You are using a legacy command line system that waw replaced with the introduction of ImageMagcik version 6.
See IM Examples Basics for details
http://www.cit.gu.edu.au/~anthony/graph ... k6/basics/
In summery... command line usage in versions before version 6 was ill-defined and broken, producing numerous odd and unexpected results.
You need to update you script to do...
convert in.jpg -resize 30% out.jpg
The -antialias is the default and does not effect the -resize operator. The -size is specific to JPEG format reading and can not be a percentage (it is probably the cause of your problem).
If you are trying to thumbnail to a specific known size then you can speed up JPEG reading
by adding a -size otpion BEFORE the JPEG filename.
I also recomend you use -thumbnail rather than -resize and it will also include a -strip to remove profiles, and do some scaling of very large images before using the -resize function to finish the job. This can improve performance.
For all the details on thumbnailing image using IM see IM Examples on Thumbnailsing.
http://www.cit.gu.edu.au/~anthony/graph ... humbnails/
If your thumbnail script has something to offer other IM users, please contribute to IM examples.
Small images on resize
- anthony
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Anthony Thyssen -- Webmaster for ImageMagick Example Pages
https://imagemagick.org/Usage/
https://imagemagick.org/Usage/
- anthony
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I have a tendancy to just waffle on...
Please miss, can I leave, my brain is full!
Please miss, can I leave, my brain is full!
Anthony Thyssen -- Webmaster for ImageMagick Example Pages
https://imagemagick.org/Usage/
https://imagemagick.org/Usage/
- anthony
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- Authentication code: 8675308
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Those links are also my waffle 
however I do like to add other peoples waffle to my waffle. Many additions and updates are from these forums.
yadda yadda yadda

however I do like to add other peoples waffle to my waffle. Many additions and updates are from these forums.
yadda yadda yadda

Anthony Thyssen -- Webmaster for ImageMagick Example Pages
https://imagemagick.org/Usage/
https://imagemagick.org/Usage/