31 lines
1.3 KiB
Markdown
31 lines
1.3 KiB
Markdown
# Log-Polar PSDF
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Compute the power spectral density function in log-polar coordinates. The x-axis is the azimuthal angle (0-360 degrees) and the y-axis is log(frequency). Better than Cartesian PSDF for anisotropy analysis. Equivalent to Gwyddion's psdf_logphi.c module.
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## Inputs
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| Name | Type | Required | Description |
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|------|------|----------|-------------|
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| field | DATA_FIELD | Yes | Input spatial-domain field |
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## Outputs
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| Name | Type | Description |
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|------|------|-------------|
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| psdf | DATA_FIELD | Power spectral density in log-polar coordinates (domain=frequency) |
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## Controls
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| Name | Type | Default | Description |
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|------|------|---------|-------------|
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| n_phi | INT | 180 | Number of azimuthal angle bins (36–720) |
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| n_r | INT | 100 | Number of radial (log-frequency) bins (20–500) |
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## Notes
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- The mean value is subtracted before computing the 2D FFT, and the power spectrum is shifted so DC is at the centre.
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- Bilinear interpolation is used when sampling the Cartesian power spectrum onto the log-polar grid.
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- The output is log-scaled via `log1p` for better visual contrast.
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- Output xreal is 360.0 (degrees) and yreal is log(r_max) where r_max is half the shorter image dimension.
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- Anisotropic surfaces produce bright bands at specific azimuthal angles; isotropic surfaces appear uniform along the angle axis.
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