// HalveInplace downsamples the image by 50% using averaging interpolation.
func HalveInplace(m image.Image) image.Image {
b := m.Bounds()
switch m := m.(type) {
case *image.YCbCr:
for y := b.Min.Y; y < b.Max.Y/2; y++ {
for x := b.Min.X; x < b.Max.X/2; x++ {
y00 := uint32(m.Y[m.YOffset(2*x, 2*y)])
y10 := uint32(m.Y[m.YOffset(2*x+1, 2*y)])
y01 := uint32(m.Y[m.YOffset(2*x, 2*y+1)])
y11 := uint32(m.Y[m.YOffset(2*x+1, 2*y+1)])
// Add before divide with uint32 or we get errors in the least
// significant bits.
m.Y[m.YOffset(x, y)] = uint8((y00 + y10 + y01 + y11) >> 2)
cb00 := uint32(m.Cb[m.COffset(2*x, 2*y)])
cb10 := uint32(m.Cb[m.COffset(2*x+1, 2*y)])
cb01 := uint32(m.Cb[m.COffset(2*x, 2*y+1)])
cb11 := uint32(m.Cb[m.COffset(2*x+1, 2*y+1)])
m.Cb[m.COffset(x, y)] = uint8((cb00 + cb10 + cb01 + cb11) >> 2)
cr00 := uint32(m.Cr[m.COffset(2*x, 2*y)])
cr10 := uint32(m.Cr[m.COffset(2*x+1, 2*y)])
cr01 := uint32(m.Cr[m.COffset(2*x, 2*y+1)])
cr11 := uint32(m.Cr[m.COffset(2*x+1, 2*y+1)])
m.Cr[m.COffset(x, y)] = uint8((cr00 + cr10 + cr01 + cr11) >> 2)
}
}
b.Max = b.Min.Add(b.Size().Div(2))
return subImage(m, b)
case draw.Image:
for y := b.Min.Y; y < b.Max.Y/2; y++ {
for x := b.Min.X; x < b.Max.X/2; x++ {
r00, g00, b00, a00 := m.At(2*x, 2*y).RGBA()
r10, g10, b10, a10 := m.At(2*x+1, 2*y).RGBA()
r01, g01, b01, a01 := m.At(2*x, 2*y+1).RGBA()
r11, g11, b11, a11 := m.At(2*x+1, 2*y+1).RGBA()
// Add before divide with uint32 or we get errors in the least
// significant bits.
r := (r00 + r10 + r01 + r11) >> 2
g := (g00 + g10 + g01 + g11) >> 2
b := (b00 + b10 + b01 + b11) >> 2
a := (a00 + a10 + a01 + a11) >> 2
m.Set(x, y, color.RGBA{
R: uint8(r >> 8),
G: uint8(g >> 8),
B: uint8(b >> 8),
A: uint8(a >> 8),
})
}
}
b.Max = b.Min.Add(b.Size().Div(2))
return subImage(m, b)
default:
// TODO(wathiede): fallback to generic Resample somehow?
panic("Unhandled image type")
}
}
// ResampleInplace will resample m inplace, overwritting existing pixel data,
// and return a subimage of m sized to w and h.
func ResampleInplace(m image.Image, r image.Rectangle, w, h int) image.Image {
// We don't support scaling up.
if r.Dx() < w || r.Dy() < h {
return m
}
switch m := m.(type) {
case *image.YCbCr:
xStep := float64(r.Dx()) / float64(w)
yStep := float64(r.Dy()) / float64(h)
for y := r.Min.Y; y < r.Min.Y+h; y++ {
for x := r.Min.X; x < r.Min.X+w; x++ {
xSrc := int(float64(x) * xStep)
ySrc := int(float64(y) * yStep)
cSrc := m.COffset(xSrc, ySrc)
cDst := m.COffset(x, y)
m.Y[m.YOffset(x, y)] = m.Y[m.YOffset(xSrc, ySrc)]
m.Cb[cDst] = m.Cb[cSrc]
m.Cr[cDst] = m.Cr[cSrc]
}
}
case draw.Image:
xStep := float64(r.Dx()) / float64(w)
yStep := float64(r.Dy()) / float64(h)
for y := r.Min.Y; y < r.Min.Y+h; y++ {
for x := r.Min.X; x < r.Min.X+w; x++ {
xSrc := int(float64(x) * xStep)
ySrc := int(float64(y) * yStep)
r, g, b, a := m.At(xSrc, ySrc).RGBA()
m.Set(x, y, color.RGBA{
R: uint8(r >> 8),
G: uint8(g >> 8),
B: uint8(b >> 8),
A: uint8(a >> 8),
})
}
}
default:
// TODO fallback to generic Resample somehow?
panic("Unhandled image type")
}
r.Max.X = r.Min.X + w
r.Max.Y = r.Min.Y + h
return subImage(m, r)
}