Topic       : TOS - The Operating System
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View Ref-File7.2.5  VDI coordinate systems                                      TOS

The VDI distinguishes two different types of coordinate systems:

   . Raster coordinates (RC): When using this system one can use the 
     exact coordinate system native to the output device. For a screen 
     this corresponds to the available horizontal and vertical pixels. 
     The point of origin lies in the top left corner.
     The main advantage of raster coordinates is the absolutely exact 
     positioning, which alone enables precise work with screen rasters 
     (windows etc).

   . Normalized coordinates (NDC): This system always has a maximum 
     resolution of 32768*32768 points. When a device driver is called 
     the VDI will automatically recalculate to the device's physical 
     coordinate system. The origin lies in the bottom left corner. For 
     working with the NDC system a GDOS has to have been installed - 
     the screen driver in ROM can not handle NDC coordinates.

The following illustration makes the difference between RC and NDC 
coordinates particularly clear:




Note: In practice the NDC system is seldom used. Programs that have to 
offer exact dimensioning (desktop publishing, scientific graphics) 
must in any case use internally their own - finer - coordinate 
representation.

The use of the NDC system would mean that the coordinates would have 
to be converted twice (once from the internal representation in NDC 
coordinates, then to raster coordinates), which would lead to 
superfluous speed losses and rounding errors.

See Also: VDI fundamentals   Style guidelines