How to resample photos

Whenever I get photos from others after a trip, the photos tend to be huge.  Who the heck needs 20 megapixels anyway?  Four is plenty for me so the first thing I want to do to make the photos easier to deal with is resample them.  I use ImageMagick’s convert for this.  convert wants a percentage reduction. Here’s how you can calculate what percentage to use:

pc = ((desiredMP / ((originalpixelcountinx * originalpixelcountiny) / 1000000.)) ** 0.5) * 100

Of course, it’s helpful if all the photos that you have are the same size.  I use jhead to see the original size of the photo, but since we’re already talking ImageMagick here, identify works as well.  So, if your original photo size was 5312×2988 and you want four megapixel output, then you calculate the percentage for convert like this:

>>> ((4 / ((5312 * 2988) / 1000000.)) ** 0.5) * 100

50% for these photos.  Then you can run a little bash script to do the conversion:

for img in `ls *.JPG`
  convert -sample 50% $img small/$img
  echo "$img converted"

The reduced photos are written into a directory called “small” that must already exist.  Now you can delete those huge photos since we’re not making billboards. 🙂

Update September 18, 2018:

I quickly wrote this code to deal with a directory of photos of varying sizes. The code still needs some work.

import os
from PIL import Image

def shellquote(s):
  return "'" + s.replace("'", "'\\''") + "'"

sInDir = '.'
sFileType = 'jpg'
iTargetMP = 4
sOutDir = 'small'

lName = os.listdir(sInDir)
for sName in lName:
  if (sName[-3:].lower() == sFileType):
    mImage =
    iWidth, iHeight = mImage.size
    fPC = ((iTargetMP / ((iWidth * iHeight) / 1000000.)) ** 0.5) * 100
    iPC = int(fPC)

    #fix the filenames
    sInName = shellquote(sName)
    sOutName = shellquote('%s/%s' % (sOutDir, sName))

    if (iPC <= 99):
      sLine = 'convert -sample %i%% ' % iPC
      sLine = 'cp '
    sLine += '%s %s' % (sInName, sOutName)
    print sLine

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