I have a number of JPEG pictures which I would like to scale down. Another requirement is that the file size should not be larger than say 300kByte.
Is that possible, please help me with an example =)
2 Answers
To restrict the resulting image to a maximum file size, you can use the commandline parameter -define jpeg:extent=max_value, like this:
convert original.jpeg -define jpeg:extent=300kb output.jpg
convert original.jpeg -define jpeg:extent=300kb -scale 50% output.jpg
convert original.jpeg -define jpeg:extent=300kb [...other options...] output.jpg
Note, this will not always work to match the exact size you wanted. You may have asked for 40kb output size, where input is 300kb, and get a result of 48kb.
(Update/Clarification: Output file size may be a bit lower or higher than your file requested size.)
8 Comments
Mark Setchell
I think it is the other way around. You may get a smaller file than you ask for but not bigger - imagemagick.org/script/command-line-options.php#define
Kurt Pfeifle
@MarkSetchell: My statement about output file size was maybe a bit un-precise. I did not mean to imply it's always larger than requested. It did want to show it's not exactly as requested. Also, ImageMagick sometimes changes behaviour with new releases, and I did not specifically test if this feature changed recently.
越鸟巢南枝
this seems only affect the quality factor, not geometry scaling.
Witek
Sadly this option also makes small files bigger. Is there any way to only affect files which are larger then 300kb?
Kurt Pfeifle
@kojow7: In general, ImageMagick will execute all actions in the same order as are given on the command line. So indeed, your suspicion/suggestion is correct: you will likely get better quality results if you first scale down, then reduce the file size.
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The jpegoptim tool (actual homepage is for multiple programs) works better for me:
jpegoptim -s -S8 *.JPG
-smeans to strip all metadata (EXIF, JFIF, XMP, etc.)-S8means to target a filesize of about 8 KiB
You can crunch them even better by running it twice, because empirically, most images are smaller as progressive:
jpegoptim -s --all-progressive -S8 *.JPG; jpegoptim -s --all-normal -S8 *.JPG
jpegoptim will normally refuse to write an image that increases the size, so this will give you optimum quality/size.
As for the image dimensions part: You normally define a target size in terms of dimensions first and scale there, only then you define the target file size. This is because, when you display it, you ideally want the image dimensions being an integer multiple or fraction of the final display dimensions, to make scaling easier, or even unnecessary. So, scale to the target dimensions first (with quality 100, and possibly other things like 4:4:4 chroma, and most certainly with dct float), then downsize the file. If you can’t get to the file size range you want, choose smaller target dimensions and restart (from the original image, not the one you scaled down first).
4 Comments
brablc
The same problem as
convert -define jpeg:extent=300kb ..., it keeps image dimensions and decreases quality.
Nemo
For JPEG-2000 images you'll get an error. Asked at github.com/tjko/jpegoptim/issues/32
mirabilos
@Nemo the question was about JPEG files, not about JPEG2000 files. Convert those to JPEG first. (JPEG2000 decoders are currently in the process of being removed from a lot of software due to bugs, security holes, and lack of actual JPEG2000 images in the wild.)
Nemo
@mirabilos I know that, otherwise I wouldn't have prefixed my comment with "For JPEG-2000"
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