https://github.com/AllenDang/cimgui-go/issues/303

font_scaling_demo.go

package main

import (
	"fmt"
	imgui "github.com/AllenDang/cimgui-go"
	"os"
	"time"
	"unsafe"

	_ "embed"
)

// Embed the full ttf file contents within the golang binary at compile-time. This requires importing the `embed` package.
// Note that any TTF file will work. I got this one from https://fonts.google.com/specimen/Merriweather
// If you have a different way of loading a ttf (e.g. hard-coding the TTF bytes as a base64 string), this can be replaced.
//
//go:embed Merriweather-Regular.ttf
var defaultFontTTF []byte

const (
	DefaultFontSize int32 = 20
	MinFontSize     int32 = 10
	MaxFontSize     int32 = 100
)

var (
	selectedFontSize int32 = DefaultFontSize

	// FontsBySize is populated by a single font, rendered from MinFontSize through MaxFontSize. This allows using
	// different the font at different sizes without needing to scale them (i.e. `imgui.CurrentFont().SetScale(2)`).
	// Scaling the font usually causes blurriness or other distortion. It's preferable to load the font at all
	// of the sizes you plan to use, if you know those sizes in advance.
	FontsBySize map[int]*imgui.Font
)

// InitFonts must be called in `backend.SetAfterCreateContextHook(hookFunc)`. Before rendering, imgui may not
// have a context. During the game loop, imgui doesn't allow loading fonts.
func InitFonts() {
	fmt.Println("Loading fonts...")
	start := time.Now()

	FontsBySize = make(map[int]*imgui.Font)

	for i := MinFontSize; i <= MaxFontSize; i++ {
		FontsBySize[int(i)] = loadFont(float32(i))
	}

	imgui.CurrentIO().Fonts().Build()
	imgui.CurrentIO().SetFontDefault(FontsBySize[int(DefaultFontSize)])
	fmt.Printf("Finished loading fonts in %v\n", time.Now().Sub(start).Round(time.Millisecond))
}

func loadFont(fontSize float32) *imgui.Font {
	// NOTE: It takes an estimated 2ms per font, so for 100 sizes it's slightly noticeable to load these at startup.
	//       You may want to add a loading screen, or simply load less sizes fonts. Perhaps keep only a few different
	//       fonts, named based on their usage (e.g. Title, Heading1, etc)
	cfg := imgui.NewFontConfig()
	cfg.SetFontData(uintptr(unsafe.Pointer(imgui.SliceToPtr(defaultFontTTF))))
	cfg.SetFontDataSize(int32(len(defaultFontTTF)))
	cfg.SetSizePixels(fontSize)
	cfg.SetFontDataOwnedByAtlas(false) // Let golang manage this memory. Otherwise imgui crashes when trying to clean it up while exiting. The error is: `munmap_chunk(): invalid pointer`.
	cfg.SetPixelSnapH(true)
	return imgui.CurrentIO().Fonts().AddFont(cfg)
}

func loop() {
	imgui.Begin("Window1")
	{
		imgui.Text("Size: ")
		imgui.SameLine()
		imgui.SliderInt("##FontSizeSlider", &selectedFontSize, MinFontSize, MaxFontSize)

		imgui.PushFont(FontsBySize[int(selectedFontSize)])
		imgui.Text(fmt.Sprintf("%d default default default", selectedFontSize))
		imgui.PopFont()

		for i := MinFontSize; i <= MaxFontSize; i += 10 {
			imgui.PushFont(FontsBySize[int(i)])
			imgui.Text(fmt.Sprintf("%d  test test test test test test", i))
			imgui.PopFont()
		}
	}
	imgui.End()
}

func main() {
	backend, err := imgui.CreateBackend(imgui.NewGLFWBackend())
	if err != nil {
		fmt.Printf("Error creating backend: %v\n", err)
		os.Exit(1)
	}
	backend.SetAfterCreateContextHook(func() {
		InitFonts()
	})
	backend.CreateWindow("Resized font demo", 1500, 900)
	backend.Run(loop)
}