Add a watermark on images using golang

watermark.go

/**
* This code was written using go 1.23
*
* There are some changes you have to do, like chaging paths and file type extensions.
*
* go mod init
* go get github.com/fogleman/gg
*/

package main

import (
	"fmt"
	"image"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"
	"strings"

	"github.com/fogleman/gg"
)

func addWatermark(inputPath, outputPath, watermarkText string) error {
	imgFile, err := os.Open(inputPath)
	if err != nil {
		return fmt.Errorf("failed to open input image: %w", err)
	}
	defer imgFile.Close()

	img, _, err := image.Decode(imgFile)
	if err != nil {
		return fmt.Errorf("failed to decode input image: %w", err)
	}

	imgHeight := float64(img.Bounds().Dy())

	dc := gg.NewContextForImage(img)

  // change font path
	if err := dc.LoadFontFace("/home/luna/.local/share/fonts/ComicSansMS3.ttf", 60); err != nil {
		return fmt.Errorf("failed to load font: %w", err)
	}

	dc.SetRGB(1, 1, 1)
	dc.SetHexColor("#FFFFFF99")

	padding := 22

	_, textHeight := dc.MeasureString(watermarkText)
	dc.DrawString(watermarkText, float64(padding), imgHeight-textHeight+float64(padding/2))

	if err := dc.SavePNG(outputPath); err != nil {
		return fmt.Errorf("failed to save output image: %w", err)
	}

	return nil
}

func addWatermarkForAllFiles(watermarkText string) error {
	outputDir := "./output"
	if _, err := os.Stat(outputDir); os.IsNotExist(err) {
		if err := os.Mkdir(outputDir, 0755); err != nil {
			return fmt.Errorf("failed to create output directory: %w", err)
		}
	}

	files, err := ioutil.ReadDir(".")
	if err != nil {
		return fmt.Errorf("failed to read current directory: %w", err)
	}

	for _, file := range files {
    // change file type if needed
		if strings.HasSuffix(file.Name(), ".png") {
			inputPath := file.Name()
			outputPath := filepath.Join(outputDir, file.Name())

			fmt.Printf("Processing: %s -> %s\n", inputPath, outputPath)

			if err := addWatermark(inputPath, outputPath, watermarkText); err != nil {
				return fmt.Errorf("failed to add watermark to %s: %w", inputPath, err)
			}
		}
	}

	fmt.Println("Processing completed for all PNG files.")
	return nil
}

func main() {
  // the watermark to add
	watermarkText := "@mwlica, lunish.nl"

	if err := addWatermarkForAllFiles(watermarkText); err != nil {
		log.Fatalf("Error processing files: %v", err)
	}
}