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Shooting Venetian blinds


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#1 Dan McCormick

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Posted 29 January 2008 - 03:39 PM

Hi

When shooting thin, open venetian blinds on film will there be a movement effect (don't know the spelling for the proper name, is it something like moray phonetically?). By either closing them or overeposing behind the window enough can the effect be avoided? If they are closed but there is still some light hitting the edge of the slats leaving very sharp contrast on each what would happen?

Thanks

Dan McCormick
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#2 David Mullen ASC

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Posted 29 January 2008 - 06:06 PM

Hi

When shooting thin, open venetian blinds on film will there be a movement effect (don't know the spelling for the proper name, is it something like moray phonetically?). By either closing them or overeposing behind the window enough can the effect be avoided? If they are closed but there is still some light hitting the edge of the slats leaving very sharp contrast on each what would happen?

Thanks

Dan McCormick


Yes, it's called "moire" -- it is aliasing, an interference pattern of the horizontal lines of the blinds interacting with the lines of the TV monitor or even the fixed pattern of photosites on the sensor (though that is more subtle, it's really standard def TV viewing that is the problem).

If the blinds are out of focus, it's not so much of a problem, or if you are more diagonal to them. It's somewhat hit or miss, and it may show up in NTSC but not HDTV, depending on your luck. There are some post tricks to soften the effect in the telecine transfer. If you are shooting on video, you can adjust the angle or focus or blinds to minimize the effect when looking at a monitor, though the effect can be worse on some monitors more than others, which is why you often just want to avoid thin blinds.

Probably the worst when you look straight at the blinds in a wide shot where they are just thin lines on camera.
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#3 John Sprung

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Posted 29 January 2008 - 06:50 PM

If it's shot on film, printed, and projected, there isn't much of a problem. Film grain is a sort of random sampling structure, so the aliasing is random, and subjectively not anywhere near as objectionable.

The problem shows up with digital or video, where you have a regular sampling grid that can alias against the regular pattern of the blinds. It happens in both HD and SD, but HD has about twice as many lines, so the blinds have to be about half as big in the frame to give you the same amount of aliasing. In HD, you more often see the problem in smaller detail, like the vent slots on the side of a computer monitor, rather than things as large as venetian blinds.

SD video, also, is mostly interlaced, which means that the aliasing is both spatial (moire) and temporal (small area flicker, or to use the British term, twitter). Broadcast HD is mostly 1080i, with some 720p, but shows are almost always posted in 1080p and downconverted.

In all cases (except film) there is an optical low pass filter ahead of the sensor, which is supposed to remove high frequency (fine) detail that will alias instead of being correctly reproduced. In general, the OLPF is cheated slightly to allow a small amount of aliasing. The trade-off is that by accepting a little aliasing, the rest of the picture can look a little bit sharper. This cheat is generally pushed a little farther in SD than HD, because HD has lots of resolution, and can afford to trade a bit more away to reduce aliasing.

The OLPF has to happen ahead of the sampling, i.e., the sensor. Thereafter, there's no way to distinguish between real detail and aliasing.




-- J.S.
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