1 files changed, 101 insertions, 0 deletions
diff --git a/convert.go b/convert.go
new file mode 100644
index 0000000..ea7403c
--- /dev/null
+++ b/convert.go
@@ -0,0 +1,101 @@
+package main
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"image"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
+	"os"
+	"strings"
+
+	"golang.org/x/image/bmp"
+	"golang.org/x/image/tiff"
+	_ "golang.org/x/image/webp"
+)
+
+func load(filename string) (image.Image, error) {
+	file := os.Stdin
+	if filename != "-" {
+		var err error
+		file, err = os.Open(filename)
+		if err != nil {
+			return nil, err
+		}
+		defer file.Close()
+	}
+
+	img, _, err := image.Decode(file)
+	if err != nil {
+		return nil, fmt.Errorf("decode %q: %s", filename, err)
+	}
+
+	return img, nil
+}
+
+func run() error {
+	args := flag.Args()
+	if len(args) != 2 {
+		return errors.New("usage: convert input-file output-file")
+	}
+	inputFileName, outputFileName := args[0], args[1]
+
+	n := strings.LastIndex(outputFileName, ".")
+	if n < 0 {
+		return errors.New("unknown output file extension")
+	}
+	ext := outputFileName[n+1:]
+	switch ext {
+	case "gif", "jpeg", "jpg", "png", "bmp", "tiff":
+	default:
+		return fmt.Errorf("unknown output file extension %q", ext)
+	}
+
+	input, err := load(inputFileName)
+	if err != nil {
+		return err
+	}
+
+	outputFile, err := os.Create(outputFileName)
+	if err != nil {
+		return err
+	}
+	defer outputFile.Close()
+	switch ext {
+	case "gif":
+		err = gif.Encode(outputFile, input, nil)
+	case "jpeg", "jpg":
+		err = jpeg.Encode(outputFile, input, nil)
+	case "png":
+		err = png.Encode(outputFile, input)
+	case "bmp":
+		err = bmp.Encode(outputFile, input)
+	case "tiff":
+		err = tiff.Encode(outputFile, input, nil)
+	}
+	if err != nil {
+		return err
+	}
+	return outputFile.Close()
+}
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprint(os.Stderr, `Usage: convert input-file output-file
+Supported formats:
+  png
+  jpeg
+  bmp
+  tiff
+  webp (input only)
+`)
+	}
+	flag.Parse()
+
+	if err := run(); err != nil {
+		fmt.Fprintf(os.Stderr, "convert: %s\n", err)
+		os.Exit(1)
+	}
+}