2D Power Spectrum

Overview

The program estimates the 2D power spectra for each input section by performing one or more forward transforms, taking their magnitude, and averaging the results. This can then be smoothed if desired.

The controls in the user interface include a standard set of controls for selecting the input file, output file, and region of interest and for starting and interrupting processing. The other controls are organized as follows:

Use the button labeled "Set preprocessing" to display and change the parameters which control the processing of the data before it is split into blocks and transformed. By default, no preprocessing is done.
Use the button labeled "Set averaging/smoothing" to open a dialog with the settings for how many blocks a 2D input region is broken into and how much smoothing is applied to the average result.
Use the button labeled "Set window/taper" to select a function used to smooth out discontinuities at the edges of the data after it has been preprocessed and split into blocks. By default, no smoothing is done.
Use the button labeled "Set output format" to open a dialog with controls for the form of the output: whether or not the log of the power is taken, and whether or not the result is shifted to center the zero frequency component.

There are a couple things to note about this application:

The pixel format for the output is always floating-point reals.
For the x and y dimensions, the pixel spacings are nxs / (nx * dx) and nys / (ny * dy) where nx and ny are the dimensions, in pixels, of the block transformed, dx and dy are the input pixel spacings, and nxs and nys are the dimensions of the smoothing box in pixels.
The x and y start indices in the header are set to zero. The x and y coordinates for the origin are set to zero if the zero frequency is not centered; otherwise, they are set to the most negative frequencies present, in units of the output pixel spacing. The z start index and z coordinate for the origin are not modified.

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Preprocessing

The input region can be processed to remove an offset or trend before the data is broken into blocks, tapered, and transformed. To display the controls for this processing, press the button labeled Set preprocessing in the main dialog. There are five types of processing that can be done; select one by pressing the appropriate toggle in the dialog. The processing options are:

None: The input is not modified before the data is split into blocks, tapered, and transformed.
Subtract offset: The specified value is subtracted from all the input values.
Subtract mean: For each 2D (xy) input region, the mean of the region is subtracted from all values in the region.
Remove linear trend: An average slope for each 2D (xy) input region is estimated by averaging values from the first and last third of each dimension; the linear trend corresponding to that average slope and the mean of the region is subtracted from the input data. The algorithm used is the average slope method extended to two dimensions (Digital data analysis procedures, Bendat and Piersol, 1st Edition, 1971, page 288).
Remove trends up to order: A polynomial of the given order is fit to each 2D (xy) input region. That polynomial evaluated at each input point is then subtracted from the input before the data is split into blocks, tapered, and transformed.

If you are intending to taper the data and the data does not already have a zero mean, you should consider using the mean subtraction, linear trend removal, or polynomial fit options.

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Averaging/Smoothing

This application provides two ways to reduce the variance of the spectra estimate: breaking the input region into blocks which are handled separately and then averaged and smoothing of the spectra.

Breaking the input into blocks is controlled by the parameters of the top half of the dialog opened with the Set averaging/smoothing button. The inputs are the x and y size in pixels for each block, the percent of the block that overlaps with the block to the left and to the bottom, and the number of blocks in x and y. When the Automatically adjust for best coverage toggle is on, changing the overlap or size will cause the count to be adjusted so there is as many blocks as possible, and when the number of blocks is adjusted, the size will be changed so that it is an efficiently handled size and as much of the input region is covered as possible. When the toggle is off, the size, overlap, and count can be varied independently within the constraint that the blocks must all fit in the input region. When breaking the input into blocks, a common approach is to fully taper each block and have a 50% overlap.

The amount of smoothing done is set in the Size field at the bottom of the dialog. Each output pixel is the summation of the unsmoothed result over a rectangle of the size shown. The size in each direction must be a positive odd integer. To ensure that a purely real input results in a spectra with the correct symmetry about the origin, choose the smoothing size for a dimension such that

  (floor(n / 2) - floor(s / 2) + s - 1) / s

is even where n is the block size in that dimension.

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Window/taper

Applying a windowing function, also called tapering, multiplies the data by a function which goes to zero (or near zero) at the edges of the data. This is done after preprocessing and splitting the data into blocks to remove discontinuities at the edges which can complicate the interpretation of frequency spectra; the disadvantage of applying a window is that it essentially throws away parts of the input and it broadens features in the spectra. The former problem can be alleviated by overlapping the blocks that are transformed.

The windowing functions used are separable, i.e, the windowing function, W(x,y), can be written as the product of WX(x) and WY(y). WX and WY are calculated from the same formula but with different inputs for the number of pixels, n, and the fraction, f, of the dimension that is attenuated by the windowing function. The two values in the Coverage field are the values of f divided by two for the x and y directions respectively. Commonly used values for the coverage are 0.5 (a "full taper") and 0.1 (a 10% taper). The power, x, is used by half-sine windowing function.

The functions used to calculate the windowing functions are shown below. i is the pixel index and runs from 0 to n minus 1, and e is given by

      { floor(0.5 + f / 2.0 * n)          n even
  e = {
      { floor(0.5 + f / 2.0 * (n - 1))    n odd

Rectangular

         { 0     i < e
  w(i) = { 1     e <= i <= n - e
         { 0     i > n - e

Because of the sharp cutoffs of the rectangular window, it is not particularly useful; it's provided for completeness.

Triangular

         { i / e           i < e
  w(i) = { 1               e <= i <= n - e
         { (n - i) / e     i > n - e

Half-sine^power

         { (sin(pi * i / (2 * e)))^x           i < e
  w(i) = { 1                                   e <= i <= n - e
         { (sin(pi * (n - i) / (2 * e)))^x     i > n - e

Hamming

         { 0.54 - 0.46 * cos(pi * i / e)           i < e
  w(i) = { 1                                       e <= i <= n - e
         { 0.54 - 0.46 * cos(pi * (n - i) / e)     i > n - e

Blackman

         { cos(pi * i / e)           i < e
  a(i) = { -1                        e <= i <= n - e
         { cos(pi * (n - i) / e)     i > n - e

  w(i) = 0.34 - a(i) * (0.5 - a(i) * 0.16)

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Output format

The output spectra is scaled so that summing over the spectra gives an estimate of the variance of the input data. By default the frequencies are ordered so that zero frequency is at (0, 0) and positive Nyquist frequency in both directions is located at (nx/2, ny/2) where nx and ny are the sizes of a block. Use the button labeled Set output format in the main dialog to open a dialog with controls that allow modifications to the default format. The available options are:

Center zero frequency: The output is shifted so zero frequency is at ((nx-1)/2, (ny-1)/2).
Take log10 of result: The logarithm base 10 of the normally scaled result is reported.

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Command line

FPwrSpec2D accepts the command-line arguments described in Region.html. In addition, FPwrSpec2D accepts the options shown below (optional parts are shown in brackets). If mutually exclusive options (i.e. for selection of the windowing function), the last option that appears on the command line takes precedence. The default is to calculate the power spectrum without any preprocessing, no tapering of the data, and no averaging or smoothing.

-subtract=value or -subtract=mean: Specifies that the input data should be preprocessed by subtracting value or the mean.
-linear_detrend: Specifies that the input data should be preprocessed by subtracting the linear trend which corresponds to the estimated average slope.
-polyfit=n: Specifies that the input data should be preprocessed by subtracting the least-squares fit polynomial of order n. Valid values of n range from one to 10.
-block_size=nx:ny: Specifies the dimensions of the blocks which are extracted from the preprocessed input region, used as the source of the spectra calculation, and then averaged. The default is to use a block the size of the input region.
-block_overlap=x_overlap:y_overlap: Specifies the overlap, in percent, of the blocks. The default is to not overlap the blocks.
-block_count=nx:ny: Specifies the number of blocks in the various dimensions. The default is to use one block.
-smooth_size=nx:ny: Specifies the size of the smoothing filter applied to the spectra after averaging over the blocks. nx and ny must be odd positive integers. The default is to use a smoothing filter with dimensions of one, equivalent to no smoothing at all.
-rectangular=x_coverage:y_coverage: Selects a rectangular windowing function that attenuates 200 times x_coverage percent of the data in x and 200 times y_coverage percent of the data in y. Valid values for x_coverage and y_coverage are between 0 and 0.5.
-triangular=x_coverage:y_coverage: Selects a triangular windowing function that attenuates 200 times x_coverage percent of the data in x and 200 times y_coverage percent of the data in y. Valid values for x_coverage and y_coverage are between 0 and 0.5.
-halfsine=x_coverage:y_coverage: Selects a half-sine windowing function (consult the Window/taper topic for details) that attenuates 200 times x_coverage percent of the data in x and 200 times y_coverage percent of the data in y. Valid values for x_coverage and y_coverage are between 0 and 0.5. Use -halfsine_power to set the exponent, which controls the sharpness of the window edges.
-halfsine_power=p: Sets the exponent for the half-sine windowing function. This option is ignored when other windowing functions are used. If this option is not explicitly specified when the half-sine windowing function is used, the exponent used is one.
-hamming=x_coverage:y_coverage: Selects a Hamming window function (consult the Window/taper topic for details) that attenuates 200 times x_coverage percent of the data in x and 200 times y_coverage percent of the data in y. Valid values for x_coverage and y_coverage are between 0 and 0.5.
-blackman=x_coverage:y_coverage: Selects a Blackman window function (consult the Window/taper topic for details) that attenuates 200 times x_coverage percent of the data in x and 200 times y_coverage percent of the data in y. Valid values for x_coverage and y_coverage are between 0 and 0.5.
-center_zero: Causes the output to be shifted so zero frequency appears at the center of the image.
-log10: Causes the logarithm base 10 of the power spectrum to be written as the result.

As an example, the following estimates the power spectrum from noise.dat which is assumed to be at least 512 x 512. The input is broken into 9 blocks of 256 x 256, the blocks are individually tapered with a triangular windowing function, and the results (after centering the spectra and taking the logarithm base 10) are written to window 1.

    FPwrSpec2D noise.dat 1 -block_size=256:256 -block_count=3:3 \
        -block_overlap=50:50 -triangular=.5:.5 -log10 -center_zero

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2D Power Spectrum

Overview

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Related Priism Topics

Preprocessing

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Averaging/Smoothing

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Window/taper

Rectangular

Triangular

Half-sine^power

Hamming

Blackman

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Output format

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Command line

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