A subsetting question

[roger64]

Using calibre 7.1. linux.

When subsetting my usual fonts, the reporting little window under each of the three fonts tells me:

FFTM NOT subset; don't know how to subset; dropped

Nevertheless the subsetting process takes place.

However the downside is that I lose the display of open-type properties (like old numerotation, etc.) in the subset.

Calibre 7.1. has been installed using the linux script provided by Calibre. Maybe some fonttools software is badly installed? I have python-fonttools 4.44.3-1 installed.

[kovidgoyal]

calibre comes with its own copy of fonttools assuming you are using the official binaries. fftm is a fontforge timestamp table, it has no connection to opentype features. https://fontforge.org/docs/techref/TrueOpenTables.html

[roger64]

Thanks for your reply.

I use the binaries provided by the Calibre Linux script, currently at 7.1.

In the joint png screenshot you can see the error message I am getting when I ask Calibre to make a subset.

In the zip, I provided two examples of the same ePub3. This book was published by me on MR in 2018 and I slightly modify it for this purpose. As there are nineteenth century letters, the use of an old numerotation makes sense.

- The first one is before subsetting, where you can see, for example, that the Date at the beginning of each chapter uses the old numerotation (open-type).
- The second one named (calibre-subset) is after subsetting where you can see that the Date at the beginning of each chapter does not use anymore the old numerotation.

So this error message seems to have an impact, even if indirect, on the open-type display.

[kovidgoyal]

The error message has no connection. Subsetting simply doesn't preserve open type feature variations. Maybe someday I might add support but its not a priority for me, patches welcome.

[roger64]

Hi

I checked again. I reinstalled Calibre 7.1. (Archlinux) using the Calibre Linux custom install script and the terminal reported no problem during install.

I remember using Calibre subsetting during these last years. At least from 2021, it did save open-type variations after subsetting. But the size of the subset was bigger than today say roughly 15%.

About one or two years ago, the Calibre subsetting process was made more efficient, reducing the subset to about 5%. I noticed at that time during calibre install on my Linux distro a dependency problem with fonttools. So I reverted to the Calibre linux install script. Since that time however, the calibre subset does not save anymore the open-type variations.

Another install of Calibre reports the same error message as me and the same faulty results. I would like to know how to get rid of this message which appears when subsetting even when I perform a regular Calibre script install.

Patches

You say that patches are welcome. The current version of fonttools (the Archlinux package is python-fonttools 4.44.3-1) does indeed preserve open-type variations so there could be some benefit to use it.

Here is a test case to illustrate my point. I added enclosed in this zip a version obtained with the Sigil subset plugin which preserves the open-type variations. It uses exclusively the latest fonttools.

Note: for users of this forum, to see how open-type variations are preserved, just look for the display of the Date paragraph style which you'll find at the beginning of each chapter.

[roger64]

Hi

It seems that, despite some interest about it, this question about subsetting not preserving open type feature variations will stay unresolved.

I note that the Calibre editor issues a warning before sorting the style rules (see attachment-in French sorry).

Maybe, for the time being and as it is not an obvious behaviour, would it be useful to add such a kind of warning before launching the tool "subsetting of included fonts" which replaces existing fonts:

"Are you sure? Subsetting does not preserve open type feature variations."

[kovidgoyal]

https://github.com/kovidgoyal/calibr...7965c37447d6c8

[roger64]

Thank you.

[JSWolf]

Originally Posted by kovidgoyal

https://github.com/kovidgoyal/calibr...7965c37447d6c8

Can we have an option to do it the current way if we don't care about OpenType features? Most of the time, fonts are just there to display text without needing any of the OT features.

[nabsltd]

Originally Posted by JSWolf

Can we have an option to do it the current way if we don't care about OpenType features? Most of the time, fonts are just there to display text without needing any of the OT features.

If a font doesn't have any OpenType features tables, then this change will do nothing.

If a font has more complete OpenType features tables, then you really want them included. They don't add much to the size of the font, but will make the character subset display much better, which is what you want if you are creating books for arbitrary users (i.e., sale or other distribution).

If you are creating books only for yourself, then don't embed fonts with a full set of OpenType features, or remove the features tables from the font before embedding.

[JSWolf]

Originally Posted by nabsltd

If a font doesn't have any OpenType features tables, then this change will do nothing.

If a font has more complete OpenType features tables, then you really want them included. They don't add much to the size of the font, but will make the character subset display much better, which is what you want if you are creating books for arbitrary users (i.e., sale or other distribution).

If you are creating books only for yourself, then don't embed fonts with a full set of OpenType features, or remove the features tables from the font before embedding.

The thing is, If all I want is to just display the font and don't care about keeping OpenType features, then I want the old way as I want the font smaller not larger. That's why an option to do it the old way is a good thing to have.

Having to edit the font is not something we should have to do when all we need is a check box to decide which way to subset. Also, finding a different font with no OpenType features is also not something we should need to do when a checkbox is al we need.