9 CLIM drawing functions

CLIM generalizes the concept of a hierarchy of window in a windowing system in several different ways. A window in a windowing system generalizes to a sheet in CLIM. More precisely, a window in a windowing system generalizes to the sheet region of a sheet. A CLIM sheet is an abstract concept with an infinite drawing plane and the region of the sheet is the potentially visible part of that drawing plane.

CLIM sheet regions don't have to be rectangular the way windows in most windowing systems have to be. Thus, the width and the height of a window in a windowing system generalizes to an arbitrary region in CLIM. A CLIM region is simply a set of mathematical points in a plane. CLIM allows this set to be described as a combination (union, intersection, difference) of elementary regions made up of rectangles, polygons and ellipses.

Even rectangular regions in CLIM are generalizations of the width+height concept of windows in most windowing systems. While the upper left corner of a window in a typical windowing system has coordinates (0,0), that is not necessarily the case of a CLIM region. CLIM uses that generalization to implement various ways of scrolling the contents of a sheet. To see that, imagine just a slight generalization of the width+height concept of a windowing system into a rectangular region with x+y+width+height. Don't confuse the x and y here with the position of a window within its parent, they are different. Instead, imagine that the rectangular region is a hole into the (infinite) drawing plane defined by all possible coordinates that can be given to drawing functions. If graphical objects appear in the window with respect to the origin of some coordinate system, and the upper-left corner of the window has coordinates (x,y) in that coordinate system, then changing x and y will have the effect of scrolling.

CLIM sheets also generalize windows in that a window typically has pixels with integer-value coordinates. CLIM sheets, on the other hand, have infinte resolution. Drawing functions accept non-integer coordinate values which are only translated into integers just before the physical rendering on the screen.

The x and y positions of a window in the coordinate system of its parent window in a typical windowing system is a translation transformation that takes coordinates in a window and transform them into coordinates in the parent window. CLIM generalizes this concepts to arbitrary affine transformations (combinations of translations, rotations, and scalings). This generalization makes it possible for points in a sheet to be not only translated compared to the parent sheet, but also rotated and scaled (including negative scaling, giving mirror images). A typical use for scaling would be for a sheet to be a zoomed version of its parent, or for a sheet to have its y-coordinate go the opposite direction from that of its parent.

When the shapes of, and relationship between sheets are as simple as those of a typical windowing system, each sheet typically has an associated window in the underlying windowing system. In that case, drawing on a sheet translates in a relativly straightforward way into drawing on the corresponding window. CLIM sheets that have associated windows in the underlying windowing system are called mirrored sheets and the system-dependent window object is called the mirror. When shapes and relationships are more complicated, CLIM uses its own transformations to transform coordinates from a sheet to its parent and to its grandparent, etc., until a mirrored sheet is found. To the user of CLIM, the net effect is to have a windowing system with more general shapes of, and relationships between windows.