在Go中修改gif图片...无法正确设置透明度

I have some code which performs the following logical operations:

• Read in and decode a gif image to a *GIF using gif.DecodeAll
• Modify some pixels in each frame of the *GIF using image.Set
• Write out the resulting modified image using gif.EncodeAll

Here's some code snippets to help demonstrate what the code is doing (error handling, file closing, etc removed for brevity):

f, err := os.Open(filename)
reader := bufio.NewReader(f)
g, err := gif.DecodeAll(reader)
err = modify_image(g)

of, err := os.Create("out.gif")
writer := bufio.NewWriter(of)
err = gif.EncodeAll(writer, g)

Here's the modify_image function:

func modify_image(img *gif.GIF) error {
    for i := 0; i < len(img.Image); i++ {
        err := modify_frame(img.Image[i])
    }
    return nil
}

And modify_frame:

func modify_frame(frame *image.Paletted) error {
    xmin := frame.Rect.Min.X
    ymin := frame.Rect.Min.Y
    xmax := frame.Rect.Max.X
    ymax := frame.Rect.Max.Y

    for y := ymin; y < ymax; y++ {
        for x := xmin; x < xmax; x++ {
            if should_turn_pixel_transparent(frame, x, y) {
                frame.Set(x, y, color.RGBA64{0, 0, 0, 0})
            }
        }
    }
    return nil
}

The out.gif that this code produces has the correct pixels turned transparent, but as the animation proceeds, the pixels which I turned transparent are not "clearing"; i.e. as these transparent pixels are written over non-transparent pixels, the non-transparent pixels underneath are still displayed.

My (brief) understanding is that there are two different methods for representing transparency in gifs. I don't know if I need to use index transparency versus alpha transparency, or if I'm just doing things entirely wrong. Any advice would be appreciated.