Fred's ImageMagick Scripts



    Licensing:

    Copyright © Fred Weinhaus

    My scripts are available free of charge for non-commercial use, ONLY.

    For use of my scripts in commercial (for-profit) environments or non-free applications, please contact me (Fred Weinhaus) for licensing arrangements. My email address is fmw at alink dot net.

    If you: 1) redistribute, 2) incorporate any of these scripts into other free applications or 3) reprogram them in another scripting language, then you must contact me for permission, especially if the result might be used in a commercial or for-profit environment.

    Usage, whether stated or not in the script, is restricted to the above licensing arrangements. It is also subject, in a subordinate manner, to the ImageMagick license, which can be found at: http://www.imagemagick.org/script/license.php

FFTFILTER


Performs filtering on an image in the frequency domain.

Download Script

last modified: September 12, 2015



USAGE: fftfilter infile filtfile outfile
USAGE: fftfilter [-h or -help]

No options are required.

PURPOSE: To perform convolution on an image in the frequency domain.

DESCRIPTION: FFTFILTER performs filtering on an image in the frequency domain using a frequency domain filter image. Two inputs are required. The image and a grayscale frequency domain filter. The image is transformed to the frequency domain using -fft and the filter image is then multiplied with the Fourier transform of the image and the product is then returned to the spatial domain using -ift. Any alpha channel on the filter will be removed automatically before processing. If the image has an alpha channel it will not be processed, but simply copied from the input to the output.

ARGUMENTS:

None

The filter image must be appropriately centered and padded with black to the same size as the input image.

REQUIREMENTS: IM version 6.5.4-7 or higher. HDRI is not required, but Q8 compilations of IM are not recommended as it will not carry enough precision. Also requires the FFTW delegate library.

LIMITATIONS: This script works well only with even, square images. Otherwise, the FFT will pad them with black to conform. However, there will be excessive ringing due to the color discontinuity associated with the padding. This even, square limitation is a ramification of the current IM implementation that needs addressing at some future time. It is not a limitation of FFTW.

See Fourier Transform with ImageMagick, for more details.

CAVEAT: No guarantee that this script will work on all platforms, nor that trapping of inconsistent parameters is complete and foolproof. Use At Your Own Risk.


EXAMPLES


Gaussian Blurring - Low Pass Filtering

Input

Filter
convert -size 128x128 xc:black \
-fill white -draw "point 64,64" \
-alpha off -blur 0x8 \
-contrast-stretch 0 gaussian8.png

Arguments:
(none)



Gaussian Edge Detection - High Pass Filtering

Input

Filter
convert -size 128x128 xc:black \
-fill white -draw "point 64,64" \
-alpha off -blur 0x8 -contrast-stretch 0 \
-negate gaussian8_neg.png

Arguments:
(none)

Post Processing:
-normalize



Simple Binary Notch Filtering

Input

Spectrum:
(see script, spectrum)

Masked Spectrum:

Filter:
Binarized, Masked
Spectrum:
-fill white +opaque black

Arguments:
(none)



Notch Filtering With Tapering

Input

Spectrum:
(see script, spectrum)

Masked Spectrum:

Filter:
Binarize And Apply
Linear Taper
-fill white +opaque black \
-blur 5x65000

Arguments:
(none)



What the script does is as follows:

  • Computes the magnitude and phase of the Fourier transform
    of the image
  • Multiplies the filter by the magnitude component
  • Converts the magnitude product with the original phase back
    to the spatial domain

This is equivalent to the following IM commands.

  • width=`identify -ping -format "%w" $infile`
  • height=`identify -ping -format "%h" $infile`
  • cx=`convert xc: -format "%[fx:floor(($width+1)/2)]" info:`
  • cy=`convert xc: -format "%[fx:floor(($height+1)/2)]" info:`
  • convert \( $tmpA1 -fft \) \
    \( -clone 0 $tmpA2 -compose multiply -composite \) \
    -delete 0 +swap -ift \
    -crop ${width}x${height}+0+0 +repage $outfile