# Fourier Transform of image convoluting with kernel [closed]

edit: clarifying question. ...

## 1 Answer

The function $$\frac{\sin(x)}{x}$$ has a well-known Fourier transform: $$\int_{-\infty}^{\infty} \: \frac{\sin{x}}{x} e^{-j 2 \pi f x} dx = \begin{cases} \pi & |f| < \frac{1}{2 \pi} \\ 0 & |f| > \frac{1}{2 \pi} \\ \end{cases}$$ which is derived here.

Your function:

$$\frac{\sin(x)\sin(y)}{xy}$$

is a simple separable two dimensional version of it. The 2D Fourier transform will just be:

$$\int_{-\infty}^{\infty}\int_{-\infty}^{\infty} \: \frac{\sin{x}}{x} \frac{\sin{y}}{y} e^{-j 2 \pi f_1 x} e^{-j 2 \pi f_2 y} dx dy = \int_{-\infty}^{\infty} \: \frac{\sin{x}}{x} e^{-j 2 \pi f_1 x} dx \int_{-\infty}^{\infty} \:\frac{\sin{y}}{y} e^{-j 2 \pi f_2 y} dy =\begin{cases} \pi & |f_1| \mbox{ and } |f_2| < \frac{1}{2 \pi} \\ 0 & |f_1| \mbox{ or } |f_2| > \frac{1}{2 \pi} \\ \end{cases}$$

Below is a plot that takes a random image with a white square in the middle (top left), FFTs it, multiplies the FFT by a brick filter (top right), inverse FFTs it and plots the result (bottom right).

R Code Below

# 26383

img <- runif(128*128, 0, 1)
dim(img) <- c(128, 128)

img[32:96,32:96] <- 1

N <- 50

imgFFT <- fft(img)
imgFFT[N:(128-N+2),] <- 0.0000000000000
imgFFT[,N:(128-N+2)] <- 0.0000000000000
img2 <- fft(imgFFT, inverse = TRUE) / 128 / 128

img2[Mod(img2) > 1] <- 1

mx <- max(log(Mod(imgFFT + 0.00001)))
mn <- min(log(Mod(imgFFT + 0.00001)))

scaled_log_imgFFT <- (log(Mod(imgFFT + 0.00001)) - mn)/(mx - mn)

par(pty="s") # make the plit square
plot(0:256, type='n')
rasterImage(as.raster(img),0,128,128,256)
rasterImage(as.raster(scaled_log_imgFFT),128,128,256,256)
rasterImage(as.raster(Mod(img2)),128,0,256,128)

• Well "impossible" is too strong a word. You certainly can take the 2D FFT of an image and multiply it by a sampled box function, and take the inverse FFT to get a "low pass" filtered version of it. However, because the $\sin(x)/x$ function is infinite in extent, the result will have time aliasing. – Peter K. Oct 12 '15 at 21:01