When we sold raster to vector conversion software we were sometimes asked whether our software would convert to tolerances of hundredths, or even thousandths of an inch.
To get an idea of the tolerances that are possible, we created a drawing of a 1" square (25.4 mm) in CAD. We printed it out onto smooth high quality paper using a laser printer, scanned the printed square at 200, 400 and 600 dpi using a desktop scanner, vectorized it, imported it back into our CAD program and measured the results.
Here are the measurements:
|200 dpi||400 dpi||600 dpi|
The most accurate result we got was exactly one inch. The least accurate result we got was 0.9900000" (25.146 mm), so the error in this very small experiment varied between +/- 0" and 0.01" (0.254 mm).
This indicates that tolerances of thousandths of an inch are out, and even if you want tolerances of hundredths of an inch you are pushing it.
Further, the results above were created under ideal circumstances. In real life, as the proverb says, there's many a slip 'twixt cup and lip. Or, in this case, 'twixt original and vectorized image.
The following are all potential slips:
|a) A section of a scan of a part.||b) A zoomed in view of the hole outlined in red in (a). The hole has some shadowing and its edge is not cleanly demarcated.
|c) The hole thresholded using a value of 156.||d) The hole thresholded using a value of 250.
|e) Vectorizations of (c) and (d) superimposed over the original scan. The two different threshold settings have resulted in two differently sized vectorizations of the hole.
Although our little experiment showed that tolerances of a hundredth of an inch may be within the bounds of possibility under perfect circumstances, raster to vector conversion is not really suitable for high tolerance applications. There are just too many opportunities for error.