winauth-go/tools/gen_ico/main.go

// gen_ico produces a multi-resolution .ico file from a single PNG
// source. The output embeds each size as a PNG payload (Vista+ .ico
// format) so the result stays small and crisp.
//
// Usage:
//
//	go run ./tools/gen_ico [src.png] [dst.ico]
//
// Defaults:
//
//	src = internal/ui/assets/app_icon.png
//	dst = internal/ui/assets/app_icon.ico
//
// After generating the .ico, embed it into the Windows binary via:
//
//	go run github.com/akavel/rsrc@latest \
//	    -ico internal/ui/assets/app_icon.ico \
//	    -o internal/ui/assets/rsrc_windows.syso
//
// rsrc_windows.syso is auto-linked by `go build` on Windows.
package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"image"
	"image/png"
	"io"
	"os"

	"golang.org/x/image/draw"
)

// icoSizes are the embedded resolutions. Windows picks the largest size
// that fits the target use (title bar = 16, Alt-Tab = 32, Explorer = up
// to 256) and scales down. Skipping sizes Windows can't read anyway
// keeps the file under 200 KB.
var icoSizes = []int{16, 24, 32, 48, 64, 128, 256}

// icoEntry holds one image's dimensions and its encoded PNG bytes.
type icoEntry struct {
	w, h int
	png  []byte
}

func main() {
	srcPath := "internal/ui/assets/app_icon.png"
	dstPath := "internal/ui/assets/app_icon.ico"
	if len(os.Args) > 1 {
		srcPath = os.Args[1]
	}
	if len(os.Args) > 2 {
		dstPath = os.Args[2]
	}
	if err := run(srcPath, dstPath); err != nil {
		fmt.Fprintln(os.Stderr, "gen_ico:", err)
		os.Exit(1)
	}
}

func run(srcPath, dstPath string) error {
	srcF, err := os.Open(srcPath)
	if err != nil {
		return fmt.Errorf("open src: %w", err)
	}
	defer srcF.Close()
	src, _, err := image.Decode(srcF)
	if err != nil {
		return fmt.Errorf("decode src: %w", err)
	}

	var entries []icoEntry
	for _, size := range icoSizes {
		dst := image.NewNRGBA(image.Rect(0, 0, size, size))
		// CatmullRom gives sharper results at small sizes than the
		// default nearest-neighbour.
		draw.CatmullRom.Scale(dst, dst.Bounds(), src, src.Bounds(), draw.Over, nil)

		var buf bytes.Buffer
		if err := png.Encode(&buf, dst); err != nil {
			return fmt.Errorf("encode %dpx: %w", size, err)
		}
		entries = append(entries, icoEntry{w: size, h: size, png: buf.Bytes()})
	}

	out, err := os.Create(dstPath)
	if err != nil {
		return fmt.Errorf("create dst: %w", err)
	}
	defer out.Close()
	return writeICO(out, entries)
}

// writeICO serialises a Vista+ PNG-based .ico file: ICONDIR + one
// ICONDIRENTRY per image + the PNG bytes concatenated.
func writeICO(w io.Writer, entries []icoEntry) error {
	if _, err := w.Write([]byte{
		0, 0, // reserved
		1, 0, // type = 1 (icon)
		byte(len(entries)), 0, // count
	}); err != nil {
		return err
	}

	const dirHdrLen = 6
	const dirEntryLen = 16
	headerLen := dirHdrLen + dirEntryLen*len(entries)

	offset := headerLen
	for _, e := range entries {
		wByte := byte(e.w)
		if e.w >= 256 {
			// 0 means 256 in the .ico directory.
			wByte = 0
		}
		hByte := byte(e.h)
		if e.h >= 256 {
			hByte = 0
		}
		size := uint32(len(e.png))

		entry := make([]byte, dirEntryLen)
		entry[0] = wByte
		entry[1] = hByte
		entry[2] = 0 // color palette
		entry[3] = 0 // reserved
		binary.LittleEndian.PutUint16(entry[4:], 1)  // planes
		binary.LittleEndian.PutUint16(entry[6:], 32) // bit count
		binary.LittleEndian.PutUint32(entry[8:], size)
		binary.LittleEndian.PutUint32(entry[12:], uint32(offset))
		if _, err := w.Write(entry); err != nil {
			return err
		}
		offset += int(size)
	}

	for _, e := range entries {
		if _, err := w.Write(e.png); err != nil {
			return err
		}
	}
	return nil
}