Posted by Jan B on 04/18/06 05:55

On Mon, 17 Apr 2006 15:46:34 -0700, Gene E. Bloch <spamfree@nobody.invalid>
wrote:

>On 4/16/2006, Jan B posted this:
>> (Sorry I had to work around a FreeAgent limit by move the follow up upwards
>> in the thread).
>>
>> On Sat, 15 Apr 2006 09:45:05 -0400, Tom Stiller <tomstiller@comcast.net>
>> wrote:
>>
>>> In article <4440e4d2.109134196@news.individual.net>,
>>> nospam@nospam.se (Jan B) wrote:
>>>
>>>> When such a camera pans horisontally over a vertical object like a
>>>> pole and the video is displayed on a CRT that takes nearly a field
>>>> period to scan from top to bottom I would expect to perceive a tilted
>>>> object "moving" across the screen. This is since the scanning at the
>>>> bottom would lag the top when displaying but not at the recording.
>>>>
>>>> Have anybody noticed such effects?
>>>
>>> You overlooked the effect of the electronic shutter that effectively
>>> freezes the image before it is scanned out. There is no relative motion
>>> during the scanning process.
>>
>> But that is excactly my point. The case cited above is the case with freezed
>> momentary picture in the CCD-camera, but the scanning in the CRT takes time.
>> To get a smooth "momentary strobed" motion over the CRT surface, the camera
>> picture should be scanned the same way (I think).
>>
>> The "moving" vertical pole should have moved to a later position when the CRT
>> scans the bottom lines.
>> /Jan
>
>The CRT doesn't scan anything, it just displays what the signal says.
>In this case, the signal says the pole is straight.

By "scanning" I mean that a CRT displays the signal by illuminating the image
from top to bottom line by line. That takes time (and is designed that way).

The illuminated phosfor fades out in a couple of milliseconds.
The question here is what happens with perceived motion when the scanning at the
recording (as in no scanning) and the scanning at displaying is done
differently. The arguments that claim that the strobed moving picture from a CRT
is interpretated by our brain as a smooth motion must also consider that the
moving vertical pole would have moved between the time the CRT scanns the top
until it scans the bottom.

My thinking is that the strobed moving pole can be perceived as vertical only if
the camera was scanned in exactly the same way. So a CCD camera with instant
half field (or frame) storage would give a tilted moving pole when displayed on
a scanning display.

I suspect that an LCD panel refresh when addressing the panel row by rowalso
takes time (not discussing slow pixel response here) , so I'm interested to know
how many milliseconds it takes from top to bottom.
/Jan

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