Data Translation MACH Series, User Manual

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Principles of Operation
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For each entry in the passthru LUT, the index of the closest matching
RGB value in the Windows system palette is used. If you want an
exact color to display during passthru, you can use software to load
that color into the system palette. This guarantees that the color is
available when the board attempts to find the closest match in the
Windows system palette.
Overlays
Note:
Overlays can be used for area-scan operations only. They
cannot be used for line-scan operations.
Overlays require a 16-bit color display adapter and Direct Draw
Interface (DDI) support.
For area-scan operations only, you can add overlays to the display
using software. Overlays allow you to place an image on top of
another image that was captured using passthru.