Enlarging all images in an epub for Kobo AURA?

[MelBr]

Hi all,
I'm currently reading an epub that has a lot of equations. They're all typset as images. Problem is that images are completely unreadable on screen. They're so small. I'm using kobo.epub format so you can double-tap on an image to enlarge it. But that takes way too long since sometime I misstap images due to their small size.

I've been looking at KoboTouchExtended plugin and also have looked at covert options in Calibre but don't see an obvious way to enlarge them.

I did notice Search & Replace in Calibre's conversion options so could I somehow change each img tag to include a larger size somehow? Is anyone familiar with CSS that Kobo supports which would properly enlarge each image by 200% or something like that?

Anyone have some suggestions?

Thanks!

[davidfor]

I think it will depend on how the images are displayed in book. Most seem to have an img tag. That can have the sizing and positioning in the tag. Or there could be a class for each image to set the size. The image could also be wrapped in SVG tags to display. Whichever is used, changing the sizes seems to work. Of course, the only "testing" I have done for this is when I get the numbers wrong, usually swapped, and the image is display incorrectly.

The extended driver won't be able to do this. I don't know if a conversion could. I'd probably open the epub in the editor and change them manually. I don't think there is a way to do it automatically there.

[MelBr]

davidfor, here's what the html of these images looks like:



Any suggestions as to which element I should try adding to these equations to make them bigger?

BTW, while they do appear readable in this screenshot, they're completely unreadable on Aura. I'm guessing that's due to high density and pixel pitch on the Aura...
Thanks!

[davidfor]

From that, if open the stylesheet, there should be a style called "calibre5". This should have the size of that image. There will probably be a style like that for each image.

A hack would be to double or triple the numbers. That should increase the size, but if you go to big, it might not all fit on the page. You could also try removing the height and changing the width to "100%". I that will put the image full width on the page and adjust the height proportionally.

And that gets to pretty much the end of my knowledge on this. I'd have to experiment a lot. Either someone with better knowledge will wander along, or ask the question in the ePub subforum of the Workshop. The experts reside there and can tell you the best ways to do this.

[MelBr]

davidfor, thank you so much for tips and sharing your wisdom!

I managed to resize images through the CSS alone but was not really satisfied with how the Kobo's reader was enlarging them. I couldn't really fault it because it was doing the most sensible thing and just using bicubic for enlarging. Since the original images were extremely small and "soft", they were all fairly blurry and way too grey when viewed on Aura.

I ended up opening the ebook with Calibre and then using open-source Imagemagic to enlarge images in a way that sharpened all the edges to minimize the blur and make the images look good on Aura. I spent fiddling with parameters for 30 min, rendered images and then tested them on Kobo.

In the end, I ended up with something that was perfectly readable and looked great to me. I ended up enlarging all images by 300% and applying various sharpen filters (which ones you apply depends heavily on source material so the keys is to try and see what works).

Here's some examples...

Original:

Bicubic:

Custom:

others:









I think this is the way to go when faced with the issue of ebooks with small images. Kobo/Eink readers prefer images with better contrast.

Thanks again and hopefully this helps others as well.

[davidfor]

That's probably the best thing to do. A bit more work, but probably worth it especially if it is a reference work. It's a pity they haven't used a more resizable format.

[barf]

Originally Posted by davidfor

From that, if open the stylesheet, there should be a style called "calibre5". This should have the size of that image. There will probably be a style like that for each image.

A hack would be to double or triple the numbers. That should increase the size, but if you go to big, it might not all fit on the page. You could also try removing the height and changing the width to "100%". I that will put the image full width on the page and adjust the height proportionally.

Necroreplying because this saved my day!
Pretty easy hack, and I also learned a little bit more about the innards of ebooks. Many thanks!

[Ken Maltby]

Long pressing on an epub image in KOReader will open the image in a viewer as a full page image. Depending on the aspect ratio of the image it may display at full width, in that case you can rotate the image and it will fill the screen.

Luck;
Ken

[DNSB]

Originally Posted by Ken Maltby

Long pressing on an epub image in KOReader will open the image in a viewer as a full page image. Depending on the aspect ratio of the image it may display at full width, in that case you can rotate the image and it will fill the screen.

And if you are lucky, you won't end up with a full page blur. As the OP in the thread commented on using kepub and zooming the image, if the image is too small, zooming it is not a useful technique.

[JSWolf]

Are there any good bitmap to SVG converters out there that are easy to use? If so, that would allow the images to be resized without extra fuzz.

[Quoth]

You'd need manual adjustment. Possibly try inkscape.

[Pajamaman]

Originally Posted by Ken Maltby

Long pressing on an epub image in KOReader will open the image in a viewer as a full page image. Depending on the aspect ratio of the image it may display at full width, in that case you can rotate the image and it will fill the screen.

Luck;
Ken

Indeed. I'd try Koreader.