Re: How to get (void*) data into a Python array

On 4 Feb, 2011, at 18:16, Trevor Bentley wrote:

> Hello,
> 
> I am working on a small Python script to grab a single frame from the
> iSight camera using PyObjC.  I want to import the frame into a python
> numpy array for manipulation, and I want to write it exclusively in
> Python.  I have fairly limited experience in Python and Objective-C, and
> no prior experience with PyObjC.
> 
> I have made some progress, and currently it is able to find the camera,
> open it, and capture a frame.  The frame data is (theoretically)
> available in my QTCaptureDecompressedVideoOutput delegate's callback
> function:
> 
> - (void)captureOutput:(QTCaptureOutput *)captureOutput
> didOutputVideoFrame:(CVImageBufferRef)videoFrame
> withSampleBuffer:(QTSampleBuffer *)sampleBuffer
> fromConnection:(QTCaptureConnection *)connection
> 
> So I added "pdb.set_trace()" at the beginning of the callback, and am
> trying to figure out how to get the raw bytes available in Python.
> 
> The closest I have come is examining sampleBuffer:
> --------------------------------------------------
> (Pdb) sampleBuffer
> <QTSampleBuffer: 0x1044b42d0>
> (Pdb) sampleBuffer.numberOfSamples()
> 1
> (Pdb) sampleBuffer.lengthForAllSamples()
> 2624000
> (Pdb) sampleBuffer.bytesForAllSamples()
> 4674363392L

That's a bug, PyObjC should be smart enough to know that it supposed to return a buffer (string/bytes/...) in bytesForAllSamples. I will fix this in a future version, but don't know yet when that will be.  Sadly enough I'll have to write some C code for that.

I haven't used ctypes myself beyond simple hello-world level scripts, but it should be possible to tell ctypes that the integer you get back from bytesForSamples is actually a 'void*', and that should give you a way to copy the buffer into a python string.

Ronald

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