What a master image is versus a running average
A master image is the fixed signed sample. It answers whether the print still looks like the signed sheet. A running average updates from recent frames and can follow an allowed slow change, but a defect that enters the average becomes the new normal. An average with no cap cannot replace the master image.
A master image is the fixed signed sample. It answers whether the print still looks like the moment it was signed. A running average keeps updating from recent frames. It can follow a slow change that is allowed, but if a defect gets into the average, the average treats the defect as the new normal. A running average with no limit cannot replace the master image.
The average will learn the defect
If every frame joins the average, a dirt spot that keeps coming back looks more and more like the template, and the alarm fades. Only frames judged good may enter the average, and the average may move only a written distance from the original signed sample, for example a very small density change. Past that range a person signs again. The average does not keep walking.
A fixed master image does not change by itself. Its drawback is that a lawful slow change will also alarm. That is the signal to sign again. It is not a reason to switch to an unlimited average.
Which one
| Purpose | Use | Limit |
|---|---|---|
| Is this still the signed sheet | Fixed master image | A lawful change needs a new signature |
| A very small slow drift is allowed | A running average with a cap | Bad frames stay out. Past the cap it stops |
| Too many alarms | Do not switch to an unlimited average first | Check light, material, and threshold |
Check at a job change
- See whether the current reference is the fixed sample or the average.
- If it is the average, write the cap on how far it may leave the signed sample.
- Put in a repeating defect and confirm it does not disappear after a few dozen labels.
