Filesize changes whereas there is no change to the picture

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CheeseDurger
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Joined: Thu Jul 07, 2016 6:09 am

Filesize changes whereas there is no change to the picture

Postby CheeseDurger » Thu Jul 07, 2016 6:31 am

Hi,
I'm trying to reduce image that are > to 1 MP. However, there are already multiple images < 1 MP in the input. And when the batch goes on, those images size stays the same (height and width - so far so good) but the file size changes... So why is it so ?

See, the example below : from 61,1KB to 35,2 KB. I tried the batch on jpg and png. Also, sometimes the filesize increases, and sometimes it decreases.

I searched the forums but couldn't go with anything (well, I confess I'm quite new to all of this).


Thanks.
Attachments
2016-07-07 08_25_46-Action center.png
Action settings
2016-07-07 08_25_46-Action center.png (2.59 KiB) Viewed 841 times
2016-07-07 08_25_16-Action center.png
Output settings
2016-07-07 08_25_16-Action center.png (2.86 KiB) Viewed 841 times
2016-07-07 08_23_26-Action center.png
Input picture
2016-07-07 08_23_26-Action center.png (11.37 KiB) Viewed 841 times
2016-07-07 08_23_00-Settings.png
Output picture
2016-07-07 08_23_00-Settings.png (6.71 KiB) Viewed 841 times

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xnview
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Re: Filesize changes whereas there is no change to the pictu

Postby xnview » Sat Jul 09, 2016 7:49 am

In batch convert file are always resaved, resize action is not always executed...
Pierre.

CheeseDurger
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Joined: Thu Jul 07, 2016 6:09 am

Re: Filesize changes whereas there is no change to the pictu

Postby CheeseDurger » Sat Jul 09, 2016 8:35 am

xnview wrote:In batch convert file are always resaved, resize action is not always executed...


Thanks for your answer. However, why would filesize change (up or down) if resize isn't executed ? Is image quality still the same ?

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xnview
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Re: Filesize changes whereas there is no change to the pictu

Postby xnview » Sat Jul 09, 2016 10:43 am

perhaps the compression mode for png is not the same
Pierre.

CheeseDurger
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Joined: Thu Jul 07, 2016 6:09 am

Re: Filesize changes whereas there is no change to the pictu

Postby CheeseDurger » Sat Jul 09, 2016 5:40 pm

Ok.

I did a little test with the 2 PNGs (the original one and the one that got copied with XnConvert). I converted them to BMP with XnView then I did an hex comparison of the 2 image files. The BMP were exactly the same at the hex level. So I guess you're right and, for PNGs, it's just a matter of compression.

However, what about JPEG ? I did the same test, and the BMPs were different at the hex level.
Have you another wild guess for that ?

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xnview
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Re: Filesize changes whereas there is no change to the pictu

Postby xnview » Tue Jul 12, 2016 11:09 am

CheeseDurger wrote:However, what about JPEG ? I did the same test, and the BMPs were different at the hex level.
Have you another wild guess for that ?

PNG is lossless compression, jpeg is lossy compression
Pierre.


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