func minTransparency(image *vips.Image) (float64, error) {
if !image.HasAlpha() {
return 1.0, nil
}
band, err := image.Copy()
if err != nil {
return 0, err
}
defer band.Close()
if err := band.ExtractBand(band.ImageGetBands()-1, 1); err != nil {
return 0, err
}
// If all pixels are at least 90% opaque, we can flatten.
min, err := band.Min()
if err != nil {
return 0, err
}
return min / band.MaxAlpha(), nil
}