EPUB3 to EPUB2 Compression issues

[maddz]

I'm finding the compression changing epub3 to epub2 in v7.25 isn't working. I downloaded a 20 MB epub3 book from Kobo and ran it through the converter. No difference in file size - if anything, a little larger. I managed to get it to 16 MB by compressing the illustrations (b/w line art). Then I ran it using 3.48 on an older machine - it shrank to 3.42 MB with (as far as I can see) no loss in image quality.

Both versions are outputting to epub2. What's the difference?

[BetterRed]

Moderator Notice
@maddz

Moved to a new thread, the one you jumped into related to TXT to EPUB conversion.

BR

[JSWolf]

Originally Posted by maddz

I'm finding the compression changing epub3 to epub2 in v7.25 isn't working. I downloaded a 20 MB epub3 book from Kobo and ran it through the converter. No difference in file size - if anything, a little larger. I managed to get it to 16 MB by compressing the illustrations (b/w line art). Then I ran it using 3.48 on an older machine - it shrank to 3.42 MB with (as far as I can see) no loss in image quality.

Both versions are outputting to epub2. What's the difference?

The difference could be the settings. In 3.48, what is it set to in Preferences > Conversion > Common Options > Page Setup? I have Output profile set to Tablet and Input profile set to Default profile.

By setting Output profile to Tablet, it means that there will be no reducing hi-res images in size or compression. That's what you want set.

[kovidgoyal]

eoub3 to epub2 doesnt affect filesize. Older versions used to downgrade image sizes automatically. You now have to use the shrink images larger than option under EPUB output.

[maddz]

7.25 both are set to default profiles, 3.48 output is generic e-ink, input is default input profile.

I'll try flipping them this evening when I can get at the iMac.

[kovidgoyal]

The default profiles dont shrink images anymore, as I said, set a max image size under EPUB output or choose an output profile with a smaller screen size.

[DNSB]

Generic e-ink assumes the screen is 590x775 while the default profile assumes 1200x1600. On most newer HD e-Ink devices, a 590x775 image is noticeably softer than a larger image. My personal preference is tablet with no limitations on image size so a large map is zoomable though I think that the default profile doesn't shrink images in newer calibre versions.

Edit: should have read Kovid's message before replying...

[maddz]

Originally Posted by kovidgoyal

The default profiles dont shrink images anymore, as I said, set a max image size under EPUB output or choose an output profile with a smaller screen size.

What was the compression ratio for earlier versions? Even compressing by 50% was only reducing the file size from the original 20.13 MB to 15.75 MB.

[JSWolf]

Originally Posted by maddz

What was the compression ratio for earlier versions? Even compressing by 50% was only reducing the file size from the original 20.13 MB to 15.75 MB.

It doesn't matter what the compression is. The thing is, the image sizes are being reduced. So you want to have the settings such that the image do not get changed and we've already given you the info on how to do that earlier in this thread.

[maddz]

Originally Posted by JSWolf

It doesn't matter what the compression is. The thing is, the image sizes are being reduced. So you want to have the settings such that the image do not get changed and we've already given you the info on how to do that earlier in this thread.

But I do want the images compressed! They total 19.1 MB in the file which makes it very laggy even viewing it on the computer let alone my Kobo.

What I am trying to figure out is how to set things up so I get roughly the same size output file as I used to in 3.48 (certainly no more than 5 MB). I really don't want to have to copy to an old computer to carry out the compression then port back to the main computer.

I did wonder if it was something in the Epub 3 output that causing it (I've had previous experience with a print layout Epub 3 file where every letter had it's own css).

@Kovid, was there a reason why the new conversion doesn't compress images automatically? It's rather annoying to have to run books through multiple processes before they get transferred to one's device.

[maddz]

I think I've got it: setting the output to Kobo Reader, then setting the image size to 500x700 drops to a bit over 3 MB - which is roughly what I got when I ran the conversion on the old installation. Now to get this to stick!