Golang vips: How to render text with custom truetype font? - go

Question

When we want to render text an vips image, you do something like this with vips_text:
import "C"
var textImage *C.VipsImage
cText := C.CString("Some text")
cFont := C.CString("Arial 12px")
C.cgo_vips_text(&textImage, cText, cFont)
But here, Arial 12px is a fontconfig string name and assumes that the system has this font installed.
How do I let the program use a custom truetype font file, e.g., Roboto.ttf?
Trying
cFont := C.CString("Roboto.ttf")
probably won't work.
We could put the Roboto.ttf file in the system directory, for example, in Ubuntu you put the file in /usr/share/fonts/truetype, but the code makes the assumption that this font exists, which is not a solution to write machine-independent program. So I wonder if there is a better way to do this so that we can ship the executables with the font file together.

Answer 1

vips uses standard fontconfig font names, so you need to set up fontconfig to add your custom font to its search path.
From the manual it looks like you can set the environment variable XDG_CONFIG_HOME to point to your new font, then from go refer to that font as Roboto 48pt (for example).
It's easy to test from the command-line. Try:
$ fc-match "Roboto"
Roboto-Regular.ttf: "Roboto" "Regular"
$ vips text x2.png "hello" --font "Roboto 48px"
$ vips text x.png "hello" --font "Arial 48px"
$ eog x.png x2.png
And make sure you see two different images.