While analysing randomly rough surfaces we often need a statistical approach to determine some set of representative quantities. Within Gwyddion, there are several ways of doing this. In this section we will explain the various statistical tools and modules offered in Gwyddion, and also present the basic equations which were used to develop the algorithms they utilize.
Scanning probe microscopy data are usually represented as a two-dimensional data field of size N×M, where N and M are the number of rows and columns of the data field, respectively. The real area of the field is denoted as L_{x}×L_{y} where L_{x} and L_{y} are the dimensions along the respective axes. The sampling interval (distance between two adjacent points within the scan) is denoted Δ. We assume that the sampling interval is the same in both the x and y direction and that the surface height at a given point (x, y) can be described by a random function ξ(x, y) that has given statistical properties.
Note that the AFM data are usually collected as line scans along the x axis that are concatenated together to form the two-dimensional image. Therefore, the scanning speed in the x direction is considerably higher than the scanning speed in the y direction. As a result, the statistical properties of AFM data are usually collected along the x profiles as these are less affected by low frequency noise and thermal drift of the sample.
Statistical quantities include basic properties of the height values distribution, such as variance, skewness and kurtosis. The quantities accessible within Gwyddion by means of the Statistical Quantities tool are divided into several groups.
Moment based quantities are expressed using integrals of the height distribution function with some powers of height. They include the familiar quantities:
More precisely, RMS (σ), skewness (γ_{1}), and kurtosis (γ_{2}) are computed from central moments of i-th order μ_{i}
where z̄ denotes the mean value. The are expressed by the following formulas:
Note that Gwyddion computes the excess kurtosis, which is zero for Gaussian data distribution. Add 3 to obtain the area texture parameter Sku.
Mean roughness S_{a} is similar to RMS value, the difference being that it is calculated from the sum of absolute values of data differences from the mean, instead of their squares.
Order based quantities represent values corresponding to specific ranks if all the values were ordered. They include:
Hybrid quantities combine heights and spatial relations in the surface and include:
The surface area is estimated by the following method. Let z_{i} for i = 1, 2, 3, 4 denote values in four neighbour points (pixel centres), and h_{x} and h_{y} pixel dimensions along corresponding axes. If an additional point is placed in the centre of the rectangle which corresponds to the common corner of the four pixels (using the mean value of the pixels), four triangles are formed and the surface area can be approximated by summing their areas. This leads to the following formulas for the area of one triangle (top) and the surface area of one pixel (bottom):
The method is now well-defined for inner pixels of the region. Each value participates on eight triangles, two with each of the four neighbour values. Half of each of these triangles lies in one pixel, the other half in the other pixel. By counting in the area that lies inside each pixel, the total area is defined also for grains and masked areas. It remains to define it for boundary pixels of the whole data field. We do this by virtually extending the data field with a copy of the border row of pixels on each side for the purpose of surface area calculation, thus making all pixels of interest inner.
In addition, the tool calculates a few quantities that do not belong in any of the previous categories:
One-dimensional statistical functions can be accessed by using the Statistical Functions tool. Within the tool window, you can select which function to evaluate using the selection box on the left labelled Output Type. The graph preview will update automatically. You can select in which direction to evaluate (horizontal or vertical), but as stated above, we recommend using the fast scanning axis direction. You can also select which interpolation method to use. When you are finished, click to close the tool window and output a new graph window containing the statistical data.
The simplest statistical functions are the height and slope distribution functions. These can be computed as non-cumulative (i.e. densities) or cumulative. These functions are computed as normalized histograms of the height or slope (obtained as derivatives in the selected direction – horizontal or vertical) values. In other words, the quantity on the abscissa in “angle distribution” is the tangent of the angle, not the angle itself.
The normalization of the densities ρ(p) (where p is the corresponding quantity, height or slope) is such that
Evidently, the scale of the values is then independent on the number of data points and the number of histogram buckets. The cumulative distributions are integrals of the densities and they have values from interval [0, 1].
The height and slope distribution quantities belong to the first-order statistical quantities, describing only the statistical properties of the individual points. However, for the complete description of the surface properties it is necessary to study higher order functions. Usually, second-order statistical quantities observing mutual relationship of two points on the surface are employed. These functions are namely the autocorrelation function, the height-height correlation function, and the power spectral density function. A description of each of these follows:
The autocorrelation function is given by
where z_{1} and z_{2} are the values of heights at points (x_{1}, y_{1}), (x_{2}, y_{2}); furthermore, τ_{x} = x_{1} − x_{2} and τ_{y} = y_{1} − y_{2}. The function w(z_{1}, z_{2}, τ_{x}, τ_{y}) denotes the two-dimensional probability density of the random function ξ(x, y) corresponding to points (x_{1}, y_{1}), (x_{2}, y_{2}), and the distance between these points τ.
From the discrete AFM data one can evaluate this function as
where m = τ_{x}/Δx, n = τ_{y}/Δy. The function can thus be evaluated in a discrete set of values of τ_{x} and τ_{y} separated by the sampling intervals Δx and Δy, respectively.
For AFM measurements, we usually evaluate the one-dimensional autocorrelation function based only on profiles along the fast scanning axis. It can therefore be evaluated from the discrete AFM data values as
The one-dimensional autocorrelation function is often assumed to have the form of a Gaussian, i.e. it can be given by the following relation
where σ denotes the root mean square deviation of the heights and T denotes the autocorrelation length.
For the exponential autocorrelation function we have the following relation
We can also introduce the radial ACF G_{r}(τ), i.e. angularly averaged two-dimensional ACF, which of course contains the same information as the one-dimensional ACF for isotropic surfaces:
The difference between the height-height correlation function and the autocorrelation function is very small. As with the autocorrelation function, we sum the multiplication of two different values. For the autocorrelation function, these values represented the different distances between points. For the height-height correlation function, we instead use the power of difference between the points.
For AFM measurements, we usually evaluate the one-dimensional height-height correlation function based only on profiles along the fast scanning axis. It can therefore be evaluated from the discrete AFM data values as
where m = τ_{x}/Δx. The function thus can be evaluated in a discrete set of values of τ_{x} separated by the sampling interval Δx.
The one-dimensional height-height correlation function is often assumed to be Gaussian, i.e. given by the following relation
where σ denotes the root mean square deviation of the heights and T denotes the autocorrelation length.
For the exponential height-height correlation function we have the following relation
In the following figure the height-height correlation function obtained for a simulated Gaussian surface is plotted. It is fitted using the formula shown above. The resulting values of σ and T obtained by fitting the HHCF are practically the same as for the ACF.
The two-dimensional power spectral density function can be written in terms of the Fourier transform of the autocorrelation function
Similarly to the autocorrelation function, we also usually evaluate the one-dimensional power spectral density function which is given by the equation
This function can be evaluated by means of the Fast Fourier Transform as follows:
where P_{j}(K_{x}) is the Fourier coefficient of the j-th row, i.e.
If we choose the Gaussian ACF, the corresponding Gaussian relation for the PSDF is
For the surface with exponential ACF we have
In the following figure the resulting PSDF and its fit for the same surface as used in the ACF and HHCF fitting are plotted. We can see that the function can be again fitted by Gaussian PSDF. The resulting values of σ and T were practically same as those from the HHCF and ACF fit.
We can also introduce the radial PSDF W_{r}(K), i.e. angularly integrated two-dimensional PSDF, which of course contains the same information as the one-dimensional PSDF for isotropic rough surfaces:
For a surface with Gaussian ACF this function is expressed as
while for exponential-ACF surface as
The spectral density can also be integrated radially, giving the angular spectrum. Its peaks can be used to identify the image texture direction.
The Minkowski functionals are used to describe global geometric characteristics of structures. Two-dimensional discrete variants of volume V, surface S and connectivity (Euler-Poincaré Characteristic) χ are calculated according to the following formulas:
Here N denotes the total number of pixels, N_{white} denotes the number of “white” pixels, that is pixels above the threshold. Pixels below the threshold are referred to as “black”. Symbol N_{bound} denotes the number of white-black pixel boundaries. Finally, C_{white} and C_{black} denote the number of continuous sets of white and black pixels respectively.
For an image with continuous set of values the functionals are parametrized by the height threshold value ϑ that divides white pixels from black, that is they can be viewed as functions of this parameter. And these functions V(ϑ), S(ϑ) and χ(ϑ) are plotted.
The range distribution is a plot of the growth of value range depending on the lateral distance. It is always a non-decreasing function.
For each pixel in the image and each lateral distance, it is possible to calculate the minimum and maximum from values that do not lie father than the given lateral distance from the given pixel. The local range is the difference between the maximum and minimum (and in two dimensions can be visualised using the Rank presentation function). Averaging the local ranges for all image pixels gives the range curve.
The area scale graph shows the ratio of developed surface area and projected area minus one, as the function of scale at which the surface area is measured. Because of the subtraction of unity, which would be the ratio for a completely flat surface, the quantity is called “excess” area.
Similar to the correlation functions, the area excess can be in principle defined as a directional quantity. However, the function displayed in the tool assumes an isotropic surface – the horizontal or vertical orientation that can be chosen there only determines the primary direction used in its calculation.
Most of the statistical functions support masking. In other words, they can be calculated for arbitrarily shaped image regions. For some of them, such as the height or angle distribution, no further explanation is needed: only image pixels covered (or not covered) by the mask are included in the computation. However, the meaning of the function is less clear for correlation functions and spectral densities.
We will illustrate it for the ACF (the simplest case). The full description can be found in the literature [1]. The discrete ACF formula can be rewritten
where Ω_{m} is the set of image pixels inside the image region for which the pixel m columns to the right also lies inside the region. If we calculate the ACF for a rectangular region, for example the entire image, nothing changes. However, this formula expresses the function meaningfully for regions of any shape. The only missing part is how to perform the computation efficiently.
For this we define c_{k,l} as the mask of valid pixels, i.e. it is equal to 1 for pixels we want to include and 0 for pixels we want to exclude. It then follows that
Furthermore, if image data are premultiplied by c_{k,l} the ACF sum over the irregular region can be replaced by the usual ACF sum over the entire image. Both are efficiently computed using FFT.
It is important to note that the amount of information available in the data about the ACF for given m depends on Ω_{m}. Unlike for entire rectangular images, it does not have to decrease monotonically with increasing m and can vary almost arbitrarily. There can even be holes, i.e. horizontal distances m for which the region contains no pair of pixels exactly this apart. If this occurs Gwyddion replaces the missing values using linear interpolation.
The height-height correlation function is calculated in a similar manner, only the sums have to be split into parts which can be then evaluated using FFT. For PSDF, this is not possible because each Fourier coefficient depends on each data value. So, instead, the cyclic autocorrelation function is calculated in the same manner as ACF, and PSDF is then obtained as its Fourier transform.
The Row/Column Statistics tool calculates numeric characteristics of each row or column and plots them as a function of its position. This makes it kind of complementary to Statistical Functions tool. Available quantities include:
In addition to the graph displaying the values for individual rows/columns, the mean value and standard deviation of the selected quantity is calculated from the set of individual row/column values. It must be emphasised that the standard deviation of the selected quantity represents the dispersion of values for individual rows/columns and cannot be interpreted as an error of the analogous two-dimensional quantity.
The Correlation Length tool can be used for a quick estimation of image autocorrelation length T from scan lines. The autocorrelation length is estimated using several common methods. The results of all are displayed, allowing the user to choose the most appropriate one (and compare immediately how much they differ):
The tool also displays the relation between autocorrelation length T and image size L (more precisely scan line length):
The tool can performs scan line corrections during the estimation. This is controlled by Scan line levelling. By default, the offset is subtracted, which is usually a good choice. You can switch to no levelling if you want the scan lines used exactly as they are. Sometimes it can be also useful to do on-the fly tilt correction. If you need higher order or more complex corrections, apply it explicitly using Align Rows.
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The full two-dimensional autocorrelation function introduced above is sometimes used to evaluate anisotropy of rough surfaces. The 2D ACF module calculates the function and can also plot its section in selected directions or calculate several roughness parameters that are based on 2D ACF.
Options in the ACF tab control how the function is calculated and displayed. In most cases the mean value should be subtracted before the calculation (as would Mean Zero Value do). This is selected by Mean value subtraction in Data adjustment. However, Plane levelling can be also useful. If you already adjusted the zero level correctly, select None to calculate the ACF from unaltered data. If there is a mask on the image the dialogue offers the standard masking options. See Masked Statistics Functions for an overview of masked ACF calculation.
The Graph tab contains settings for extracted ACF sections. They are the same as for normal profile extraction.
Finally, Parameters displays various numerical parameters derived from 2D ACF. They are determined from the decay of ACF in different directions and depend on Threshold, below which data points are considered essentially uncorrelated. The threshold is a fraction with respect to the maximum and is usually chosen as 0.2. The area above the selected threshold is marked using a mask on the ACF image.
The autocorrelation length S_{al} is the shortest distance at which the ACF falls below the threshold. The corresponding direction is also shown, as well as the distance and direction of the slowest decay. The ration between the longest and smallest distances is the texture aspect ratio S_{tr}.
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The full two-dimensional spectral density function is used to evaluate characteristic spatial frequencies occurring in surfaces. The 2D PSDF module calculates the function and can also plot its section in selected directions.
Options in the PSDF tab control how the function is calculated and displayed. Option Windowing selects the windowing type. If there is a mask on the image the dialogue offers the standard masking options. See Masked Statistics Functions for an overview of masked PSDF calculation.
The Graph tab contains settings for extracted PSDF sections. They are the same as for normal profile extraction.
Finally, Parameters displays some numerical parameters derived from 2D PSDF. Two are calculated using the angular spectrum, texture direction S_{td} and texture direction index S_{tdi}. S_{td} is given by the direction in which the radial PSDF integral is the largest and determines the dominant texture direction. S_{tdi} measures how dominant is the dominant direction by comparing the average over all directions to the dominant direction. Note that smaller S_{tdi} means more spectral weight in the dominant direction (not less).
Several functions in
→ operate on two-dimensional slope (derivative) statistics.Use local plane fitting is enabled, by fitting a local plane through the neighbourhood of each point and using its gradient.
calculates a plain two-dimensional distribution of derivatives, that is the horizontal and vertical coordinate on the resulting data field is the horizontal and vertical derivative, respectively. The slopes can be calculated as central derivatives (one-side on the borders of the image) or, iffacet inclination angles given by the following formula:
can also plot summary graphs representing one-dimensional distributions of quantities related to the local slopes andThree different plots are available:
function is a visualization aid that does not calculate a distribution in the strict sense. For each derivative v the circle of points satisfying
is drawn. The number of points on the circle is given by Number of steps.
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This function can estimate the differential entropy of value and slope distributions and also provides a visualisation of how it is calculated.
The Shannon differential entropy for a probability density function can be expressed
where X is the domain of the variable x. For instance for the height distribution x represents the surface height and X is the entire real axis. For slope distribution x is a two-component vector consisting of the derivatives along the axes, and X is correspondingly the plane.
There are many more or less sophisticated entropy estimation methods. Gwyddion uses a relatively simple histogram based method in which the above formula is approximated with
where p_{i} and w_{i} are the estimated probability density, i.e. normalized histogram value, and width of the i-th histogram bin.
Of course, the estimated entropy value depends on the choice of bin width. With the exception of uniform distribution, for which the estimate does not depend on the bin size. From this follows that for reasonable distributions a suitable bin width is such that the estimated entropy does not change when the width changes somewhat. This is the criterion used for choosing a suitable bin width.
In practice this means the entropy is estimated over a large range of bin widths (typically many orders of magnitude), obtaining the scaling curve displayed in the graph on the right side of the dialogue. The entropy is then estimated by finding the inflexion point of the curve. If there is no such point, which can happen for instance when the distribution is too close to a sum of δ-functions, a very large negative value is reported as the entropy.
The entropies are displayed as unitless since they are logarithmic quantities. For absolute comparison with other calculations, it should be noted that in Gwyddion they are always calculated from quantities in base SI units (e.g. metres as opposed to for instance nanometres) and natural logarithms are used. The values are thus in natural units of information (nats).
The absolute entropy depends on the absolute width of the distribution. For instance, two Gaussian distributions of heights with different RMS values have different entropies. This is useful when we want to compare the absolute narrowness of structures in the height distribution. On the other hand, it can be useful to compare them in a manner independent on the overall scale. For this we can utilise the fact that the Gaussian distribution has the maximum entropy among all distributions with the same RMS value. The difference between this maximum and the estimated entropy is displayed as entropy deficit. By definition, it is always positive (apart from numerical issues).
[1] D. Nečas and P. Klapetek: One-dimensional autocorrelation and power spectrum density functions of irregular regions. Ultramicroscopy 124 (2013) 13-19, doi:10.1016/j.ultramic.2012.08.002
[2] D. Nečas, P. Klapetek and M. Valtr: Estimation of roughness measurement bias originating from background subtraction. Measurement Science and Technology (2020) 094010, doi:10.1088/1361-6501/ab8993
[3] D. Nečas, M. Valtr and P. Klapetek: How levelling and scan line corrections ruin roughness measurement and how to prevent it. Scientific Reports 10 (2020) 15294, doi:10.1038/s41598-020-72171-8