ImageJ - compare original and resized image on similarity - image

Question

Dear Colleagues.
Could you help me please with next question.
I want to resize huge amount of images and substitute original images with resized for saving disk space. But before substitution I want to be sure, that resized image is the same original image but with different dimensions (not white list, not Malevich's square and so on).
Is there a way to check such similarity to be sure that resizing was successful?
Thanks.

Answer 1

One idea might be to downscale your image into a tentative down-resed version, then scale it back up to the original size and compare that with the original. If they seem pretty similar, overwrite the original with the tentative conversion, if not, report error.
Here's how you might do that in bash with comments. It can be rehashed into other languages of course, or you can use a system() to shell out and use this command line version from another language.
#!/bin/bash
# Downscale an image and check if correct
# Supply image name as parameter
original="$1"
tentative="t-$$-$original"
echo DEBUG: tentative filename=$tentative
# Get size of original so we can resize back up to that size
origsize=$(identify -format "%G" "$original")
echo DEBUG: origsize=$origsize
# Do downsizing of image, saving result tentatively
convert image.jpg -resize 800x800 "$tentative"
# Test quality/success of conversion by looking at PSNR
PSNR=$(convert "$tentative" -resize $origsize\! "$original" -metric PSNR -format "%[distortion]" -compare info:)
echo DEBUG: PSNR=$PSNR
# PSNR above 20 is pretty indicative of good similarity - use "bc" as shell doesn't do floats
if [ $(echo "$PSNR>20" | bc) -eq 1 ]; then
echo $original looks good
else
echo $original something wrong
fi
One thing to beware of is transparency - don't convert from a GIF or PNG (both of which support transparency) to a JPEG (which doesn't) and then resize and compare - you are asking for trouble. You will see in my script above that I retain the image extension and pre-pend bits to the front rather than the end of the filename.

Answer 2

well as far as the size is concerned you can simply check the dimensions
in matlab
[x,y]=size(im);
this will let you know that the if the image has been resized
To check if the image is the same you can use feature extraction such as SURF features.
Extract the surf features, match them if you get 100% ~ 90% match you have the same image!!