Subtle hot pixels throughout blue channel

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Policar
Posts: 5
Joined: Mon May 01, 2017 4:08 am

Hi, using an Alexa Plus 4:3 to a Gemini 444 recorder shooting ArriRAW. Viewing LogC with no LUT in After Effects.

I have noticed subtle but visible hot pixels throughout the image, most visible in the blue channel. Attaching an image zoomed in 400% and another similar image with frames blended together but no other noise reduction to highlight where the hot pixels are. They're persistent between clips, int the same place.

These don't seem severe enough to mask individually – there are too many of them, anyway. But this is 24fps, I know the Alexa gets hot pixels with long shutter speeds, but this is unexpected. However, it is also my first time working with ArriRAW and I've noticed it's noisier than either 2K or 3.2K ProRes, which might be masking similar issues with compression.

What should I do to fix this? (Or is it normal?)

Loving the image otherwise, even all these years later.
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Anders Holck
Posts: 51
Joined: Mon Dec 14, 2015 9:40 pm
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It's hard to judge from the screengrabs how pronounced the variation is, but I would definitely send the camera to ARRI Service for a factory sensor calibration, and a following image QC judgement.

We have had to replace a few sensor blocks with ARRI refurbished ones over the years as the sensors age. But all our Alexa EV, EV Plus, Studio, 4:3, XT and SXTs are gone now and replaced by newer cameras. Oldest camera is the Mini, and we even have had to exchange a few sensors here as well due to aging and photosite defects become a problem, and user pixel mapping was becoming unfeasible.

Did you double check in the ARRI Reference tool https://www.arri.com/en/learn-help/lear ... rence-tool if you see the same issue?
Anders Holck DFF

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Policar
Posts: 5
Joined: Mon May 01, 2017 4:08 am

Thanks. Yes, the pixels are still visible in Arri reference tool.

How do I render in LogC3 out of Arri reference tool btw? The only option I see is for LogC4, which is not compatible with cameras before the Alexa35. So I can only render in rec709, etc.

The variation is relatively small. It's almost not really an issue, very hard to notice unless you look for it. I bought this camera for personal use, so it's not the end of the world. But it's enough to bother me.

It's an old 4:3 with 5000 hours on it, but when I received it there was no issue and it had just been calibrated by Arri I believe. Anyway, it's been a year or so since then and I haven't used it since – I was gearing up to use it and was shooting ArriRAW and noticed this issue.

It reminds me a bit of the "hot pixels" you get when doing step printing with the Alexa (long shutter speeds in excess of 1/4th second or whatever), except far more subtle. Those can be awful. So I was wondering if it was normal, but my old ArriRAW clips from like a year ago don't have it. So whatever happened developed while the camera was sitting around.

Very curious how to render LogC3 out of Arri reference tool. I used to be able to! Maybe it was an old version. (Edit: it was, I mean I was using ArriRAW converter. It's been a while.)
Last edited by Policar on Mon Nov 07, 2022 1:59 am, edited 1 time in total.
Policar
Posts: 5
Joined: Mon May 01, 2017 4:08 am

Also – my old clips that were fine come into After Effects as 2880X2160, the new ones are 2880X1614, but they render as 2880X2160 in ART.

This is my first time working in ArriRAW. Certainly more confusing than ProRes. Using a Gemini444.
Anders Holck
Posts: 51
Joined: Mon Dec 14, 2015 9:40 pm
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Hi. LogC/AWG export of LogC3/AWG3 from Legacy cameras doesn't seem to work in the reference tool yet. SCENE linear/AWG3 in EXR does seem to work. Seems you need the legacy ARRIRAW converter to do a native LogC3/AWG3 ProRes export https://www.arri.com/en/learn-help/lear ... y-software
Anders Holck DFF

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info@andersholck.com
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Policar
Posts: 5
Joined: Mon May 01, 2017 4:08 am

Thanks. Well, now ARC won't open, it just crashes. After Effects can read ArriRAW but for some reason is cutting off pixels at the top and bottom. ART renders correctly but doesn't support LogC3. I'll worry about all this later (or just shoot ProRes if I have to).

Is it possible this is a result of the camera sitting around too long (the hot pixels)? It doesn't bother me now for personal projects, but I'd like to fix it later. Thanks.
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