Thanks for introducing this feature of 'non-destructive updates' into calibre. Is the plan to migrate some of kiwidude's 'Modify Epub' features over into full calibre or are they now divergent projects?
Regarding Polish update cover, I think there is a bug.
I have
Prefs - Output options - preserve cover aspect ratio unchecked, i.e. I want my covers edge-to-edge, no white bars. If I use Polish to update the cover then it is no longer edge-to-edge, but if I use Modify Epub it is. My understanding is that Modify Epub gives me the cover I would have got if I'd done an epub-epub conversion.
On looking more closely at the titlepage.xhtml svg code I see that with a new cover size of 987x1500:
Polish copies the fullsize 987x1500 jpg into the epub and outputs this svg code
Code:
<svg version="1.1" xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
width="100%" height="100%" viewBox="0 0 600 800"
preserveAspectRatio="none">
<image width="600" height="800" xlink:href="cover.jpg"/>
</svg>
whereas Modify Epub scales down the 987x1500 jpg to 513x779 (probably because my output profile is Sony) and outputs this svg code
Code:
<svg version="1.1" xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
width="100%" height="100%" viewBox="0 0 513 779"
preserveAspectRatio="none">
<image width="513" height="779" xlink:href="images/cover.jpeg"/>
</svg>
The problem would appear to be that Polish is setting fixed dimensions of 600, 800 in the viewbox and <image> settings. If the new cover does not have 3:4 aspect ratio then the Polish-ed epub will display the new cover with white bars. I checked this out by manually editing the Polish-ed svg code to match the actual jpg dimensions and indeed the cover is, once again, edge-to-edge.
To be honest, neither Modify Epub nor Polish currently produces my 'perfect result' which would be to update with the fullsize cover jpg and make the svg dimensions match it, but maybe there are reasons why that's not possible.