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Arri Alexa and "pixel gridding"


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Hi,

Is anyone familiar and have experienced "pixel gridding" from Alexa footage? I see this appearing around bright objects in the frame. Footage is from an XT Plus.

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Regards,

Kristian Sundsvalen

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Posted (edited)

Ah! Now I see what you’re referring to. Sorry, I didn’t see the texture before now. Interesting. I guess the first question would be is it showing in the ARRI RAW footage? This could be from a variety of reasons. What sensor mode were you in?
 

G

Edited by Gregory Irwin
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This was ArriRaw and 16:9 2.8K. 25fps, 180 shutter. But it is also visible in ProRes 3.2K, and with difference fps and shutter. I have not tested it in Open Gate.
 

It is visible pre grading, and in different software. The images is exported from DaVinci, but the patterns is also visible with ArriRaw Converter.

I have been given feedback on that this is a normal behavior when there is objects clipping in the frame. But if this is a behavior that is normal I would be reassured that this is observd by others shooting with Alexa. In other words that it is not a quality deviation. 

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Got it. I’m consulting with my DIT because you’ve peaked my curiosity. I’ve shot a lot of open gate Alexa and have never experienced this. Let me get back to you. 
 

G

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Posted (edited)

Curious what you find. I was tempted to look through my library of Alexa footage but realized how infrequently it blows out!

What do you think of 2.8k ArriRAW vs 3.2k ProRes btw? My work is ProRes 3.2k but I couldn't afford an Amira so I bought a 4:3 Plus and Gemini recorder instead. But I like 2k ProRes enough I think for me. 😞

Edited by M Joel W
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8 hours ago, M Joel W said:

Curious what you find. I was tempted to look through my library of Alexa footage but realized how infrequently it blows out!

What do you think of 2.8k ArriRAW vs 3.2k ProRes btw? My work is ProRes 3.2k but I couldn't afford an Amira so I bought a 4:3 Plus and Gemini recorder instead. But I like 2k ProRes enough I think for me. 😞

Hi Joel,

So you are observing the same patterns?

Whatever difference there may be between ArriRaw and ProRes 4444, the difference is as far as I know negligible. In situations where heavy grading is necessary I have read that ArriRaw has it advantages. 

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I’ll have several questions for you regarding this as we go along. For now, we are interested in what the light sources were, if any filters were used? If so, which filters? Also, does the low pass filter look clean and clear?

G

 

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Posted (edited)
10 hours ago, Kristian Sundsvalen said:

Hi Joel,

So you are observing the same patterns?

Whatever difference there may be between ArriRaw and ProRes 4444, the difference is as far as I know negligible. In situations where heavy grading is necessary I have read that ArriRaw has it advantages. 

I don't see it in the footage I'm working with now. 3.2k Alexa. But it's all pretty high key lighting without too many light sources in frame so I only found one or two clips with much clipping. But I don't see this happening there even around blown out lights.

But the areas that are blown out are smaller so I can't tell.

Edited by M Joel W
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Posted (edited)
1 hour ago, Gregory Irwin said:

I’ll have several questions for you regarding this as we go along. For now, we are interested in what the light sources were, if any filters were used? If so, which filters? Also, does the low pass filter look clean and clear?

G

 

The light sources was a 8 by 8 ultrabounce key. I can`t remember if i used a LED with fresnel or a ETC Leko Source 4 (750w tungsten) to bounce from it. The image with the light fixture in it as a backlight is a tungsten 650w fresnel. I used ND´s for sure. .6 or .9 Schneider as far as i remember. Can´t remember the stop on the lens (SLR Magic APO Hyperprime). The low pass filter is clean and have no visible debrises. This patterns is also visible on outside footage with the sun as the only light source. I can see the patterns appearing around the sun for instance.

Edited by Kristian Sundsvalen
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Are we sure this isn't rolling shutter banding? It'd have to be a very, very fast flicker, although it's hard to be specific becuase I can't see how big this artefact is compared to the height of the frame. Either way, it is possible for this to be caused by flickering lights.

P

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21 hours ago, Phil Rhodes said:

Are we sure this isn't rolling shutter banding? It'd have to be a very, very fast flicker, although it's hard to be specific becuase I can't see how big this artefact is compared to the height of the frame. Either way, it is possible for this to be caused by flickering lights.

P

Hi Phil,

I have been getting feedback that this patterns can appear whenever there is objects clipping in the frame, and that it is considered normal. Which I believe, considered this comes from Arri . My agenda with this post is to get an acknowledgement that others shooting Alexa is experience the same. I have been shooting 25 fps (180 shutter) with European current (50 Hz). So flicker should not be occurring. 

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Just now, Kristian Sundsvalen said:

I have been shooting 25 fps (180 shutter) with European current (50 Hz). So flicker should not be occurring. 

Unfortunately with modern lighting that's not necessarily, or even often, the solution anymore. 

Still, bit odd, I've not seen it happen before.

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3 minutes ago, Phil Rhodes said:

Unfortunately with modern lighting that's not necessarily, or even often, the solution anymore. 

Still, bit odd, I've not seen it happen before.

Totally agree with you there Phil. I have been experience flicker from LED fixtures, even if it was shoot with "safe" shutters. However I can´t observe any flicker on the clips I have shoot regarding this topic. 

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1 hour ago, Kristian Sundsvalen said:

Totally agree with you there Phil. I have been experience flicker from LED fixtures, even if it was shoot with "safe" shutters. However I can´t observe any flicker on the clips I have shoot regarding this topic. 

The point is that with rolling shutter cameras, once the flicker gets to be fast enough, it stops looking like flicker and starts looking like banding.

This may absolutely be an Alexa-specific issue, but it's worth bearing in mind that flicker may manifest as stripes.

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Is it a net behind the lens or something? 

I don't see this in the footage I'm working with now, but there are only smaller areas in it that are blown out in this footage. But some windows and point sources do blow out here and there.

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20 hours ago, M Joel W said:

Is it a net behind the lens or something? 

I don't see this in the footage I'm working with now, but there are only smaller areas in it that are blown out in this footage. But some windows and point sources do blow out here and there.

No net behind the lenses on this footage.

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I've seen this on other lower-end digital cameras, but I have never seen this on an Alexa. 

To me, it looks like the de-bayer process isn't working in areas which are near clipping. But I wouldn't think those areas are "clipping" at all. 

In the past when I've had this problem, when I've edited in Rec 709, the problems have gone away. 

If you put it in a 2k ( I assume you're going 2k since you shot 2.8k) Rec 709 sequence and the problem is still there, I would absolutely contact an Arri tech and show them. It could be an issue with the imager.

Sometimes heat can cause imagers to do weird things like this. 

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22 minutes ago, Tyler Purcell said:

I've seen this on other lower-end digital cameras, but I have never seen this on an Alexa. 

To me, it looks like the de-bayer process isn't working in areas which are near clipping. But I wouldn't think those areas are "clipping" at all. 

In the past when I've had this problem, when I've edited in Rec 709, the problems have gone away. 

If you put it in a 2k ( I assume you're going 2k since you shot 2.8k) Rec 709 sequence and the problem is still there, I would absolutely contact an Arri tech and show them. It could be an issue with the imager.

Sometimes heat can cause imagers to do weird things like this. 

Hi Tyler,

I have been in contact with Arri Service regarding this. They are saying it is a normal behavior whenever there is objects klipping in the frame. And that there is nothing that can be done against it. 
Based on Arri's feedback I reckon this should be visible on all Alexa cameras. But I am starting to worrying that there is a quality deviation with my camera.

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6 hours ago, Kristian Sundsvalen said:

They are saying it is a normal behavior whenever there is objects klipping in the frame.

I just don't see how that could be the case. I've pointed Alexa's at lights before, had them entirely blown out and never seen this problem. The shot of the window, no way that was "clipping". Did you see the histogram or waveform when you were shooting? I usually keep those on when shooting to insure I'm not going off some reason. I also use zebra at 85% quite a bit.

Edited by Tyler Purcell
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