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Comparing Images with Overlays
Displaying Comparisons to Standard Images Using Pseudo Color Image Overlays
David Fitzpatrick
VP Engineering
ORS Automation, Princeton, NJ
Abstract
Several Optical Inspection functions require displaying the results of comparisons of a known good part, a "golden standard", to the part under test, the production part. Deviations from the standard must be reported in a way that highlights any discrepancies to make them obvious to the inspector. The use of Pseudo Color Image Overlays can quickly identify missing areas, such as missing components or incomplete traces. Excess image areas, such as solder bridges or under etched traces, are also easily marked. Finally misaligned areas, such as misregistered substrates or components can be easily recognized where they differ from the learned standard.
Introduction
In performing optical inspection of a production part, it has long been a common practice to compare the production part to a known good part. This known good part is often referred to as a "golden standard". To enable the inspector to make comparisons between the golden standard and the part under test, there has to be a system to present both images to the inspector. One way that is very effective is to generate an "Image Overlay" of the standard that is compared and transparently superimposed on the part under test. The comparison process between the standard image and the part under test image produces highlights on the inspected part that point out deviations that are indicative of errors in the manufacture of the part under test.
Microsoft provides some basic routines for manipulating graphics that can be used to highlight differences between the standard and the test image. These functions are called ROP or Raster Operators. They are useful for combining images on a bit by bit raster basis. When the computer system supports these operations in hardware they can be very fast. With the current fast processors, these operations appear as real time (without flickering) even when performed in software.
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