PNG to JPG

[ableal]

Sometimes ebooks will be embiggened with large PNG images of photos which should have been much smaller JPGs

The Calibre editor built-in "compress image" tool does not offer that conversion, and it seems to me, after some search, that no plug-in does it either.

I would not mind at all being corrected on this

[Doitsu]

Originally Posted by ableal

Sometimes ebooks will be embiggened with large PNG images of photos which should have been much smaller JPGs

If you're familiar with command line tools, you could do a lossy png optimization with pngquant.
Shameless plug: I created a Sigil plugin for lossy (and lossless) .png optimization that uses pngquant for lossy optimization.

[compurandom]

Originally Posted by ableal

Sometimes ebooks will be embiggened with large PNG images of photos which should have been much smaller JPGs

It's not PNG vs JPG that makes it big, it is quality and resolution.

JPG is generally lower quality. A PNG of the same quality as the JPG likely would be smaller than the JPG.

[retiredbiker]

I often see this. Given the quality of images on an e-ink reader, I don't care much about absolute image quality. On the Kindle or Kobo I usually can't tell the difference after I seriously change and compress images. I use ImagMagick from the command line:

1. Open the book in the editor, export all the png images to a working directory.
2. In that directory, run mogrify -format jpg *.png from a terminal, of course ImageMagick has to be installed.
3. Back in the editor, delete all the png files and import all the jpg files you made
4. In search & replace, look for .png and replace with .jpg in all text files
5. Run compress images lossily at a factor of 50.

That actually takes only a minute or two after you have done it a couple of times. When you are dealing with 40 or 50 or more images, it saves hours compared to one-by-one processing.

[ownedbycats]

Originally Posted by compurandom

It's not PNG vs JPG that makes it big, it is quality and resolution.

JPG is generally lower quality. A PNG of the same quality as the JPG likely would be smaller than the JPG.

JPG also doesn't support transparency if that's important to you. (JPEG2000 does -- shame it isn't more widely used.)

[Quoth]

Most ereaders don't support PNG Transparency and dark mode can be a problem.

Line art, graphic presentations & some non-photographic images can look worse in JPG or even in extreme cases be larger. But natural images can look fine in jpeg with a big file space saving.

Transparency is for fancy websites, or multimedia or apps. It should usually be replaced by white for images to be used on paper or in real eBooks.

[ableal]

Originally Posted by retiredbiker

I often see this. Given the quality of images on an e-ink reader, I don't care much about absolute image quality. On the Kindle or Kobo I usually can't tell the difference after I seriously change and compress images. I use ImagMagick from the command line:

1. Open the book in the editor, export all the png images to a working directory.
2. In that directory, run mogrify -format jpg *.png from a terminal, of course ImageMagick has to be installed.
3. Back in the editor, delete all the png files and import all the jpg files you made
4. In search & replace, look for .png and replace with .jpg in all text files
5. Run compress images lossily at a factor of 50.

That actually takes only a minute or two after you have done it a couple of times. When you are dealing with 40 or 50 or more images, it saves hours compared to one-by-one processing.

Thanks, yeah, I'm with with you on all points, though I'm squeamish and usually stick to 85% lossy. This should work for me, no problem with CLI tools, started there donkey's years ago. Got WSL on Windows for some occasional Unix mojo.

Step 3 is the key, hadn't thought of that. Replacing manually each one of a few dozen images in a particular large book tripped my lazyness threshold

Many thanks.

[ableal]

Thanks also to those who mentioned the quality and transparency issues, but that's not at play in the case that made me post.

It's particularly egregious, there's a bunch of photos as pngs, and to add insult to injury, they are shown on page smaller than original size ...

[Quoth]

Originally Posted by ableal

Thanks also to those who mentioned the quality and transparency issues, but that's not at play in the case that made me post.

It's particularly egregious, there's a bunch of photos as pngs, and to add insult to injury, they are shown on page smaller than original size ...

Set a % width if a landscape image. Not sure about portrait. I set 90% wide & keep aspect in the Wordprocessor before export to docx and conversion to epub2. I make mobi, dual mobi or azw3 from the epub.

[ableal]

Just to confirm that retiredbiker's kindly provided recipe works flawlessly.

I managed to create a snag - the png photos were dumped in a folder three deep (hmtl?/?/images), and I imported the jpegs back directly, instead of in a folder. Being lazy, I just fixed the paths by replacing prefix strings, after doing the same for the file extensions.

[Liudprand]

Originally Posted by retiredbiker

I often see this. Given the quality of images on an e-ink reader, I don't care much about absolute image quality. On the Kindle or Kobo I usually can't tell the difference after I seriously change and compress images. I use ImagMagick from the command line:

1. Open the book in the editor, export all the png images to a working directory.
2. In that directory, run mogrify -format jpg *.png from a terminal, of course ImageMagick has to be installed.
3. Back in the editor, delete all the png files and import all the jpg files you made
4. In search & replace, look for .png and replace with .jpg in all text files
5. Run compress images lossily at a factor of 50.

That actually takes only a minute or two after you have done it a couple of times. When you are dealing with 40 or 50 or more images, it saves hours compared to one-by-one processing.

I know this is from a while ago, but I'm hoping it'll be useful for me, but ran into a problem. I'm not too familiar with command-line processes, but when I tried to do what I think you're suggesting in #2, I just get the response:

"'mogrify' is not recognized as an internal or external command,
operable program or batch file."

I've got ImageMagick installed, but obviously there's something I'm missing... Any tips gratefully received.

[retiredbiker]

"'mogrify' is not recognized as an internal or external command,
operable program or batch file."
I've got ImageMagick installed, but obviously there's something I'm missing... Any tips gratefully received.

This will be a problem with the ImageMagick installation - not in your path or something. If you are on Ubuntu/Linux try the command
:~$ which mogrify
and see if you get
/usr/bin/mogrify

I'm no expert but ImageMagick is a complex beastie and as I recall I had to mess with it for a while to get it working a few years ago, but I can't recall what it needed. The OS forums are your friends here!

[Liudprand]

Actually I worked out the problem from a youtube video, which told me to use:

"magick mogrify... [etc.]"

rather than starting with "mogrify". That worked.

Trouble is, I have another problem now. I've created a new thread about it, here: https://www.mobileread.com/forums/sh...62#post4263062

I'd certainly be grateful if you could offer any further advice...