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Old 02-26-2016, 05:07 AM   #1
gianni_1
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images format changing and resizing

Hello to all,
I'ld like to know if there is a way to convert images in Calibre e-books from png to jpg and to downsize them to 72 dpi.
Thanks.
Gianni
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Old 02-28-2016, 11:11 PM   #2
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I don't know why you would want to use the lossy JPG format (and do a conversion in the process), but sure -- you can open the book in the E-Book Editor (shortcut key is "T").

Then export the PNG images, use your favorite image editing software to resize the image and save as JPG, and import the new image into the ebook.
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Old 02-29-2016, 02:11 PM   #3
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Internal plugin Unpack book is maybe a solution with fewer steps.

Edit:
Ups. Just recognize I did'nt read careful. You want to change file format too. In this case eschwartz has the better solution.

Last edited by Divingduck; 02-29-2016 at 02:16 PM.
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Old 02-29-2016, 02:13 PM   #4
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Internal plugin Unpack book is maybe a solution with fewer steps
Overall that has more steps. You then have to delete the images from the eBook, edit the OPF to fix the filenames/filetype and also fix all the extensions in all the XHTML files that reference these images.

This works so much easier using the eBook editor.
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Old 02-29-2016, 02:15 PM   #5
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The E-book editor allows resizing Images.
Click the image (file) , now you get image tools in that window
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Old 02-29-2016, 02:19 PM   #6
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@JSWolf, you are right. Had recognize it in the moment I send my answer
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Old 02-29-2016, 06:58 PM   #7
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Quote:
Originally Posted by theducks View Post
The E-book editor allows resizing Images.
Click the image (file) , now you get image tools in that window
That tool doesn't have conversion or re-sampling.

I think they're what the OP effectively asked for. And there's no Open with... option to use an external program - as there is for the cover in the library manager. Or if there is I can't find it

BR
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Old 02-29-2016, 07:22 PM   #8
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calibre does not have an "Open with external program" option.
It does have "Export file" and "Replace ____ with file".

You do not want to update a JPG with the contents of a PNG anyway, that is a recipe for disaster...

This is exactly why I specified in my original reply:
  1. Open ebook in calibre's Editor
  2. Export image.
  3. Do xxx to image.
  4. Import new image into book.
  5. Update references of the old image, to refer to the new image instead. (This goes without saying... right???)

You need the Editor, because it can handle the work of importing and deleting new EPUB resource files, and applying changes to all the text files that might invoke that image.
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Old 02-29-2016, 09:04 PM   #9
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Sigil has an Open with on its Images right click menu - seems to work. Not if you save what was a PNG to a TGA of course, nor if you change the file name from say fred.tif to fred.tiff

But resizing, resampling, scribbling on, blurring etc, etc, are OK. I suppose if one resized an image and text is wrapped around the image then I one might have some work to do.

BR
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Old 02-29-2016, 09:50 PM   #10
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I am sure the Open With option is useful in general -- just not in this case, since we already know the OP wants to convert PNG ==> JPG instead of limiting himself to resizing, resampling, scribbling on, blurring etc, etc.

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Old 05-24-2016, 01:45 PM   #11
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Quote:
Originally Posted by eschwartz View Post
I don't know why you would want to use the lossy JPG format (and do a conversion in the process),
Replying to an older post but I have a rationale for wanting to change format and re-size.

Many eBook novels are effectively 100kb+ of compressed text attached to a 3mb+ of image(s). The images, in all their glory, are often viewed for seconds or in thumbnail while the text is pored over for hours. A lossy reasonably sized jpg can give an excellent image presentation on the screens they are viewed on, whereas large lossless print-quality 24bit PNGs are something I consider overkill in many casual read/paperback-like cases, though desirable for archives and art books.

Would be nice if Calibre's Polish, or a Plugin, or the Editor could batch resize and change format to a "paperback" spec, reliably without artifacts (bicubic, etc...). Throw in a "combine identical images" function and a lot of epub novels could go from ~3mb to ~300k. A 90% filesize reduction that doesn't noticeably affect readability is nothing to sneeze at.

RIOT Radical Image Optimization Tool, used judiciously, is a nice in-place batch tool for Unpacked images. Format changes in the epub still have to be managed individually in Edit eBook though. Take care on black-and-white gifs/pngs, it can try to optimize them all the way down to 2-color dithered. Major size changes can produce ugly compression artifacts as well.

Photoshop's "Image Processor" script is still the consistently cleanest batch resize/resample that I've seen.
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Old 09-07-2018, 10:14 AM   #12
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Hello,
I found a great program to reduce the weight of images without loosing quality, you know, epub are read on a display so, 72 dpi are enough for them.
The program is "FILEminimizer Pictures", you must to go in Settings (right lower corner), check the box "custom compression" and, in the next window, you can choose the resolution and the quality you want.
of course there is always the problem that you have to extract all the images you want to modify, until someone integrates this or another similar program into Calibre to make it more effective and easier to use.
Bye.
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Old 09-07-2018, 05:56 PM   #13
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@gianni_fed - FWIW: the Sigil EPUB editor has an option for opening/editing/saving images within an EPUB using a user specified editor, this eliminates the need to extract and replace. I use it with Irfanview to compress images in much the same way.

BR

Last edited by BetterRed; 09-07-2018 at 05:59 PM.
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Old 09-07-2018, 06:50 PM   #14
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Quote:
Originally Posted by gianni_fed View Post
Hello,
I found a great program to reduce the weight of images without loosing quality, you know, epub are read on a display so, 72 dpi are enough for them.
The program is "FILEminimizer Pictures", you must to go in Settings (right lower corner), check the box "custom compression" and, in the next window, you can choose the resolution and the quality you want.
of course there is always the problem that you have to extract all the images you want to modify, until someone integrates this or another similar program into Calibre to make it more effective and easier to use.
Calibre already has a utility for lossless compression of images.

You can batch run for multiple books using the built-in Polish plugin or for a single book inside the calibre Editor from an option in its Tools menu.
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Old 09-07-2018, 08:35 PM   #15
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And you can use the "Unpack book" function to unpack to a directory. Then you can do whatever is needed to the files and rebuild the book.
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