changing pixel value, saving and reading again returns the original color

Question

I want to change all Blue values of pixels to 255, if it is equal to 20. I read the source image, draw.Draw it to new image.RGBA, so that i can modify pixels.

But, when I take output image(after executing the program) and feed it as the input, and put a debug point inside the IF block, and run program in debug mode, i see in multiple points debugger stops inside there. Which means, I am not correctly modifying image.

Can anyone tell me, how can I modify pixels and save correctly? Thanks a lot

func changeOnePixelInImage() {
    imgPath := "./source.png"
    f, err := os.Open(imgPath)
    check(err)
    defer f.Close()
    sourceImage, _, err := image.Decode(f)

    size := sourceImage.Bounds().Size()
    destImage := image.NewRGBA(sourceImage.Bounds())
    draw.Draw(destImage, sourceImage.Bounds(), sourceImage, image.Point{}, draw.Over)

    for x := 0; x < size.X; x++ {
        for y := 0; y < size.Y; y++ {
            pixel := sourceImage.At(x, y)
            originalColor := color.RGBAModel.Convert(pixel).
            (color.RGBA)

            b := originalColor.B

            if b == 20 {
                b = 255 // <--- then i swap source and destination paths, and debug this line
            }

            c := color.RGBA{
                R: originalColor.R,
                G: originalColor.G,
                B: b,
                A: originalColor.A,
            }
            destImage.SetRGBA(x, y, c)
        }
    }
    ext := filepath.Ext(imgPath)
    newImagePath := fmt.Sprintf("%s/dest%s", filepath.Dir(imgPath), ext)
    fg, err := os.Create(newImagePath)
    check(err)
    defer fg.Close()
    err = jpeg.Encode(fg, destImage, &jpeg.Options{100})
    check(err)
}

Answer 1

I found the answer to my question. The thing is, I was Decoding jpeg image, and I found out that JPEG images loss quality(so, pixel values are modified during the process) from this stackoverflow question: Is JPEG lossless when quality is set to 100?

So, I should have used PNG images.(and even though I am using source.png as source image, it was actually jpg image :/)

So i changes last lines to:

if ext != ".png" {
    panic("cannot do my thing with jpg images, since they get compressed")
}
err = png.Encode(fg, destImage)