Problem with large png image

[Liudprand]

Since Calibre doesn't seem to have the native capacity to compress png files, but just hangs for ages trying to do so but failing, I routinely convert png files to jpg, using ImageMagick - usually (maybe 80% of the time) without a problem, using the following steps:

1. Export png image(s) to a separate folder.
2. Change file extension within ebook for those images to jpg.
3. Delete the images from the ebook.
4. Convert exported png files to jpg.
5. Import new jpg images back into the ebook.

I often have trouble, though, with some png files even after this procedure, which as far as I can work out should do the job. The images concerned don't show up properly in the book - just empty boxes where they should be.

Is there a step I'm missing out?

Thanks in advance!

[kovidgoyal]

1. Convert your PNG to JPEG however you like
2. Right click on the PNG file in the files browser in the editor and choose replace this file with and select your JPEG file

[Liudprand]

Many thanks for this - works like a charm.

Does it mean, though, that there's no way to batch-replace png files with jpg? Do you have to do so one image at a time? Because some of the books I'm dealing with have dozens of png files, sometimes more.

[Liudprand]

(Just signed up as a patron, by the way!)

[kovidgoyal]

1. select all files
2. right click
3. rename file extension
4. save and close
5. In calibre use the unpack book tool (press u key)
6. use whatever tool you like to do whatever you like to the image files in the resulting folder (other than renaming them of course)

[Liudprand]

Great - thanks.

[retiredbiker]

Originally Posted by Liudprand

Many thanks for this - works like a charm.

Does it mean, though, that there's no way to batch-replace png files with jpg? Do you have to do so one image at a time? Because some of the books I'm dealing with have dozens of png files, sometimes more.

Here is my process that has always worked for me, very similar:

Export all the png's to a folder
Convert them with ImageMagick
In the Editor, delete all the png's
Import all the new jpg's
Do search and replace in the Editor across all text files to change .png to .jpg

Then I can use Calibe's lossy compression on them all.

I almost never have a problem with this.