Implement resizing

1 files changed, 50 insertions, 4 deletions

diff --git a/convert.go b/convert.go
index ea7403c..432d0fc 100644
--- a/convert.go
+++ b/convert.go
@@ -12,10 +12,18 @@ import (
 	"strings"
 
 	"golang.org/x/image/bmp"
+	"golang.org/x/image/draw"
 	"golang.org/x/image/tiff"
 	_ "golang.org/x/image/webp"
 )
 
+var (
+	x       = flag.Int("x", 0, "new image width")
+	y       = flag.Int("y", 0, "new image height")
+	stretch = flag.Bool("stretch", false, "if resizing, whether to stretch the image rather than cropping (the default)")
+	format  = flag.String("format", "", "output image format, default is to use the file extension")
+)
+
 func load(filename string) (image.Image, error) {
 	file := os.Stdin
 	if filename != "-" {
@@ -35,6 +43,37 @@ func load(filename string) (image.Image, error) {
 	return img, nil
 }
 
+func divRound(n, d int) int {
+	return (n + d/2) / d
+}
+
+func resize(img image.Image) image.Image {
+	x, y := *x, *y
+	if x == 0 {
+		x = divRound(y*img.Bounds().Dx(), img.Bounds().Dy())
+	}
+	if y == 0 {
+		y = divRound(x*img.Bounds().Dy(), img.Bounds().Dx())
+	}
+	output := image.NewRGBA64(image.Rect(0, 0, x, y))
+	srcRect := img.Bounds()
+	if !*stretch {
+		if img.Bounds().Dx()*y >= img.Bounds().Dy()*x {
+			scaledX := divRound(img.Bounds().Dy()*x, y)
+			xOffset := (img.Bounds().Dx() - scaledX) / 2
+			srcRect.Min.X += xOffset
+			srcRect.Max.X = srcRect.Min.X + scaledX
+		} else {
+			scaledY := divRound(img.Bounds().Dx()*y, x)
+			yOffset := (img.Bounds().Dy() - scaledY) / 2
+			srcRect.Min.Y += yOffset
+			srcRect.Max.Y = srcRect.Min.Y + scaledY
+		}
+	}
+	draw.CatmullRom.Scale(output, output.Bounds(), img, srcRect, draw.Src, nil)
+	return output
+}
+
 func run() error {
 	args := flag.Args()
 	if len(args) != 2 {
@@ -42,11 +81,14 @@ func run() error {
 	}
 	inputFileName, outputFileName := args[0], args[1]
 
-	n := strings.LastIndex(outputFileName, ".")
-	if n < 0 {
-		return errors.New("unknown output file extension")
+	ext := *format
+	if ext == "" {
+		n := strings.LastIndex(outputFileName, ".")
+		if n < 0 {
+			return errors.New("unknown output file extension")
+		}
+		ext = outputFileName[n+1:]
 	}
-	ext := outputFileName[n+1:]
 	switch ext {
 	case "gif", "jpeg", "jpg", "png", "bmp", "tiff":
 	default:
@@ -58,6 +100,10 @@ func run() error {
 		return err
 	}
 
+	if *x != 0 || *y != 0 {
+		input = resize(input)
+	}
+
 	outputFile, err := os.Create(outputFileName)
 	if err != nil {
 		return err