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Aliasing With Bright Red Edges

Published : 10th June 2005

Someone recently showed me a snapshot of a scene in which a man is wearing a bright red tie. Everything about the image looked fine except the edges of the tie exhibited extreme stair-stepping. I've seen this, though not that often, in other images as well and experienced it when I was playing around with painting a Varicam and pulled out the red and green and pumped up the blue.

The shot I first mentioned however was shot straight. Any ideas what's going on here?

I know red has a tendency to smear, but the heavy aliasing really surprised me.

Dan Coplan
Cinematographer / DIT
www.dancoplan.com


Dan Coplan wrote :

>A bright red tie. Everything about the image looked fine except the >edges of the tie exhibited extreme stair-stepping.

G’day Dan,

I was just looking at some footage this afternoon that had the same issue. The difference being it was MiniDV. I have seen this before on other Mini DV productions and have always assumed it was *undocumented feature* of the format. Compression ? It's almost like the compressor can't deal with the red and you end up with chunky macro clocks of artifact, especially when the camera is moving.

Although the shot I saw it on this arvo was a DVX100a shot, I've also seen it on Canon XL's. Is the Varicam using DCT compression like the other DV formats ? Perhaps it's a DCT compression artifact ???

John Brawley
DOP
Melbourne Australia
www.viciousmedia.com


>Everything about the image looked fine except the edges of the tie >exhibited extreme stair-stepping.

I'm assuming this was 4:1:1 or 4:2:0 footage? That would be the chroma information being sampled at a lower frequency and showing its particular drawbacks. DV can also have a tendency to offset the chroma in multi-generational encoding so that might exacerbate it further.

> Is this something that you would see on a monitor during shooting or >only upon playback?

You might see it in very high chroma contrast situations - especially when it is pure red against a light - or even better/worse - a white background. It really depends on the scene. Furthermore it really matters how you are monitoring your camera. Are you looking at the compressed DV coming through onto a PC monitor, or are you seeing the analog signal coming off the RCA jacks or other connections on your
camera.

If it is the compressed DV stream (as in using something like DV Rack) then yes - you would see it as the results on screen are the actual DV file. With a field monitor it would probably depend on its quality (better calibrated and quality might show it more), but I don't really know for sure.

Regards,
Christopher Mills
Visual Fx TD
Wellington, New Zealand


I should clarify that the red example did in fact come from a miniDV camera, not a Varicam which was what I was messing with when I pulled the green and red and pumped up the blue.

That would be the chroma information being sampled at a lower>frequency and showing its particular drawbacks.

Is this something that you would see on a monitor during shooting or only upon playback? Like I said, I just saw a snapshot, I wasn't present during the shoot. I'm assuming, then, that the same would happen if it were a bright blue tie?

Dan Coplan
Cinematographer / DIT


 

 

 

 

 

 

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