ID Number: Q74044
3.00
WINDOWS
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.
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.