convert_image.go

// Copyright 2016 Martin Hebnes Pedersen (LA5NTA). All rights reserved.
// Use of this source code is governed by the MIT-license that can be
// found in the LICENSE file.

package main

import (
	"bytes"
	"image"
	"image/jpeg"
	"io"
	"mime"
	"path"
	"strings"

	_ "image/gif"
	_ "image/png"

	"github.com/nfnt/resize"
)

func isImageMediaType(filename, contentType string) bool {
	var mediaType string
	if contentType != "" {
		mediaType, _, _ = mime.ParseMediaType(contentType)
	}
	if mediaType == "" {
		mediaType = mime.TypeByExtension(path.Ext(filename))
	}

	return strings.HasPrefix(mediaType, "image/")
}

func convertImage(r io.Reader) ([]byte, error) {
	img, _, err := image.Decode(r)
	if err != nil {
		return nil, err
	}

	// Scale down
	if img.Bounds().Dx() > 600 {
		img = resize.Resize(600, 0, img, resize.NearestNeighbor)
	}

	// Re-encode as low quality jpeg
	var buf bytes.Buffer
	err = jpeg.Encode(&buf, img, &jpeg.Options{Quality: 40})
	return buf.Bytes(), err
}