[Numpy-discussion] is there a faster way to get a buffer interface than ndarray.tostring()?

Chris Colbert sccolbert@gmail....
Tue Feb 24 20:27:31 CST 2009


thanks for both answers!

Lisandro, you're right, I should have declared the array outside the loop.
Thanks for catching that!

Robert, as always, thanks for the answer. Quick and to the point! You've
helped me more than once on the enthought list :)



On Tue, Feb 24, 2009 at 8:06 PM, Lisandro Dalcin <dalcinl@gmail.com> wrote:

> When you do pixeldata[::-1,:,::-1], you just got a new array with
> different strides, but now non-contiguous... So I believe you really
> need a fresh copy of the data... tostring() copies, but could be
> slow... try to use
>
> revpixels = pixeldata[::-1,:,::-1].copy()
>
> ...
>
> rgbBMP = wx.BitmapFromBuffer(640, 480, buffer(revpixels))
>
>
> Perhaps you could optimize this by declaring revpixels outside de
> loop, and then inside de loop doing
>
> revpixels[...] = pixeldata[::-1,:,::-1]
>
>
> This way you will save the mem allocation of revpixels at each step of the
> loop.
>
>
>
>
>
> On Tue, Feb 24, 2009 at 9:15 PM, Chris Colbert <sccolbert@gmail.com>
> wrote:
> > Hi all,
> >
> >  I'm new to mailing list and relatively new (~1 year) to python/numpy. I
> > would appreciate any insight any of you may have here. The last 8 hours
> of
> > digging through the docs has left me, finally, stuck.
> >
> > I am making a wxPython program that includes webcam functionality. The
> > script below just brings up a simple display window that shows the webcam
> > feed. The problem I am running into is in the WebCamWorker.run() method.
> > This method reads the raw pixel data (in string form) from the webcam
> > buffer. This data (thank you microsoft!) comes in BGR and bottom-to-top.
> I
> > feed this buffer to the array constructor and then swap the pixel order
> to
> > RGB and top-to-bottom. This all happens very fast, ~1ms on a Quad Core,
> and
> > the majority of that time is spent constructing the array (the numpy
> pixel
> > swapping is on the order of E-5s !). The tostring() method, however,
> takes a
> > whopping 10ms to execute. Unfortunately, the wx.BitmapFromBuffer needs a
> > single segment buffer interface as an argument. Is there a way to expose
> my
> > numpy array as a buffer to cut down on this conversion time?
> >
> > I have tried sending the array to the PIL Image.fromarray(), but that
> > eventually requires me to do a Image.tostring() anyway, which negates any
> > benefit.
> >
> > Thanks in advance for the help!
> >
> > S. Chris Colbert
> > Rehabilitation Robotics Laboratory
> > University of South Florida
> >
> >
> > ####  Code #######
> >
> > import VideoCapture
> > import wx
> > import time
> > import threading
> > import numpy as np
> > import Image
> >
> >
> > class WebCamWorker(threading.Thread):
> >     _abort = 0
> >
> >     def __init__(self, parent):
> >         super(WebCamWorker, self).__init__()
> >
> >         self._parent = parent
> >         self.panel = wx.Panel(parent)
> >
> >     def run(self):
> >
> >         while not self._abort:
> >
> >             #numpy arrays reverse pixel data that comes in from CCD as
> BGR
> > and bottom to top
> >
> >             pixeldata = np.ndarray((480,640,3),
> > buffer=webcam.getBuffer()[0], dtype='u1')
> >             revpixels = pixeldata[::-1,:,::-1].tostring()      #tostring
> is
> > an order of magnitude slower than the entire array manipulation. need a
> > faster method.
> >
> >             rgbBMP = wx.BitmapFromBuffer(640, 480, revpixels)
> >             dc = wx.ClientDC(self.panel)
> >             dc.DrawBitmap(rgbBMP, 0, 0)
> >             #b = time.clock()
> >             #print b-a
> >             time.sleep(0.015)
> >
> >
> >         return
> >
> >
> >
> > class TestWxWebCam(wx.Frame):
> >     def __init__(self, parent):
> >         wx.Frame.__init__(self, parent, -1, size=(640,480))
> >
> >         self.worker = WebCamWorker(self)
> >         self.worker.start()
> >
> >
> >
> > class TestApp(wx.App):
> >     def OnInit(self):
> >         self.frame = TestWxWebCam(None)
> >         self.frame.Show()
> >
> >         return True
> >
> >     def OnExit(self):
> >         WebCamWorker._abort = 1 #this is a hack
> >
> >
> > if __name__ == '__main__':
> >     webcam = VideoCapture.Device()
> >     webcam.setResolution(640, 480)
> >     webcam.displayCapturePinProperties()
> >     app = TestApp()
> >     app.MainLoop()
> >     del webcam
> >
> >
> > ### /Code ######
> >
> > _______________________________________________
> > Numpy-discussion mailing list
> > Numpy-discussion@scipy.org
> > http://projects.scipy.org/mailman/listinfo/numpy-discussion
> >
> >
>
>
>
> --
> Lisandro Dalcín
> ---------------
> Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
> Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
> Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
> PTLC - Güemes 3450, (3000) Santa Fe, Argentina
> Tel/Fax: +54-(0)342-451.1594
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