The unpatched firmware allows adjustment of the line spacing via a slider with 15 positions. However most people use only the lowest few settings, as the highest settings are far too wide to be useful. The
`My 15 line spacing values` patch allows the 15 stock choices to be replaced with your own 15 custom values.
The final line spacing on the page depends on a number of factors, including choice of font face, publisher's settings in the book's stylesheet, and whether the EPUB or KEPUB reader is used (see the `ePub unitless line height` patch to fix a problem affecting line spacing in the EPUB reader's stylesheet.)
My understanding is that the line spacing value set by the slider is a scale factor by which the device multiplies the base line height. The scale factors used by the unpatched firmware range from 1.0 - 3.0. The default values in the patch change this range to 0.7 - 1.1, but you can choose any custom range you want.
The screenshots below (Glo, 3.15.0) show 5 out of the 15 line spacing settings with a sideloaded epub using the Amasis font. From left to right, line spacing values are:
0.7,
0.8,
0.9,
1.0 (unpatched minimum),
1.1:
And if you are wondering why anyone would need a line spacing value as small as 0.7, it is because some fonts come with an extremely large base line height: the screenshots below (Glo, 3.15.0) show an EPUB with the Kobo Nickel font selected, line spacing values of
0.7 (default patch minimum) and
1.0 (unpatched minimum):
Beware that the KEPUB reader has a limitation which affects the use of small line spacing values. The screenshots below (Glo 3.15.0) show pages of an EPUB and a KEPUB respectively, using a line spacing value of 0.8. Note the dots at the bottom of the KEPUB page: these are the ascenders of the top line of text from the following page. How narrow you can make the KEPUB line spacing before this problem shows up depends on which font face is used. There is currently no fix for this problem, other than increasing the line spacing or switching to the EPUB reader.