PNG writer stores creation date unconditionally?

[wkoszek]

Hello,

I attempted to create myself some test data as a part of my testdata GitHub repo:

https://github.com/wkoszek/testdata/blob/master/gen.rb

I see Image Magick's PNG writer behaving in an unexpected way, which doesn't match JPG writer. Issue is related to conditions in which ImageMagick will embed a timestamp in the image.

Replicated the case through this snippet:

https://gist.github.com/wkoszek/376b93a29ee4cd9c91d6

Explanation:
- made date1.png and date2.png at different times. I see MD5 differ, meaning creation date is embedded in the image.
- made date1.jpg and date2.jpg at different times. I see MD5 be the same, meaning creation date isn't embedded in the image.
- made nodate1.png and nodate2.png at different times, but with an explicit request to not put the date in the PNG. I see MD5 is still different.

I use `png:exclude-chunk=time` since that's what I found on this forum. So far I'm not having luck with it.

My target is to make my script generate the file that is identical each time it's run, regardless of time. Shouldn't such behavior be the default?

Thanks,

Wojciech Koszek
wojciech@koszek.com

[fmw42]

I am not a PNG expert, but see +set date at http://www.imagemagick.org/Usage/formats/#png_write

[glennrp]

I use `png:exclude-chunk=time` since that's what I found on this forum. So far I'm not having luck with it.

Try `-define png:exclude-chunk=date'

[wkoszek]

It's either a bug, or I'm doing something very wrong. I tried -png:exclude-chunk=date, and +set and nothing seems to work. My updated script and the output logs:

https://gist.github.com/wkoszek/fa13a4dc0d1237554f62

[fmw42]

-png:exclude-chunk=date

try -define png:exclude-chunk=date, if glennrp is correct, not -png:...

[glennrp]

-png:exclude-chunk=date

try -define png:exclude-chunk=date, if glennrp is correct, not -png:...

The log says wkoszek did that. But it was followed with

Code: Select all

-define png:exclude-chunks=time

which replaced the list of chunks to be excluded (date) with a new list (time). To exclude
multiple chunks, use

Code: Select all

-define png:exclude-chunks=date,time

instead of multiple "-define png:exclude-chunks=something" options. I'll look into improving the documentation (or the behavior) of that feature.

[fmw42]

Glenn,

Thanks for clarifying that.

[wkoszek]

Hi guys

https://gist.github.com/wkoszek/fa13a4dc0d1237554f62

I tried all the versions of flags/switches from this block:

Code: Select all

#f="+set date:create +set date:modify"
#f="-define png:exclude-chunks=time"
#f="-define png:exclude-chunks=tIME"
#f="-define png:exclude-chunks=date,time"
f="-define png:exclude-chunks=date -define png:exclude-chunks=time" 

e.g.: I tried line 4 by itself too (with line 5 commented out). It doesn't work.

Wojciech

[glennrp]

Put the "$f" last (just before the output filename).

[wkoszek]

Result is the same. Seems like it doesn't work either that way. What I've run:

http://pastebin.com/eAtWmQ0S

[glennrp]

I'm testing a fix. The bug is in coders/png.c; we test for "Date:" instead of "date:".

[wkoszek]

Is there a workaround for this? Something I could use with ImageMagick shipped with "brew" or "apt-get" ?

Thanks for confirming,
W.

[glennrp]

I'd use the "signature" instead of MD5 to identify identical images.

identify -verbose file1.png | grep ignature
identify -verbose file2.png | grep ignature

I omit the first letter of "signature" because I can never remember whether
it is capitalized or not.

[wkoszek]

While I don't know what identify does, I know md5/sha1 utilities do pretty much what git will when I commit a file.

I was asking about a workaround for the PNG generation, since I think brew will catch up from the upstream with slight delay.

[glennrp]

While I don't know what identify does, I know md5/sha1 utilities do pretty much what git will when I commit a file.

The difference is that the "signature" reported by "identify" is calculated over the pixel date, not over the file. Therefore, the presence of "Creation date" metadata doesn't change the signature. In fact, even representing the image in a different format doesn't change the signature:

• glenn.rp> convert logo: logo.gif
  glenn.rp> convert logo: logo.png
  glenn.rp> identify -verbose logo: logo.gif logo.png | grep ignature
  signature: 5c701306a9a985a0c93c8d11a1e761d7f8637577697fc60d7189b221388f8edf
  signature: 5c701306a9a985a0c93c8d11a1e761d7f8637577697fc60d7189b221388f8edf
  signature: 5c701306a9a985a0c93c8d11a1e761d7f8637577697fc60d7189b221388f8edf
  glenn.rp>