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Step 1Theory of operation
This multitouch display screen design is based on the description in Jeff Han's paper,
Han, J. Y. 2005. Low-Cost Multi-Touch Sensing through Frustrated Total Internal Reflection. In Proceedings of the 18th Annual ACM Symposium on User Interface Software and Technology
The figure below comes from his web site.
An acrylic panel is edge lit with infrared leds. When your finger comes in contact with the acrylic, it scatters infrared light out the back where it is visible via infrared camera. As long as nothing is touching the acrylic, very little of the light escapes, instead just reflecting around inside. Image processing takes care of detecting tips of fingers and relaying their location to application software. Since the camera "reads" the whole display in parallel, it is easy to detect multiple fingertips at once, even those belonging to multiple users. All this sensing goes on in the infrared spectrum, leaving us free to utilize the visible spectrum to display interactive software.
Since most hobbyists can't afford multiple projectors (i don't even own one, just borrowed it from dr.eel), my design uses a ceiling mount that swivels so that the projector can be used either in standard mode (say for watching movies) or can be aimed downwards, bouncing off a reflector and onto the multitouch display screen.
The screen itself can be constructed from hardware store materials and hand tools. Excluding the projector and modified webcam (commodity items these days), the only thing complicated is the software. Halfway through this project, I was happy to discover that there is thriving DIY community which has already undertaken the task of writing the image processing code and several cool open source demos which can be found here:
touchlib
Han, J. Y. 2005. Low-Cost Multi-Touch Sensing through Frustrated Total Internal Reflection. In Proceedings of the 18th Annual ACM Symposium on User Interface Software and Technology
The figure below comes from his web site.
An acrylic panel is edge lit with infrared leds. When your finger comes in contact with the acrylic, it scatters infrared light out the back where it is visible via infrared camera. As long as nothing is touching the acrylic, very little of the light escapes, instead just reflecting around inside. Image processing takes care of detecting tips of fingers and relaying their location to application software. Since the camera "reads" the whole display in parallel, it is easy to detect multiple fingertips at once, even those belonging to multiple users. All this sensing goes on in the infrared spectrum, leaving us free to utilize the visible spectrum to display interactive software.
Since most hobbyists can't afford multiple projectors (i don't even own one, just borrowed it from dr.eel), my design uses a ceiling mount that swivels so that the projector can be used either in standard mode (say for watching movies) or can be aimed downwards, bouncing off a reflector and onto the multitouch display screen.
The screen itself can be constructed from hardware store materials and hand tools. Excluding the projector and modified webcam (commodity items these days), the only thing complicated is the software. Halfway through this project, I was happy to discover that there is thriving DIY community which has already undertaken the task of writing the image processing code and several cool open source demos which can be found here:
touchlib
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Apr 22, 2010. 5:34 AMdanny9893
says:
Excuse me but i have a question about the diffuser...What does it actuaiiy do?
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1
Jun 11, 2010. 10:15 AMadaminc
says:
The acrylic is clear, without the diffuser, the image from the projector will pass right through, the diffuser is acting like a screen.
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Sep 12, 2008. 3:39 PMPyrozz
says:
Very awesome. Uhm, how is it that the projected image doesn't interfere with the IR light that the webcam is receiving? Does the filter on the camera filter ALL light except the IR?
Reply
Sep 14, 2008. 3:47 PMPyrozz
says:
Never mind, I get it (after doing some research).
Reply
Jan 13, 2010. 7:51 AMmankros_dakren
says:
Please, share your findings with us . I, for one, am quite interested, but do not want to do my own research.
Reply
Aug 5, 2009. 10:01 AMmrsmither
says:
hi i love the hole thing but i have a very important question... how long does the thing last cose i have been told the silicon stuff wears out??????? thanx
Reply
Nov 11, 2009. 6:07 AMMadMonso
says:
Regular caulking wears out, the new silicon caulking comes with a 65-year guarantee and that's with water exposure so plan for the silicon to outlive the use of the screen.
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