QgsDerivativeFilter class

Adds the ability to calculate derivatives in x- and y-directions.

Needs to be subclassed (e.g. for slope and aspect)

Base classes

class QgsNineCellFilter: Base class for raster analysis methods that work with a 3x3 cell filter and calculate the value of each cell based on the cell value and the eight neighbour cells.

class QgsAspectFilter: Calculates aspect values in a window of 3x3 cells based on first order derivatives in x- and y- directions.
class QgsSlopeFilter: Calculates slope values in a window of 3x3 cells based on first order derivatives in x- and y- directions.

auto processNineCellWindow(float* x11, float* x21, float* x31, float* x12, float* x22, float* x32, float* x13, float* x23, float* x33) -> float override: Calculates output value from nine input values.

auto calcFirstDerX(float* x11, float* x21, float* x31, float* x12, float* x22, float* x32, float* x13, float* x23, float* x33) -> float: Calculates the first order derivative in x-direction according to Horn (1981)
auto calcFirstDerY(float* x11, float* x21, float* x31, float* x12, float* x22, float* x32, float* x13, float* x23, float* x33) -> float: Calculates the first order derivative in y-direction according to Horn (1981)

Calculates output value from nine input values.

Parameters
x11	surrounding cell top left
x21	surrounding cell central left
x31	surrounding cell bottom left
x12	surrounding cell top central
x22	the central cell for which the value will be calculated
x32	surrounding cell bottom central
x13	surrounding cell top right
x23	surrounding cell central right
x33	surrounding cell bottom right
Returns	the calculated cell value for the central cell x22

The input values and the output value can be equal to the nodata value if not present or outside of the border. Must be implemented by subclasses.

First index of the input cell is the row, second index is the column