Golang PNG颜色处理不会达到255

Question

The image I'm using as image.png is:

When running my program with 254, it works and the output is:

Here is my code. If I change the 254 to 255, the output file becomes completely empty (0 bytes in size):

package main

import "fmt"
import "os"
import "bufio"
import "image/png"
import "image"

func main() {
    f, _ := os.Open("image.png")
    im, _ := png.Decode(bufio.NewReader(f))
    img := im.(*image.RGBA)
    s := img.Bounds().Size()
    w := s.X
    h := s.Y
    fmt.Println(w, h)
    newPix := make([]uint8, len(img.Pix))
    iy := 0
    for ; iy < h; iy++ {
        ix := 0
        for ; ix < w; ix++ {
            i := img.PixOffset(ix, iy)
            r := img.Pix[i+0]
            g := img.Pix[i+1]
            b := img.Pix[i+2]
            a := img.Pix[i+3]
            var max uint8 = 254 /*255*/
            newPix[i+0] = uint8(max - r)
            newPix[i+1] = uint8(max - g)
            newPix[i+2] = uint8(max - b)
            newPix[i+3] = uint8(max - a)
        }
    }
    newImg := image.NewRGBA(img.Bounds())
    newImg.Pix = newPix
    o, _ := os.Create("out.png")
    defer o.Close()
    png.Encode(bufio.NewWriter(o), newImg)
    fmt.Println("DONE")
}

Why is this and how can I fix it? 255 is still uint8 so it should work...

Answer 1

I found a discussion on the golang-nuts group where this issue had come up before.

Basically, the output is too small to automatically trigger the writer to flush. Manually flushing the writer (recommended in any case) fixes the issue.

package main

import "fmt"
import "os"
import "bufio"
import "image/png"
import "image"

func main() {
    f, _ := os.Open("image.png")
    im, _ := png.Decode(bufio.NewReader(f))
    img := im.(*image.RGBA)
    s := img.Bounds().Size()
    w := s.X
    h := s.Y
    fmt.Println(w, h)
    newPix := make([]uint8, len(img.Pix))
    iy := 0
    for ; iy < h; iy++ {
        ix := 0
        for ; ix < w; ix++ {
            i := img.PixOffset(ix, iy)
            r := img.Pix[i+0]
            g := img.Pix[i+1]
            b := img.Pix[i+2]
            a := img.Pix[i+3]
            var max uint8 = 255
            newPix[i+0] = uint8(max - r)
            newPix[i+1] = uint8(max - g)
            newPix[i+2] = uint8(max - b)
            newPix[i+3] = uint8(max - a)
        }
    }
    newImg := image.NewRGBA(img.Bounds())
    newImg.Pix = newPix
    o, _ := os.Create("out.png")
    defer o.Close()
    writer := bufio.NewWriter(o)
    png.Encode(writer, newImg)
    writer.Flush()
    fmt.Println("DONE")
}

Normal caveats regarding ignoring errors apply.