I happened to come across how "Keystone", one of the menus on a remote controller for a projector, has been translated as 台形歪み. This might be referenced.

Here are some examples:

When the projector placement angle against the screen is changed while projecting the image, the picture will undergo keystone (trapezoidal) distortion.

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Depending on the amount of keystone adjustment needed and the image content, some information may be lost, or the picture quality may suffer.

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Projection sizes are approximate values for full-size picture with no keystone adjustment.


Please consider this is just for you information. Hope it may help.

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Note added at 5 hrs (2009-01-25 00:07:12 GMT)
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I'm sorry I mis-spelled the answer, which is correctly "keystone (trapezoidal) distortion".

Here are some existing osages of trapezoidal distortion:
http://www.wipo.int/pctdb/en/wo.jsp?wo=2004104668
http://www.pairtext.com/disp-2006-14233.html

Keystone distortion is also translated as キーストン歪み. Here are some related links:
http://www2.aimnet.ne.jp/nakahara/3dart/3seisak9.html
http://www.optronics.co.jp/lex/detail.php?id=1747
http://ci.nii.ac.jp/naid/110003678615/

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Note added at 3 days7 hrs (2009-01-28 01:51:10 GMT) Post-grading
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Harvey, just to clarify, this phrase appeared in the projector manual I was handling last week. It's like coincidence! -