good good study, day day up!

vendor/gopl.io/ch8/thumbnail/thumbnail.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 234.
+
+// The thumbnail package produces thumbnail-size images from
+// larger images.  Only JPEG images are currently supported.
+package thumbnail
+
+import (
+	"fmt"
+	"image"
+	"image/jpeg"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// Image returns a thumbnail-size version of src.
+func Image(src image.Image) image.Image {
+	// Compute thumbnail size, preserving aspect ratio.
+	xs := src.Bounds().Size().X
+	ys := src.Bounds().Size().Y
+	width, height := 128, 128
+	if aspect := float64(xs) / float64(ys); aspect < 1.0 {
+		width = int(128 * aspect) // portrait
+	} else {
+		height = int(128 / aspect) // landscape
+	}
+	xscale := float64(xs) / float64(width)
+	yscale := float64(ys) / float64(height)
+
+	dst := image.NewRGBA(image.Rect(0, 0, width, height))
+
+	// a very crude scaling algorithm
+	for x := 0; x < width; x++ {
+		for y := 0; y < height; y++ {
+			srcx := int(float64(x) * xscale)
+			srcy := int(float64(y) * yscale)
+			dst.Set(x, y, src.At(srcx, srcy))
+		}
+	}
+	return dst
+}
+
+// ImageStream reads an image from r and
+// writes a thumbnail-size version of it to w.
+func ImageStream(w io.Writer, r io.Reader) error {
+	src, _, err := image.Decode(r)
+	if err != nil {
+		return err
+	}
+	dst := Image(src)
+	return jpeg.Encode(w, dst, nil)
+}
+
+// ImageFile2 reads an image from infile and writes
+// a thumbnail-size version of it to outfile.
+func ImageFile2(outfile, infile string) (err error) {
+	in, err := os.Open(infile)
+	if err != nil {
+		return err
+	}
+	defer in.Close()
+
+	out, err := os.Create(outfile)
+	if err != nil {
+		return err
+	}
+
+	if err := ImageStream(out, in); err != nil {
+		out.Close()
+		return fmt.Errorf("scaling %s to %s: %s", infile, outfile, err)
+	}
+	return out.Close()
+}
+
+// ImageFile reads an image from infile and writes
+// a thumbnail-size version of it in the same directory.
+// It returns the generated file name, e.g. "foo.thumb.jpeg".
+func ImageFile(infile string) (string, error) {
+	ext := filepath.Ext(infile) // e.g., ".jpg", ".JPEG"
+	outfile := strings.TrimSuffix(infile, ext) + ".thumb" + ext
+	return outfile, ImageFile2(outfile, infile)
+}