INFO: Logical and Physical Coordinate Relationship (74044)

SUMMARY

Windows is designed to use a logical coordinate system when doing many of its graphics operations. Many of the Windows functions take logical units as parameters. Windows translates these units to physical device units based on the mapping mode of the device, window origin, window extents, viewport origin, and viewport extents.

This article discusses the relationship between logical and physical coordinates.

MORE INFORMATION

The default mapping mode for a device context (DC) is the MM_TEXT mode. The point (0,0) is the origin of the logical and physical coordinate systems in this mode. It also has a 1 to 1 mapping ratio of logical to physical device units.

The programmer can change the mapping mode of a DC by using the SetMapMode() function.

The following six mapping modes have predefined logical to physical mapping ratios:

MM_TEXT
MM_LOENGLISH
MM_HIENGLISH
MM_LOMETRIC
MM_HIMETRIC
MM_TWIPS

The following two mapping modes can be set to have user defined ratios of logical to physical units:

MM_ANISOTROPIC
MM_ISOTROPIC

In the last two modes, coordinates are transformed from logical to physical and vice versa based on the current window and viewport extents in the DC. The functions SetWindowExt() and SetViewportExt() allow an application to set the window and viewport extents. These functions should only be used in the MM_ANISOTROPIC and MM_ISOTROPIC mapping modes because they are the only ones in which the application can define the mapping ratio.

The formulae used to convert to and from the different coordinate systems are documented in section 2.5.3, page 2-18, in the "Microsoft Windows Software Development Kit Reference Volume 1" for version 3.0.

REFERENCES

For more information about the relationship between the logical and physical planes, refer to the following:

"Microsoft Windows Software Development Kit Reference Volume 1" version 3.0, section 2.5 "Mapping Functions," pages 2-15 through 2-20.

"Programming Windows," first edition, by Charles Petzold Chapter 12, pages 520-540.

"Programming Windows," second edition, by Charles Petzold Chapter 11, pages 517-539.

"Peter Norton's Windows 3.0 Power Programming Techniques," by Peter Norton and Paul Yao, Chapter 20, c. 1990, Bantam Books.