
> >> Also, note that (foo) is the same as just foo. To create a tuple of
> >> length 1 you must say (foo,)
>
> >> HTH,
> >> ~Simon
>
> > I'm sorry, I'm not sure what you're referring to when you say "Also,
> > note that (foo) is the same as just foo. To create a tuple of length
> > 1 you must say (foo,)".
>
> Sorry about that. All I mean is that in python putting parentheses
> around an expression "(some_object)" does not create a tuple unless
> you also use a comma ","
>
> I hope the following examples show what I mean:
>
> In [1]: (2)
> Out[1]: 2
>
> In [2]: (2,)
> Out[2]: (2,)
>
> In [3]: 2,
> Out[3]: (2,)
>
> In [4]: n = 2,
>
> In [5]: "%s" % (n,)
> Out[5]: '(2,)'
>
> In [6]: "%s" % (n)
> Out[6]: '2'
>
> In [7]: "%s" % n
> Out[7]: '2'
>
> Your code:
>
> "%dB" % (fft_length)
>
> is the same as
>
> "%dB" % fft_length
>
> But what you probably meant was
>
> "%dB" % (fft_length,)
>
> Note the comma. People do this to prevent string formatting errors if
> fft_length should ever happen to be a tuple rather than an int (or
> whatever.)
>
> > The 256 is passed into the script as an
> > argument and then assigned to the variable fft_length, which you can
> > see in that line. So, whatever value I pass in comes out in the error.
> > What you're saying is that the error happens because tempb isn't the
> > length of any of those values (i.e., 128, 256, 512, etc)?
>
> Yes, whatever tempb is, it isn't "a string argument of length 256" as
> the traceback says.
>
> > (1) How
> > could I determine the length of tempb?
>
> If it's a string, len(tempb) will return it's length.
>
> > and (2) It's doing this ->
> > tempb = fp.readframes(fft_length); right before doing the struct.unpack
> > (); could the splitting of the frames into frames of length fft_length
> > be causing this error?
>
> See MRAB's reply. :]
>
> > Thanks for the help!
>
> You're very welcome.
>
> One other thing, in the code you posted it has "Float", but in the
> traceback it has "float". Generally speaking when you post code to a
> newsgroup for help you should paste in the exact code, rather than
> retyping it manually.
>
> Warm regards, and happy hacking!
Ah, yes, makes so much more sense now. Thanks very much for the help!
The discrepancy between Float and float has to do with the fact that
in the code I linked to, he's using an old version of numpy. In my
code, I'm using the new version of numpy, which uses float instead of
Float. Sorry about that, thanks again.
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Re: Simple Python struct issue
On Oct 2, 12:49 pm, MRAB wrote:
> Carlo DiCelico wrote:
> > I saw an article on O'Reilly about using NumPy and Dislin to analyze
> > and visualize WAV files. It's a really fantastic article but was a
> > little out of date. I updated the script to work with the newer
> > modules &etc but am still having trouble getting it working.
>
> > The line
>
> > temp[i,:] = array(struct.unpack("%dB"%(fft_length), tempb),Float) -
> > 128.0
>
> > always returns the same error: "Traceback (most recent call last):
> > File "pysono.py", line 31, in
> > temp[i,:] = array(struct.unpack("%dB"%(fft_length),tempb),float) -
> > 128.0
> > struct.error: unpack requires a string argument of length 256" when I
> > do python pysono.py test.wav 256
>
> > I'm sure it's probably something simple but I just can't see what it
> > is!
>
> > Here's the original
> > code:http://onlamp.com/python/2001/01/31/graphics/pysono.py
>
> .readframes(n) returns (at most) n _frames_ as a bytestring (str), not n
> bytes. I tried reading 256 frames from a .wav file containing stereo at
> 16 bits per channel and got 1024 bytes (4 bytes per frame, not
> surprisingly!).
Yes! Thank you very much for pointing this out, don't know how I
missed it!
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Image to SVG conversion with Python
I need to convert JPEG and PNG files to SVG. I'm currently using PIL to generate the JPEG/PNG files to begin with. However, I need to be able to scale the generated images up in size without a loss of image quality. Using SVG seems to be the best way to do this, other than generating the images in the maximum size/resolution in the first place, which would be too resource intensive. I've looked around online and have found some tools for creating SVGs but none for converting an image into SVG. Does anyone have any experience doing this? What would be the best way to go about this? Thanks, Carlo -- http://mail.python.org/mailman/listinfo/python-list
Re: Image to SVG conversion with Python
On Nov 16, 11:48 am, Dave Angel wrote: > Carlo DiCelico wrote: > > I need to convert JPEG and PNG files to SVG. I'm currently using PIL > > to generate the JPEG/PNG files to begin with. However, I need to be > > able to scale the generated images up in size without a loss of image > > quality. Using SVG seems to be the best way to do this, other than > > generating the images in the maximum size/resolution in the first > > place, which would be too resource intensive. > > > I've looked around online and have found some tools for creating SVGs > > but none for converting an image into SVG. > > > Does anyone have any experience doing this? What would be the best way > > to go about this? > > > Thanks, > > Carlo > > I have no direct experience with SVG, but have used and analyzed other > formats. > > I expect it's unreasonable to convert jpg or png files to SVG. The > latter is a vector format, and can't efficiently represent pixels, which > is all you have in the jpg files. And even if you did it brute force, > it still wouldn't scale any better than the original jpg. If the jpg > file was generated from lines, and wasn't too crowded, it *might* be > possible to analyze it to reconstruct the vectors, but it would be both > very slow, and inaccurate. > > In Photoshop PSD files, you can have vector layers and RGB layers (plus > other kinds). You convert a vector layer (such as text) to bits by > rasterizing (or flattening). And once you do, it no longer scales > cleanly. For instance, when I'm sending composite images to a printer, > I get much better quality sending the raster portion separate from the > text, either as layers in a PSD file, or in a PDF file, or even as two > separate files that they will merge later. (In that case, I usually > send a low-res flattened file, so they can see how it's supposed to look) > > I'd say you should change your python code to generate the svg files > first (perhaps usinghttp://code.activestate.com/recipes/325823/) > > Then you might want to use (http://www.imagemagick.org/script/index.php > ) to convert it to jpg or other format. > > DaveA Thanks, this makes perfect sense. -- http://mail.python.org/mailman/listinfo/python-list
