使用image.Image或* image.RGBA的Set（）方法

Question

I am doing some practice with the Go image package with my free time this summer.

package main

import (

    "os"
    "image"
    "image/png"
    "image/color"
    "log"
    "fmt"
    "reflect"

)

func main(){

    file , err := os.OpenFile("C:/Sources/go3x3.png", os.O_RDWR, os.FileMode(0777))
    if err != nil {
        log.Fatal(err)
    }

    img , err := png.Decode(file)
    if err != nil {
        log.Fatal(err)
    }


    img.At(0,0).RGBA()
    fmt.Println("type:", reflect.TypeOf(img))

    m := image.NewRGBA(image.Rect(0, 0, 640, 480))
    fmt.Println("type:", reflect.TypeOf(m))
    m.Set(5, 5, color.RGBA{255, 0, 0, 255})
    img.Set(0, 0, color.RGBA{136, 0, 21, 255})
}

The problem here is when I run it with the img.Set commented out I get this result

type: *image.RGBA
type: *image.RGBA

but when it's uncommented I get an error saying

img.Set undefined (type image.Image has no field or method Set)

I'm assuming I'm using reflect wrong, I'm still fully grasping the whole interface and type definitions in Go.

Answer 1

reflect.TypeOf(img) gives you the reflection type of the value in the interface img, if its an interface. In this case, img is an interface, an image.Image which contains an *image.RGBA.

You can fix your code by converting img to an *image.RGBA, or more robustly, define an interface type with the right Set method, and convert img to that (the draw.Image interface in "image/draw" works perfectly for this, as noted by @DaveC). The interface type is preferable if you aren't sure that png.Decode will always give you an *image.RGBA for the .png files you have.

img.(*image.RGBA).Set(0, 0, color.RGBA{136, 0, 21, 255})

or

type Setter interface {
    Set(x, y int, c color.Color)
}

img.(Setter).Set(0, 0, color.RGBA{136, 0, 21, 255})

or (probably best):

import "image/draw"

...

img.(draw.Image).Set(0, 0, color.RGBA{136, 0, 21, 255})