func resize(image *vips.Image, iw, ih int, fastResize bool, blurSigma float64, sharpen bool) error {
m := format.MetadataImage(image)
// Interpolation of RGB values with an alpha channel isn't safe
// unless the values are pre-multiplied. Undo this later.
// This also flattens fully transparent pixels to black.
premultiply := image.HasAlpha()
if premultiply {
if err := image.Premultiply(); err != nil {
return err
}
}
// A box filter will quickly get us within 2x of the final size, at some quality cost.
if fastResize {
// Shrink factors can be passed independently here, which
// may not be sane since Resize()'s blur and sharpening
// steps expect a normal aspect ratio.
wshrink := math.Floor(float64(m.Width) / float64(iw))
hshrink := math.Floor(float64(m.Height) / float64(ih))
if wshrink >= 2 || hshrink >= 2 {
// Shrink rounds down the number of pixels.
if err := image.Shrink(wshrink, hshrink); err != nil {
return err
}
m = format.MetadataImage(image)
}
}
// If necessary, do a high-quality resize to scale to final size.
if iw < m.Width || ih < m.Height {
// Vips 8.3 sometimes produces 1px smaller images than desired without the rounding help here.
if err := image.Resize((float64(iw)+vips.ResizeOffset)/float64(m.Width), (float64(ih)+vips.ResizeOffset)/float64(m.Height)); err != nil {
return err
}
}
if blurSigma > 0.0 {
if err := image.Gaussblur(blurSigma); err != nil {
return err
}
}
if sharpen {
if err := image.MildSharpen(); err != nil {
return err
}
}
// Unpremultiply after all operations that touch adjacent pixels.
if premultiply {
if err := image.Unpremultiply(); err != nil {
return err
}
}
return nil
}