102: About My Grayscale Image Blog Page (#34)

The raspistill command for the Pi camera has 100-ish command line options but (oddly) no way to produce a black and white image file. It took me about 10 minutes to come up with a kludge to convert RGB to grayscale. Then I made a blog post about it.

Here's the mystery: With over 100 posts in this blog what is the enduring allure of post #34? Over 6% of my page views still go to that 2 year old entry. And here I thought I'd written about seemingly more important issues.

Anyway, here's a more interesting snippet of Python for a security cam app:

import subprocess

import os

import StringIO

from PIL import Image

threshold = 10 # interesting pixel value diff

numDiffs = 20  # number of diff pixels to assume motion

wide = 120     # size of test images (arbitrary)

high = 90

# Capture a small test image (for motion detection)

def captureImage():

  command = "raspistill -w %s -h %s -t 5 -e bmp -o -" % (wide, high)

  imageData = StringIO.StringIO()

  imageData.write(subprocess.check_output(command, shell=True))

  imageData.seek(0)

  im = Image.open(imageData).convert("L") # convert to B/W!

  buffer = im.load()

  imageData.close()

  return im, buffer

def compareImage(buff1, buff2): # latest vs. previous image

  changedPixels = 0

  for x in xrange(0, wide):

    for y in xrange(0, high):

      pixdiff = abs(buffer1[x,y] - buffer2[x,y])

      if pixdiff > threshold:

        changedPixels += 1

  return (changedPixels > numDiffs) # True return indicates motion

... (code that calls captureImage & compareImage)