[PD] horizontal video lines

sara kolster sara at x-i.net
Fri Dec 3 10:05:11 CET 2004

Hi Chris / list,

At http://www.umatic.nl/pdp_picts.html you can see what a such a horizontal line looks like. 
Sometimes you can get similar results when you change the field 
dominance when you edit / compress video. See quote below from a mail i 
got from Miguel Carvalhais.

usually it comes from swapping the field order in videos, so if your  
video standard/codec uses lower field before upper field and you  
compress the video the other way around (upper first) then that means  
that by reversing the fields you get a bit of a flickering in the image  
as within each frame you're seeing reversed fields..

in a simpler explanation: when you have a sequence of frames 1 - 2 - 3  
- 4 - 5 - etc you have a sequence of fields within each frame:

1L - 1U - 2L - 2U - 3L - 3U - 4L - 4U - etc... each field containing  
half of a frame... if you number this correct order of fields then you  
F1 - F2 - F3 - F4 - F5 - F6 - F7 - F8 - etc
when they get swapped, this happens:
F2 - F1 - F4 - F3 - F6 - F5 - F8 - F7 - etc
so whenever we advance in time, we also skip backwards within each  
this effect is not too visible if there's not much motion but in takes  
with more movement it can become rather obvious...

I don't know how pd handles the field dominance of the video-input, but 
maybe it could help if you could manually change the field dominance 
from: not set, to upper and lower.

Just a thought, don't know if it's possible and if it would fix the problem.



chris clepper wrote:

> On Dec 1, 2004, at 2:52 AM, sara kolster wrote:
>> Hi Chris, Jamie a.o.
>> The horizontal lines don't really look like interlacing, it's really 
>> a field-swapping thing. The more you move the camera the more you see 
>> those horizontal lines going down your screen. You have 1 or 2 lines 
>> on the screen that seems to break the image in half and it seems the 
>> video cannot keep up with the speed in which it's moving.
> I haven't seen anything like that since the old Nato/OS9 days!
> Can you post a picture of this if possible?  The OSX Grab application 
> might be able to get the frame, but SnapzPro has a movie recording 
> capability that's probably easier.  They have a demo version that 
> works for a month or so.
> http://www.ambrosiasw.com/utilities/snapzprox/
> This is also a pretty good way to record the output of GEM windows, 
> especially on dual CPU machines.
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