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JPG cropping and compression MYSTERY~~~ (woooo~)





quex
Frihost, I am desperate. Please help me.

I have some image files. They are scans of letter-size pages. They are JPGs, and they average 950kb in size, each.

In an image editor (Preview, which is original equipment in MAC OSX), I have cropped the edges off of these images. I would estimate the cropping reduces the overall area of the visible image by 20 to 40%, maybe even more in some cases. I have saved the cropped images under the exact same JPG format.... and they seem to have ballooned in file size to 1.8MB each.

What is happening? Why does cropping the images make them take up MORE disk space? Is there a way to solve this? Please help me. ;_;
Ankhanu
It's possible that while you are saving a smaller image size, you're actually saving at a higher quality, lower level of compression (e.g. saving a former 120dpi image at 300dpi). So, while you have a smaller image, and the same amount of actual information, there's less compression.

I just tried cropping a 2.9Mb file in Preview and it saved to 221kb...
I took a look in the preferences and I can't see where you might change the quality of the saved images, though. If you go in to preferences and click on images, do you have it set for 1 image pixel = 1 screen pixel, or size on screen equals size on print?? That *might* have something to do with it (mine is at 1px = 1px)
Radar
While I believe Preview should work, trying another program might save time.

Cropping should definitely make the file size smaller.
ankur209
You can try out a free program for doing this - Photoscape

You can have then a dialog box during the saving of the file asking for the size you want the pic to be.. Wink
quex
Ankhanu wrote:
It's possible that while you are saving a smaller image size, you're actually saving at a higher quality, lower level of compression (e.g. saving a former 120dpi image at 300dpi). So, while you have a smaller image, and the same amount of actual information, there's less compression.

I just tried cropping a 2.9Mb file in Preview and it saved to 221kb...
I took a look in the preferences and I can't see where you might change the quality of the saved images, though. If you go in to preferences and click on images, do you have it set for 1 image pixel = 1 screen pixel, or size on screen equals size on print?? That *might* have something to do with it (mine is at 1px = 1px)


I'm pretty sure you're right; that's the only way I can fathom this would make any sense. Unfortunately, I'm running 10.4 right now, though, and I'm fairly sure my version of Preview doesn't have the option to set the image pixel/screen pixel ratio. At least, not in preferences. I can only change the jpg quality with the standard compression slider in the save as dialog, etc.

I'm just gonna have to use other tools for my cropping activities, I guess. ;_;

But thank you (and all) for the input.
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