The QR would be of a URL. The QR should be big enough like 4 A4 size papers. I though of cutting out the white boxes from a printed paper and spray on it but what about white dot islands. The precision needs to be good enough for a cell phone camera to detect it on the floor from standing distance.
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3How about making two copies of the QR code and in each version cut out the boxed from every second row only. Then spray one stencil first and place the second stencil at the same position before you spray it (the result will depend on how exact you can position both stencil at the same spot...)– samcarter_prepared_for_xmasCommented Jun 23, 2019 at 17:37
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5Related - with what are you spraying? What's the drying time and bleed characteristics? Do you expect this tag to last an hour, a day/week/month or as long as possible? Sprinkled chalk dust could last as little as minutes depending on the traffic level. You may consider overspraying your tag with a clear-coat laquer for increased durability. Do you have permission to tag the pavement like this - and if not, should you ? Consider, this may diminish your brand not enhance it.– CriggieCommented Jun 24, 2019 at 4:27
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4Not directly related to answering your question, but depending on the type of footpath (indoor/outdoor), consider using reverse graffiti (en.wikipedia.org/wiki/Reverse_graffiti) instead of spraying paint for a much more environmental friendly solution. (also solves some concerns of @Criggie)– BrtHCommented Jun 25, 2019 at 8:33
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7I might be out of touch, but do people these days tend to scan random QR codes that they see in the street? It doesn't seem to be much different than clicking random links in spam emails.– Aaron FCommented Jun 25, 2019 at 9:17
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6@thevikas "Its [...] to encourage citizen activism." Oh, great. Somebody who thinks vandalism is a good way to promote civic responsibility.– David RicherbyCommented Jun 25, 2019 at 13:14
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6 Answers
One approach to avoid the "island" problem could be to split the pattern into two stencils. In each stencil only the holes of every other row are cut out.
First colour is applied to one of the stencils. Then the stencil is exchanged and the colour is applied to the other stencil.
The weak point of this approach is that both stencil need to be placed at the same position.
answered Jun 23, 2019 at 19:20
samcarter_prepared_for_xmassamcarter_prepared_for_xmas
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12Real QR codes have registration marks in three corners. You should be able to make use of those to align the stencils. Instead of adhering strictly to the every-other row thing, you can include overlapping opposite "C" shapes of the registration marks, without violating the no-islands constraint.– hobbsCommented Jun 24, 2019 at 2:51
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2@thevikas If you look at a QR code, it has nested Black and White squares in the corners (3 large, 1 small). On both stencils, include 3 sides of each square, so that 2 sides overlap and are shared between stencils. Then you can use those to line the second stencil up Commented Jun 24, 2019 at 15:21
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2Alignment could be done using a cardbard box with no top, and the bottom flaps folded out. User stands on two flaps to hold box position between sprays, and the sides will help stop overspray. OR a flat frame with two flaps held in on one side by tape hinges, so they can be folded out.– CriggieCommented Jun 25, 2019 at 4:39
Here's a QR Code Stencil Generator which automatically generates thin bridges between islands.
As mentioned by @Aloisdg : It is a Processing project and it is hosted on GitHub.
I tested it successfully on the pavement last year. The pavement should be as slick as possible, though.
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4Great answer. It is a Processing project. Link to the GitHub repository– aloisdgCommented Jun 24, 2019 at 16:06
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What about a custom 3D printed embossing roller?
answered Jun 24, 2019 at 12:36
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1The target size being asked for is 4 sheets of A4 paper. That'd be a huge roller and well beyond what typical consumer 3rd printers can do. Commented Jun 24, 2019 at 17:36
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3Unlike the other examples given on this page, my QR code reader is unable to read this one, or even identify it as a QR code.– MarkCommented Jun 24, 2019 at 21:27
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@Danielillo: Shouldn't flipping not have any effect on QR code? Aren't the 3 identical corners for orientation? Commented Jun 25, 2019 at 14:00
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Instead of spraypaint - consider a "pavement sticker" or "footpath decal" that looks something like these.
A brighter background colour will help it stand out much more than paint on a dingy pavement/sidewalk. And you can peel, stick, stomp and go, whereas paint takes at least a minute to be touch dry, risking people standing in your tag.
Paper won't last very long, which might be suitable for your needs. A solid vinyl sticker could last up to years.
How about buying some grid-shaped welded wire metal mesh (commonly used for rabbit or chicken hutches), and using sticky tape to cover the squares you want to mask?
The wires would be thin enough that they wouldn't affect the scan, but strong enough to support isolated islands (much stronger than relying on tiny thin pieces of paper or card, which might break once they've been soaked in paint). More importantly, it would be quicker to make than a hand-cut stencil and you wouldn't need any expensive machines like a CNC router or 3D printer.
I recommend buying a flat sheet, the stuff that comes on a roll is usually twisted and won't lie flat.
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1I've used this, worked pretty well. Covered the whole grid in tape and then used a sharp knife to cut out the stencil. Commented Jun 26, 2019 at 11:15
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@NotAGenie - that's interesting, I was thinking that you'd use (for example) a 1" grid with 1" tape, and just wrap tape around the squares or lines you want to mask Commented Jun 26, 2019 at 11:26
Your islands can become insignificant by creating stencil like bridges to the more solidly founded segments. A bridge of a few millimeters will bleed paint underneath but also will be small enough to be ignored by the QR software from a distance appropriate to the size of the code print. If the few millimeters is excessive, hot glue applied to ordinary sewing thread in more locations (spider web?) will provide the support and almost assure bleed under.
answered Jun 23, 2019 at 18:09
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2QR codes are specifically designed to withstand a moderate number of single-pixel errors, so even if the few millimeters is excessive, you may be able to leave it there anyway.– KevinCommented Jun 24, 2019 at 6:24