Golang Converting Image magick object to []byte

Question

I am using the following code which fetch the object from Amazon s3 and after performing resizing and cropping. I want to store it on s3. But the problem is i am not able convert the mw (Image maigck object) to byte array. Which will be used for storing it on s3. Moreover in current method it uses jpeg.Encode. What if the image in .png or .gif format. How will we convert it to []byte?

Could you please also tell me how to evenly crop an image just passing the aspect ratio not cropping coordinates. imgStream.Crop((int)originalWidth, ((int)(originalWidth / masterAspectRatio)), Gravity.Center) like we do it in .net. Reason i am asking is there is no method in library which provides this flexibility.

s3Client := s3.New(session.New(), &aws.Config{Region: aws.String(region)})
            params := &s3.GetObjectInput{
            Bucket: aws.String(bucketName),
            Key: aws.String(keyName),
            }

        out, err := s3Client.GetObject(params)

        if err != nil {
             log.Fatal(err)
        }

        img, err := ioutil.ReadAll(out.Body)
        if err != nil {
            log.Fatal(err)
        }      

        mw := imagick.NewMagickWand()   

        err = mw.ReadImageBlob(img)
        if err != nil {
            log.Fatal(err)
        }

        //Perform resizing and cropping on mw object

        buf := new(bytes.Buffer)
        err = jpeg.Encode(buf, mw, nil)
        sendmw_s3 := buf.Bytes()

         paramsPut := &s3.PutObjectInput{
                        Bucket:         aws.String(masterBucketName),
                        Key:            aws.String(keyName),
                        Body:         bytes.NewReader(sendmw_s3),
                }

        resp, err := s3Client.PutObject(paramsPut)
        if err != nil {
            log.Fatal(err)
        }

Error :

 cannot use mw (type *imagick.MagickWand) as type image.Image in argument to jpeg.Encode:
    *imagick.MagickWand does not implement image.Image (missing At method)

Answer 1

You need to use the func (mw *MagickWand) GetImageBlob() []byte function.

It returns a slice of bytes containing a complete encoded image for the current file format (JPEG, gif, PNG...).

The returned data can therefore be saved to disk, or sent to s3 as-is.

See https://gowalker.org/github.com/gographics/imagick/imagick#MagickWand_GetImageBlob for the documentation.

Answer 2

This question is actually two questions, and @SirDarius answered one of them, by suggesting the use of GetImageBlob(). You can also use SetImageFormat() to change the image format before generating the blob.

For the part about the crop, I am sure there are a bunch of ways to do this with ImageMagick. The way I have done it, to achieve a center crop is to first transform the image so that the smaller dimension fits into my desired target resolution. And then to crop away the parts that overflow.

// Create a new image where smallest dimension is fit
// and the rest overflows the dimensions
size := fmt.Sprintf("%dx%d^+0+0", w, h)
tx := wand.TransformImage("", size)

// Center Crop away the extra parts of the image, to perform
tx.SetImageGravity(imagick.GRAVITY_CENTER)
offsetX := -(int(w) - int(tx.GetImageWidth())) / 2
offsetY := -(int(h) - int(tx.GetImageHeight())) / 2
err := tx.ExtentImage(w, h, offsetX, offsetY)
...