As the title says, I am making a custom font and am having trouble getting it to support CJK. They just show up as blank.

Currently using FontForge.

CID-keyed OpenType/CFF fonts are limits, such as the number of glyphs (65,535 is the architectural limit) and the number of mappings in the ‘cmap‘ table (when the surrogates and non-characters are factored out, Unicode has 1,111,998 possible mappings in its 17 planes).

I see. For OpenType fonts, is there anyway for a blank glyph entry to use a glyph from another font? It seems when my font is generated as TrueType, it will pick some other Windows to show a CJK character, but when generated as an OpenType font, the character will just be blank. Is this just a property of OpenType?

Opentype CID ?

https://blogs.adobe.com/CCJKType/2012/04/cid-vs-gid.html