Bug#1037256: debian-installer: GUI font for Japanese was incorrectly rendered

Because of Han unification, wrong font typefaces are rendered by default when you choose Japanese language using GUI installer.

Most of typefaces are correct, but there are wrong typefaces (Simplified Chinese) which is used for widget rendering.

This issue [1] will not be solved by using DroidSansFallback.ttf (which is shipped by fonts-android udeb) for Japanese.

It means that we need to switch font itself which contains Japanese typeface to fix this issue.

[1] Your Code Displays Japanese Wrong

Info

The following patch is Poc to use with compressed Japanese font (MotoyaLCedar). When building installer, generate fake font cache and d-i dynamically load decompressed font on demand. It need to modify debian-installer, rootskel-gtk and localechooser.

Info

Disk Usage: MotoyaLCedar (1.9MB) < VL Gothic (3.9MB) < BIZ UDGothic (4.5MB)
Readability and Visibility (by default 9pt): MotoyaLCedar = VL Gothic < BIZ UDGothic (best)

With tweaking default font size (e.g. 9pt to 10pt), readability and visibility might be good enough with MotoyaLCedar 10pt in practical use case.

Warning

The following screenshot were provided to check the result of gtk-font-set tweaks. With customized version of rootskel-gtk and mini.iso (make build_netboot-gtk)

M PLUS 1 Medium (9pt)

M PLUS 2 (9pt)

M PLUS 2 (10pt)