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Filling modes.

The coordinates for most graphical operations are provided in the form of logical coordinates. Logical coordinates are translated into device coordinates through what is called coordinate mapping.

Mapping defines a linear relationship between the logical and the physical coordinate space. Mapping matches the origin of the logical coordinate space to the origin of the physical coordinate space, and also matches logical and physical coordinate units. Mapping in the horizontal and vertical directions may be independent of each other.

Windows defines a set of mapping modes. These mapping modes can be set using the SetMapMode member function.

On a raster device such as the screen or printer, device coordinates represent pixel coordinates. The upper-left corner is assigned the coordinates [0,0]; the horizontal coordinate increases from left to right, the vertical coordinate increases from top to bottom.

Of the many predefined mapping modes, MM_TEXT matches logical coordinates to physical coordinates. Other predefined mapping modes reverse the direction of the horizontal coordinate, so it grows from bottom to top. These mapping modes are listed in Table 1.

Table 1. Mapping modes.

Mapping mode

Description

In all of these mapping modes, applications can use the SetWindowOrg and SetViewportOrg functions to set the origin of the logical coordinate space (window) and physical coordinate space (viewport). The significance of these settings is that the two origins are mapped onto each other when coordinates are transformed.

In addition to MM_TEXT and the mapping modes in Table 1, Windows also defines the MM_ISOTROPIC and MM_ANISOTROPIC mapping mode. In these mapping modes, applications can not only specify the origin, but also the extent of the window and viewport coordinate space. By specifying the extent, applications define how many logical units are mapped to how many physical units. The difference between MM_ISOTROPIC and MM_ANISOTROPIC is that in the former mode, applications only define extents in the horizontal direction, while Windows calculates the vertical extent preserving the device aspect ratio. In the latter mode, applications can freely define any extents in both directions.

Figure 3 illustrates the effects of a typical mapping from logical to physical coordinates.