Get image size with golang

I'm new to golang and I'm trying to get the image size of all the images listed in a directory. That's what I've done

package main

import (
	"fmt"
	"image"
	_ "image/jpeg"
	"io/ioutil"
	"os"
)

const dir_to_scan string = "/home/da/to_merge"

func main() {
	files, _ := ioutil.ReadDir(dir_to_scan)
	for _, filepath := range files {

		if reader, err := os.Open(filepath.Name()); err != nil {
			defer reader.Close()
			im, _, err := image.DecodeConfig(reader)
			if err != nil {
				fmt.Fprintf(os.Stderr, "%s: %v\n", filepath.Name(), err)
				continue
			}
			fmt.Printf("%s %d %d\n", filepath.Name(), im.Width, im.Height)
		} else {
			fmt.Println("Impossible to open the file")
		}
	}
}

I've an error when it comes to image.DecodeConfig, that says image: unknown format
Has someone an idea about the proper way to do it?
In the docs here http://golang.org/src/pkg/image/format.go?s=2676:2730#L82 says that i should pass a io.Reader as argument, and that's what i'm doing.

There are two problems with your code.

The first one is that you have inverted the test err != nil, so you try to decode the image only in the case where you have an error. It should be err == nil.

The second one, as said by jimt, is that you use filepath.Name(), which contains only the file name in os.Open(), this is what makes err to always be set, thus always entering in the if, and decoding a file that doesn't exists.

Here is the corrected code :

package main

import (
    "fmt"
    "image"
    _ "image/jpeg"
    "io/ioutil"
    "os"
    "path/filepath"
)

const dir_to_scan string = "/home/da/to_merge"

func main() {
    files, _ := ioutil.ReadDir(dir_to_scan)
    for _, imgFile := range files {

        if reader, err := os.Open(filepath.Join(dir_to_scan, imgFile.Name())); err == nil {
            defer reader.Close()
            im, _, err := image.DecodeConfig(reader)
            if err != nil {
                fmt.Fprintf(os.Stderr, "%s: %v\n", imgFile.Name(), err)
                continue
            }
            fmt.Printf("%s %d %d\n", imgFile.Name(), im.Width, im.Height)
        } else {
            fmt.Println("Impossible to open the file:", err)
        }
    }
}

Also, don't forget to add other imports than image/jpeg if you have other images formats in your dir.

In case if you want to get width and height of image by use image.Decode(reader) without image.DecodeConfig

m, _, err := image.Decode(reader)
if err != nil {
	log.Fatal(err)
}
bounds := m.Bounds()
w := bounds.Dx()
h := bounds.Dy()

fmt.Printf("width: %d, height: %d\n", w, h)