Uncommon noise on Alexa footage

Kevin Laurent · October 15, 2011

Hello guys!

I'm not used to post here, but I've an issue with some footage shot on Alexa.

I was DoP on a short film a few weeks ago, shot on Alexa, and a great part of the rushes are noisy.

It's not like video noise, but it's too much for me.

I know that the Alexa is supposed to have a very low level of noise, so I'm wondering if I did something wrong on set.

Due to post-production computers, we shot on SxS cards, Prores422 (not HQ). We shot in LogC, 800ASA.

So my questions are:

Is the format Prores422 responsible for this noise?

Is there any post-production "denoising step" I should be aware of?

If you have any question to get some info that could help me, I'm here.

Thanks.

Kevin

Oliver Temmler · October 17, 2011

Hi Kevin,

it sounds to me like you took the Log C material and applied a grading on this without using a Log C to Video conversion Look Up Table. Please visit the ARRI website's digital camera section and use the LUT Generator to create a conversion LUT for whatever system you apply your grade on.

Typically, you want to switch to the Advanced Tab, then set

Camera (source): ALEXA, V3 color, Log C, Extended Range

Image (destination): Video, Extended Range

Conversion param.: Rec709 color space, Photometric, Knee shoulder 1 for a harder/ 2 for a softer highlight and blacks roll off.

LUT Param: Auto (will create a 3D LUT), default Dimension and Bits, File Format for your Grading Tool.

> Generate

Then put the created LUT into your grading tool as a viewing & output/rendering LUT to go to Rec709 Video color space. Or just as a viewing LUT if you want to leave the graded material in Log C for output to film.

Hope this helps.

Kevin Laurent · October 17, 2011

Hi Oliver,

Thanks for the answer. We used DaVinci Resolve for colorgrading, with its 'Alexa LogC to Rec709' LUT convertion.

I will look at this tool for LUTs, but the problem is not here.

In fact, I already have noise on the LogC footage.

So I wonder if it would have been better to record Prores4444 or 422HQ (I'm talking only about noise).

Unfortunately, the film was shot in just one day, so I didn't have time to do all my measures.

I will talk to my colorist about it and see if I can get some more info about the low lights of my scenes...

I was also wondering if it's better to shot at 400ASA or even 200 in low-light scenes. I know that the latitude is better, but what about the noise?

Thank you!

Kevin

John Sprung · October 17, 2011

We've never used anything but 422HQ and 4444. What was the bit rate of the mode you used? Perhaps the problem is too much compression.

-- J.S.

Kevin Laurent · October 18, 2011

We used Prores422 - 25fps, so according to Arri Specs, it should be something like 122Mbit/s.

Do you think this bitrate could cause noise?

John Sprung · October 18, 2011

That's about half the bitrate of 4444 and HQ, so it could be part of the problem. It was intended as a quick source for offline, rather than the final product.

-- J.S.

Kevin Laurent · October 19, 2011

That's what I think too, but I thought it would be visible as compression artifact, not as noise...

We shot in 422 due to editing computer that wasn't powerful enough.

Thanks for the answer.

Vladimir Usoltsev · August 11, 2012

Hello. I think the problem is in the ARRI LUT Generator. With the standard parameters it doesn't make correct LUTs.

I have the same problem - very noisy image in Da Vinci and After Effects with LUTs from ARRI LUT Generator.

You can generate good LUT for Da Vinci by this way:

1) Use "Advanced" bookmark

2) Set "Source Gamma = Log-C"

3) Color Space = None (1D) - !!!!!!!!!

4) Destination Gamma = Video

5) LUT Parameters = 3D LUT Forced

6) File Format = Da Vinci Resolve

It looks very strange, but it works. You will have good and clean ProRes images from Your SxS cards.

If You could compare these Da Vinci grades with FCP Glue Tools Log-C->Rec709 plugin You can see very little less saturation in Da Vinci with LUT. But it looks fine.

It works!!!

But the main problem is the impossibility to make Your own Look file for Alexa playback on the set and use it further as the LUT in Your posproduction.

You can use ARRI Look Creator for this. And after shooting You can convert Your Look file to the Da Vinci LUT by the ARRI LUT Generator.

But really it doesn't work - the image will be very noisy again!!! Because in the bookmark "Look" (ARRI LUT Generator) You can upload Your Look file to convert to the Da Vinci LUT.

But You can'c choose "None(1D)" in the "Color Space"!!!

I don't understand what's wrong? Can somebody explain me how to use it?

Oliver Temmler · August 13, 2012

Hello. I think the problem is in the ARRI LUT Generator. With the standard parameters it doesn't make correct LUTs.

…

OK, sounds like you two spotted a problem no one else is having. Please describe in what way the output does not look correct.

You can generate good LUT for Da Vinci by this way:

1) Use "Advanced" bookmark

2) Set "Source Gamma = Log-C"

3) Color Space = None (1D) - !!!!!!!!!

4) Destination Gamma = Video

5) LUT Parameters = 3D LUT Forced

6) File Format = Da Vinci Resolve

It looks very strange, but it works. You will have good and clean ProRes images from Your SxS cards.

If You could compare these Da Vinci grades with FCP Glue Tools Log-C->Rec709 plugin You can see very little less saturation in Da Vinci with LUT. But it looks fine.

…

It looks very strange because a 1D LUT can only provide the tonal mapping, but not the conversion of ALEXA Wide Gamut Color Space to Rec 709. Hence also the very little less saturation you describe. That's why the color space should be Rec709 and not "None", i.e. ALEXA Wide Gamut. Selecting "3D LUT Forced" makes no sense if you are not touching the color space.

So, the correct conversion would use the parameters (Advanced) Log C Extended Input, Video Extended Output, Rec 709 Color Space Output, 3D LUT is set automatically, file format DaVinci Resolve.

But the main problem is the impossibility to make Your own Look file for Alexa playback on the set and use it further as the LUT in Your posproduction.

You can use ARRI Look Creator for this. And after shooting You can convert Your Look file to the Da Vinci LUT by the ARRI LUT Generator.

But really it doesn't work - the image will be very noisy again!!! Because in the bookmark "Look" (ARRI LUT Generator) You can upload Your Look file to convert to the Da Vinci LUT.

But You can'c choose "None(1D)" in the "Color Space"!!!

I don't understand what's wrong? Can somebody explain me how to use it?

Well, it should work and a 1D LUT, as said, could not apply a color transformation. Therefore a 3D LUT is required. There's nothing wrong with it.

You find a LUT, generated with the settings I described above, in the attachment to this post. Please let me know if you still are seeing this problem, send me a screenshot if possible. If the problem persists, we'd be happy to have a look at one of the original camera clips (not graded, trimmed or anything). Please send us an email to digitalworkflow[at]arri.de and we'll open up an FTP account for you.

Kind regards,

Oliver

AlexaV3_K1S1_LogC2Video_rec709_EE_davinci.zip