[Numpy-discussion] is there a faster way to get a buffer interface than ndarray.tostring()?
Chris Colbert
sccolbert at gmail.com
Wed Feb 25 01:50:18 EST 2009
thanks!
On Wed, Feb 25, 2009 at 1:22 AM, Andrew Straw <strawman at astraw.com> wrote:
> Given what you're doing, may I also suggest having a look at
> http://code.astraw.com/projects/motmot/wxglvideo.html
>
> -Andrew
>
> Chris Colbert wrote:
> > As an update for any future googlers:
> >
> > the problem was with revpixels = pixeldata[::-1,:,;:-1] which apparently
> > returns an array that is discontinuous in memory. What Lisandro
> > suggested worked. pixeldata[::-1,:,;:-1].copy() returns a continuous
> > array object which natively implements a single-segment buffer
> > interface. i.e. no buffer(revpixels) is needed; just simply revpixels.
> >
> > array.copy() is also 50% faster than array.tostring() on my machine.
> >
> > Chris
> >
> > On Tue, Feb 24, 2009 at 9:27 PM, Chris Colbert <sccolbert at gmail.com
> > <mailto:sccolbert at gmail.com>> wrote:
> >
> > 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 at gmail.com
> > <mailto:dalcinl at 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 at gmail.com <mailto:sccolbert at 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 at scipy.org <mailto:Numpy-discussion at 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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> >
> >
> >
> >
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