7.1 Coordinate systems

CLIM uses a number of different coordinate systems and transformations to transform coordinates between them.

The coordinate system used for the arguments of drawing functions is called the user coordinate system, and coordinate values expressed in the user coordinate system are known as user coordinates. Each sheet has its own coordinate system called the sheet coordinate system, and positions expressed in this coordinate system are said to be expressed in sheet coordinates. User coordinates are translated to sheet coordinates by means of the user transformation also called the medium transformation. This transformation is stored in the medium used for drawing. The medium transformation can be composed temporarily with a transformation given as an explicit argument to a drawing function. In that case, the user transformation is temporarily modified for the duration of the drawing.

Before drawing can occur, coordinates in the sheet coordinate system must be transformed to native coordinates, which are coordinates of the coordinate system of the native windowing system. The transformation responsible for computing native coordinates from sheet coordinates is called the native transformation. Notice that each sheet potentially has its own native coordinate system, so that the native transformation is specific for each sheet. Another way of putting it is that each sheet has a mirror, which is a window in the underlying windowing system. If the sheet has its own mirror, it is the direct mirror of the sheet. Otherwise its mirror is the direct mirror of one of its ancestors. In any case, the native transformation of the sheet determines how sheet coordinates are to be translated to the coordinates of that mirror, and the native coordinate system of the sheet is that of its mirror.

The composition of the user transformation and the native transformation is called the device transformation. It allows drawing functions to transform coordinates only once before obtaining native coordinates.

Sometimes, it is useful to express coordinates of a sheet in the coordinate of its parent. The transformation responsible for that is called the sheet transformation.