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In engineering/architectural drafting, many people consider grey lines - usually used to indicate existing work or reference work belonging to other disciplines - as "screened back". When older engineers ask for something to be lightened to indicate it is existing, they will ask that the drafter "screen back" the lines.

Newer software simply calls this feature "halftone", but I have been unable to find any sources on google that indicate how hand drafting worked previously that would explain the "screen" concept.

  • LESS has a color function for "screen". If I'm reading the documentation right, the first color channels (red, green, blue for CSS) are divided by the corresponding channels to screen against, then multiplied by 255. (I will edit this after I've checked). – VampDuc Jul 19 '18 at 16:58
  • I missed my window to edit my comment, but this might help. It explains how the math works for determining the final value of each channel. It also offers this tidbit: The effect is similar to projecting multiple photographic slides simultaneously onto a single screen. – VampDuc Jul 19 '18 at 17:18
  • I think we're talking about two different things. The term I was wondering about seems to be a carryover from the days of hand drafting, so I don't think it's related to a video screen. I suspect it is something along the lines of screens used for screenprinting or like you would have to keep the bugs from coming in your window. – Secundus Jul 19 '18 at 18:15
  • I shouldn't try and answer questions after no sleep. Wikipedia's article on halftones quotes the patent as using "photographic screens or veils." So, quite literally, a mesh screen as you said. If the drafts you're referring to would have originally been old-school blueprints, they could have used a similar process when creating the copy, since it's similar to processing a photograph. For what I mentioned earlier, that's now how we can mathematically duplicate the physical process using much smaller dots. – VampDuc Jul 19 '18 at 20:00
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    You could do it by stacking two (registered) vellums (old and new drawings), and then removing the old vellum part way through the exposure process. – AmI Sep 15 '18 at 8:02

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