The effects of transformation can be minimized by defaulting to the identity transformation. This means that no deliberate action is requested by the application.
Graphic primitives are drawn in their own coordinate scale in world coordinate space. The primitives are then combined in model space. Neither the world coordinate space or the model space have "real world" units associated with them. This means that if an application draws a line on the x-axis from -300000 to +300000, the line exists as a line 600000 Cartesian units long in both world coordinate and model space.
Page space is the first of the coordinate spaces to be associated with units such as millimeters or inches. These units are set with the GpiCreatePS option PS_UNITS. If the application specifies the option PS_UNITS with the value PU_ARBITRARY, the page space remains unitless; units are applied when the output is drawn in device space.
As an example, assume that the units PU_LOMETRIC, (0.1 mm) have been chosen. Again, assuming the identity transformation between model and page space, the primitive appears in the page space as a line that extends
300000 * 0.1mm to the right of the origin 300000 * 0.1mm to the left of the origin
A presentation page, a rectangle in page space, has its size defined when a presentation space is created with GpiCreatePS. The presentation page format is defined with the GpiCreatePS option, PS_FORMAT. The three choices for that option are:
PS_FORMAT affects the number of units permitted along the coordinate axes of the presentation page. If the choice was GPIF_LONG or GPIF_DEFAULT, and, for example, PS_UNITS remains PU_LOMETRIC, the presentation page axes could range from 0 to 134217727 0.1mm. If the choice was GPIF_SHORT, the coordinate axes could range only from 0 to 32767 0.1mm. These limits are the maximum number of units, presentation pages are usually much smaller.
The presentation page does not affect the primitive in presentation space, it determines what parts of the primitive are visible. With no transformation to scale down the primitive from the model to the page space, the presentation page acts like a sheet of cardboard with a rectangular hole. Only the part of the page space behind the "hole" is visible in page space. The view of everything else is blocked by the "cardboard." Only what is visible in the page space is drawn to the next coordinate space.
Continuing with the example of the 600000 units line above, if GPIF_LONG (or GPIF_DEFAULT) is selected, the 600000 unit line could easily be viewed in the presentation page. If GPIF_SHORT is selected, the maximum presentation page size is 65534 * 0.1 mm units long. In the case, much of the 600000 unit line would not be drawn into the next coordinate space.
The device space is not affected by the PS_FORMAT option. Whether GPIF_LONG, or GPIF_SHORT, the device space is a rectangle with maximum coordinates of (32767, 32767) and minimum coordinates of (-32768, -32768). Note that not all of this rectangle is visible, since real devices are not that big. The device space relates to the physical device.
The transformation between the page and device space is handled automatically by the PM. The presentation page is mapped into a rectangle in the device space called the page viewport. This transformation is based on the parameters and options of GpiCreatePS.
Confusion can arise because the effects of the presentation page and the page viewport appear to be a clipping of the primitives in page and device space. While the action taken is identical, the term clipping is reserved for the activity deliberately designated by the application with clipping functions.