how to read a png file in color and output as gray scale using the Go programming language?

Question

How do I read in a color .png file in the Go programming language, and output it as an 8-bit grayscale image?

Answer 1

I had this problem myself and came up with a slightly different solution. I introduced a new type, Converted, which implements image.Image. Converted consists of the original image and the color.Model.

Converted does the conversion every time it is accessed, which could give slightly worse performance, but on the other hand it is cool and composable.

package main

import (
    "image"
    _ "image/jpeg" // Register JPEG format
    "image/png"    // Register PNG  format
    "image/color"
    "log"
    "os"
)

// Converted implements image.Image, so you can
// pretend that it is the converted image.
type Converted struct {
    Img image.Image
    Mod color.Model
}

// We return the new color model...
func (c *Converted) ColorModel() color.Model{
    return c.Mod
}

// ... but the original bounds
func (c *Converted) Bounds() image.Rectangle{
    return c.Img.Bounds()
}

// At forwards the call to the original image and
// then asks the color model to convert it.
func (c *Converted) At(x, y int) color.Color{
    return c.Mod.Convert(c.Img.At(x,y))
}

func main() {
    if len(os.Args) != 3 { log.Fatalln("Needs two arguments")}
    infile, err := os.Open(os.Args[1])
    if err != nil {
        log.Fatalln(err)
    }
    defer infile.Close()

    img, _, err := image.Decode(infile)
    if err != nil {
        log.Fatalln(err)
    }

    // Since Converted implements image, this is now a grayscale image
    gr := &Converted{img, color.GrayModel}
    // Or do something like this to convert it into a black and
    // white image.
    // bw := []color.Color{color.Black,color.White}
    // gr := &Converted{img, color.Palette(bw)}


    outfile, err := os.Create(os.Args[2])
    if err != nil {
        log.Fatalln(err)
    }
    defer outfile.Close()

    png.Encode(outfile,gr)
}

Answer 2

The program below takes an input file name and an output file name. It opens the input file, decodes it, converts it to grayscale, then encodes it to the output file.

Thie program isn't specific to PNGs, but to support other file formats you'd have to import the correct image package. For example, to add JPEG support you could add to the imports list _ "image/jpeg".

If you only want to support PNG, then you can use image/png.Decode directly instead of image.Decode.

package main

import (
    "image"
    "image/png" // register the PNG format with the image package
    "os"
)

func main() {
    infile, err := os.Open(os.Args[1])
    if err != nil {
        // replace this with real error handling
        panic(err.String())
    }
    defer infile.Close()

    // Decode will figure out what type of image is in the file on its own.
    // We just have to be sure all the image packages we want are imported.
    src, _, err := image.Decode(infile)
    if err != nil {
        // replace this with real error handling
        panic(err.String())
    }

    // Create a new grayscale image
    bounds := src.Bounds()
    w, h := bounds.Max.X, bounds.Max.Y
    gray := image.NewGray(w, h)
    for x := 0; x < w; x++ {
        for y := 0; y < h; y++ {
            oldColor := src.At(x, y)
            grayColor := image.GrayColorModel.Convert(oldColor)
            gray.Set(x, y, grayColor)
        }
    }

    // Encode the grayscale image to the output file
    outfile, err := os.Create(os.Args[2])
    if err != nil {
        // replace this with real error handling
        panic(err.String())
    }
    defer outfile.Close()
    png.Encode(outfile, gray)
}