// CorrBankFFT computes the correlation of an image with a bank of filters.
// h_p[u, v] = (f corr g_p)[u, v]
func CorrBankFFT(f *rimg64.Image, g *Bank) (*rimg64.Multi, error) {
out := ValidSize(f.Size(), g.Size())
if out.X <= 0 || out.Y <= 0 {
return nil, nil
}
// Determine optimal size for FFT.
work, _ := FFT2Size(f.Size())
// Re-use FFT of image.
fhat := fftw.NewArray2(work.X, work.Y)
copyImageTo(fhat, f)
fftw.FFT2To(fhat, fhat)
// Transform of each filter.
curr := fftw.NewArray2(work.X, work.Y)
fwd := fftw.NewPlan2(curr, curr, fftw.Forward, fftw.Estimate)
defer fwd.Destroy()
bwd := fftw.NewPlan2(curr, curr, fftw.Backward, fftw.Estimate)
defer bwd.Destroy()
h := rimg64.NewMulti(out.X, out.Y, len(g.Filters))
alpha := complex(1/float64(work.X*work.Y), 0)
// For each output channel.
for p, gp := range g.Filters {
// Take FFT.
copyImageTo(curr, gp)
fwd.Execute()
// h_p[x] = (G_p corr F)[x]
// H_p[x] = conj(G_p[x]) F[x]
scaleMul(curr, alpha, curr, fhat)
bwd.Execute()
copyRealToChannel(h, p, curr)
}
return h, nil
}
// CorrFFT computes the correlation of an image with a filter.
// h[u, v] = (f corr g)[u, v]
func CorrFFT(f, g *rimg64.Image) (*rimg64.Image, error) {
out := ValidSize(f.Size(), g.Size())
if out.X <= 0 || out.Y <= 0 {
return nil, nil
}
// Determine optimal size for FFT.
work, _ := FFT2Size(f.Size())
fhat := fftw.NewArray2(work.X, work.Y)
ghat := fftw.NewArray2(work.X, work.Y)
// Take forward transforms.
copyImageTo(fhat, f)
fftw.FFT2To(fhat, fhat)
copyImageTo(ghat, g)
fftw.FFT2To(ghat, ghat)
// Scale such that convolution theorem holds.
n := float64(work.X * work.Y)
scaleMul(fhat, complex(1/n, 0), ghat, fhat)
// Take inverse transform.
h := rimg64.New(out.X, out.Y)
fftw.IFFT2To(fhat, fhat)
copyRealTo(h, fhat)
return h, nil
}
// CorrMultiBankFFT computes the correlation of
// a multi-channel image with a bank of multi-channel filters.
// h_p[u, v] = sum_q (f_q corr g_pq)[u, v]
func CorrMultiBankFFT(f *rimg64.Multi, g *MultiBank) (*rimg64.Multi, error) {
out := ValidSize(f.Size(), g.Size())
if out.X <= 0 || out.Y <= 0 {
return nil, nil
}
// Determine optimal size for FFT.
work, _ := FFT2Size(f.Size())
// Cache FFT of each channel of image.
fhat := make([]*fftw.Array2, f.Channels)
for i := range fhat {
fhat[i] = fftw.NewArray2(work.X, work.Y)
copyChannelTo(fhat[i], f, i)
fftw.FFT2To(fhat[i], fhat[i])
}
curr := fftw.NewArray2(work.X, work.Y)
fwd := fftw.NewPlan2(curr, curr, fftw.Forward, fftw.Estimate)
defer fwd.Destroy()
sum := fftw.NewArray2(work.X, work.Y)
bwd := fftw.NewPlan2(sum, sum, fftw.Backward, fftw.Estimate)
defer bwd.Destroy()
h := rimg64.NewMulti(out.X, out.Y, len(g.Filters))
alpha := complex(1/float64(work.X*work.Y), 0)
// For each output channel.
for p, gp := range g.Filters {
zero(sum)
// For each input channel.
for q := 0; q < f.Channels; q++ {
// Take FFT of this input channel.
copyChannelTo(curr, gp, q)
fwd.Execute()
// h_p[x] = (G_qp corr F_p)[x]
// H_p[x] = conj(G_qp[x]) F_p[x]
addScaleMul(sum, alpha, curr, fhat[q])
}
bwd.Execute()
copyRealToChannel(h, p, sum)
}
return h, nil
}
