Transformations
The Transformations filter is split into three dialogs: Transform Metrics, Transformations, and Interpolate with Background.
Transform Metrics
Glyph → Transform Metrics assigns either a new width or new sidebearings to the selected glyph layers.
Enable Width to assign a new width to all selected glyphs. Check on both sides to modify both sides equally to match the new width. Otherwise, the points on the layer keep their original coordinates.
Uncheck Width to set new sidebearings. Control whether the left sidebearing (LSB), the right sidebearing (RSB), or both should be changed using the checkboxes next to their fields. If Relative is selected, the LSB and RSB values are added to the current sidebearings (or subtracted if the entered values are negative). Otherwise, the LSB and RSB values overwrite the existing sidebearings.
Transformations
Path → Transformations applies a linear transformation to the selected points or selected layers.
Translate moves points by a given X and Y value. Origin defines the point from which the transformation originates. Scale scales the selection along the horizontal and vertical axis. If the lock button is locked , the horizontal value is used for both axes. Slant skews the selection without optical correction. Click the Slant label and choose Cursivy for an optically corrected skew. Note that Cursivy requires horizontal and vertical metrics to be set in File → Font Info… → Masters → Stems.
horizontal and 
vertical axis.
If the lock button is locked 
, the horizontal value is used for both axes.
Slant skews the selection without optical correction.
Click the Slant label and choose Cursivy for an optically corrected skew.
Note that Cursivy requires horizontal and vertical metrics to be set in File → Font Info… → Masters → Stems.Interpolate with Background
Path → Interpolate with Background results in an interpolation of the foreground and background layers.
A value of 0 % leaves the foreground unchanged; 100 % replaces the foreground with the background, and values in-between result in an interpolated shape. Use a value below 0 % or above 100 % to extrapolate. Note that this filter does not work if the foreground and background are not compatible.
Custom Parameter
The custom parameter rule is as follows:
Transformations; LSB:LSB, RSB:RSB; Width:Width; ScaleX:Scale X; ScaleY:Scale Y; Slant:Slant; SlantCorrection:Cursivy; OffsetX:X; OffsetY:Y; Origin:Origin Metric
LSB and RSB set the left and right sidebearings.
Prefix these values with a + (plus), - (minus), * (asterisk, multiply), or / (forward slash, divide) symbol to change the sidebearings relative to their current values.
Width can be prefixed by + or -.
For example, to multiply the LSB by 2 and add 15 to the RSB, use the following filter rule:
Transformations; LSB:*2; RSB:+15
Scale X and Scale Y are percentages where a value of 100 does not scale, < 100 scales down, and > 100 scales up. Set Cursivy to 0 to disable slant correction; it is 1 by default. Set Origin Metric to 0 for cap height, 1 for ½ cap height, 2 for x-height, 3 for ½ x-height (default), or 4 for baseline.
For example, the following rule sets the width of all glyphs to 700, scales the glyph outlines to 120 % (both horizontally and vertically), cursivies the outlines to a 14° angle (adding SlantCorrection:0 would use normal slanting), and moves the outlines by 8 units to the right and 22 units down such that all these transformations originate from the x-height:
Transformations; Width:700; ScaleX:120; ScaleY:120; Slant:14; OffsetX:8; OffsetY:-22; Origin:2