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I am trying to reconstruct this interesting piece of the puzzle that a long time ago, my father brought me. It has cards from A to B, mentioning a word using the same. For example J for Joker. Nothing is visible using naked eyes. But when you place a plastic semi-transparent/translucent sheet on the card the actual figure becomes clearly visible. Unfortunately, I lost the plastic sheet. Following are the two cards:

textured cards

If you zoom in on these pictures, some patterns are slightly visible. For example, start from the left corner you would see some part of the joker.

close up of card

Has anybody come across this or something similar to this kind of puzzle? Can you give some idea how probably this could be made especially the plastic sheet that reveals the hidden images?

Note: Please note that the cards are black & white and not colored.

The closest match is "red-reveal". But the principle looks different.

This question falls under

puzzle-creation

This is called a hidden picture puzzle. All such puzzles have an associated special trick of printing. In this particular card, one has to first guess what is the hidden object by close observation (just like in the link). Then it is verified with the help of the plastic sheet. These particular cards though have a slightly different aim: to teach the children about English alphabets. Whether it is a puzzle or a teaching aid, it is for sure that the underlying technology is the same.

Update:

While the answer by SpiritFryer is useful as a digital tool to decipher the hidden information in the puzzle, I am still looking for physical version of this tool which will give me a tactile experience just like the original kit having the plastic sheet for deciphering.

Update: Kindly update your answers so that I could award the bounty to the most eligible answer.

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    $\begingroup$ The technology is called lenticular printing. $\endgroup$
    – Jaap Scherphuis
    Commented Aug 18, 2021 at 6:18
  • $\begingroup$ There was only one such plastic sheet. How could this single sheet reveal all the images? $\endgroup$
    – gpuguy
    Commented Aug 18, 2021 at 6:27
  • $\begingroup$ @JaapScherphuis In lenticular, we embed images in the plastic sheets. but here we are embedding images in paper cards, and the plastic sheet is used to reveal the hidden images. I think it's not lenticular. $\endgroup$
    – gpuguy
    Commented Aug 18, 2021 at 6:38
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    $\begingroup$ @bobble This is called a hidden image puzzle. All such puzzles have an associated special trick of printing. In this card, one has to first guess what is the hidden object by close observation (just like in the link). Then it is verified with the help of the plastic sheet. These particular cards though have a slightly different aim: to teach the children about English alphabets. Whether it is a puzzle or a teaching aid, it is for sure that the underlying technology is the same. Therefore, the post should not be closed. $\endgroup$
    – gpuguy
    Commented Aug 19, 2021 at 5:59
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    $\begingroup$ @justhalf A lenticular sheet also hides portions of the image. I vaguely do remember seeing this kind of toy in the 1970s and the image would flip from white on black to black on white when you moved your head. $\endgroup$
    – Jaap Scherphuis
    Commented Aug 24, 2021 at 8:47

3 Answers 3

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If you're scared of clowns, don't click the spoiler :D

enter image description here

You can use this tool and import your 2nd image to generate the above http://magiceye.ecksdee.co.uk/

Once imported, you can use the slider above the image on the website to fine-tune it.

I knew of this tool from this subreddit where you can find images that uncover something hidden if you manage to focus your eyes in the correct way -- you can check the subreddit sidebar for help/FAQ and for links to other similar tools: https://www.reddit.com/r/MagicEye/

And another subreddit with a similar idea: https://www.reddit.com/r/CrossView/

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    $\begingroup$ Ooh, that's a nice tool. $\endgroup$
    – justhalf
    Commented Aug 24, 2021 at 6:17
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    $\begingroup$ Perhaps worth noting that the tool isn't creating the intended image, but effectively doing an "edge detection" on it. $\endgroup$
    – Steve
    Commented Aug 24, 2021 at 14:36
  • $\begingroup$ @ SpiritFryer Wow ... thanks $\endgroup$
    – gpuguy
    Commented Aug 25, 2021 at 6:02
  • $\begingroup$ But the question remained, how to get a physical copy of this tool so as to have a tactile experience as in the original puzzle using a plastic sheet? $\endgroup$
    – gpuguy
    Commented Aug 25, 2021 at 7:35
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I think trying to get a lenticular plastic sheet printed with the exact correct spacing would be really difficult. However, if you could get the paper sheet as flat / straight as possible and scan it on a flatbed scanner, it would be reasonably easy to write a Python program or similar that cancelled out the straight lines and enlarged any deviations from the straight lines in the same way that a lenticular lens would.

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  • $\begingroup$ What you say is a way to solve it digitally. Thanks for the answer. I have updated my question a little. Kindly have a look at it. $\endgroup$
    – gpuguy
    Commented Aug 25, 2021 at 7:41
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From the description of the puzzle, the underlying physics is the optical effect called moiré.

I surmise the transparent sheet has parallel strips of altered refractive index, with the same spacing as the dark lines on the cards or very nearly so. Moiré patterns appear wherever the printed lines are not perfectly straight.

See WP for an explanation, and enter "moiré deflectogram" for a goorgling stream of examples.

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