how to process image with opencv in python?


I want to use edge detection algorithms from opencv library. Here is a piece of python code:

from import *
from opencv.highgui import *

img = cvLoadImage ('xxx.jpg')
cvNamedWindow ('img')
cvShowImage ('img', img)
cvWaitKey ()

canny = cvCreateImage (cvSize (img.width, img.height), 8, 3)
cvCanny (img, canny, 50, 200)

harris = cvCreateImage (cvSize (img.width, img.height), 8, 3)
cvCornerHarris (img, harris, 5, 5, 0.1)

Loading and showing image works fine, but canny and harris transformations fail.
cvCanny fails with:

RuntimeError:  openCV Error:
    Status=Unsupported format or combination of formats
    function name=cvCanny
    error message=

and cvCornerHarris fails with this error:

RuntimeError:  openCV Error:
    Status=Assertion failed
    function name=cvCornerHarris
    error message=src.size() == dst.size() && dst.type() == CV_32FC1

From this messages I can infer that loaded image has invalid format. But I don't understand how to convert it.
Here are values of some img fields:

img.type = 1111638032
img.nChannels = 3
img.depth = 8
2/15/2011 11:16:05 AM

Accepted Answer

Here's fixed code. See comments inline. Long story short: your data types were wrong. Read the API.

    from import *
    from opencv.highgui import *
    # Different OpenCV installs name their packages differently.
    from cv import *

if __name__ == '__main__':
    import sys
    # 1 = Force the image to be loaded as RGB
    img = LoadImage (sys.argv[1], 1)
    NamedWindow ('img')
    ShowImage ('img', img)
    WaitKey ()

    # Canny and Harris expect grayscale  (8-bit) input.
    # Convert the image to grayscale.  This is a two-step process:
    #   1.  Convert to 3-channel YCbCr image
    #   2.  Throw away the chroma (Cb, Cr) and keep the luma (Y)
    yuv = CreateImage(GetSize(img), 8, 3)
    gray = CreateImage(GetSize(img), 8, 1)
    CvtColor(img, yuv, CV_BGR2YCrCb)
    Split(yuv, gray, None, None, None)

    canny = CreateImage(GetSize(img), 8, 1)
    Canny(gray, canny, 50, 200)
    NamedWindow ('canny')
    ShowImage ('canny', canny)

    # The Harris output must be 32-bit float.
    harris = CreateImage (GetSize(img), IPL_DEPTH_32F, 1)
    CornerHarris(gray, harris, 5, 5, 0.1)
2/16/2011 1:28:49 PM

For other folks interested in the same type of problem I recommend checking out

Here is a bit of code I wrote that does line detection on an image acquired from a webcam. It will even display the image over http. beard detection

import SimpleCV
import time

c = SimpleCV.Camera(1)
js = SimpleCV.JpegStreamer() 

  img = c.getImage()
  img = img.smooth()
  lines = img.findLines(threshold=25,minlinelength=20,maxlinegap=20)
  [line.draw(color=(255,0,0)) for line in lines]
  #find the avg length of the lines
  sum = 0
  for line in lines:
      sum = line.length() + sum
  if sum:
      print sum / len(lines)
      print "No lines found!"

Check out the project I made this for at It will detect how long your neck beard is :)

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