I am trying to transform a rectangular shape to a trapezium as shown in the image below. In the Inkscape transform menu, there is an option to use a 2x3 matrix. However, that does not allow "3d" transformations. I think I read somewhere that the SVG specification supports larger matrices, but I cannot find much information about it. So my question is: how do you do this in Inkscape?
SVG transformation matrices only allow for affine transformations, i.e., something that is a combination of moving, rotating, stretching and skewing. Trapezium transformations cannot be expressed as a combination of these operations and thus they are not possible to be done with SVG transformation matrices.
However, extensions → modify path → perspective performs such a transformation. For details, see for example this question.