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

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>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

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>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

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>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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