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.
To expand on a previous answer answer:
png.Decode
may create one of several different underlying image types (*image.Gray
, *image.RGBA
, *image.Paletted
, *image.NRGBA
, etc).
It returns whatever image it created as an image.Image
interface which provides read only access to the data.
However, all (most?) of the actual image types it returns do implement the Set
method for simple write access.
The way you can safely test for and use this method is via the existing draw.Image
interface from the image/draw
package. It's just this:
// From image/draw:
// Image is an image.Image with a Set method to change a single pixel.
type Image interface {
image.Image
Set(x, y int, c color.Color)
}
So you could do something like:
func drawablePNGImage(r io.Reader) (draw.Image, error) {
img, err := png.Decode(r)
if err != nil {
return nil, err
}
dimg, ok := img.(draw.Image)
if !ok {
return nil, fmt.Errorf("%T is not a drawable image type", img)
}
return dimg, nil
}
Playground (shows an example calling all the image.Image
methods as well as Set
).
Edit for Go1.17+:
Note that Go1.17 added draw.RGBA64Image
with a SetRGBA64
method. As with draw.Image
, all of the standard image types implement this. The advantage of this method is that the color values are not boxed in the color.Color
interface type so doing many pixel operations can be faster.
Also note that Go1.18 added optimisations to the draw.Draw
and draw.DrawMask
fallback implementations for images that implement the optional draw.RGBA64Image
and image.RGBA64Image
interfaces.
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