April 14, 2017 at 9:10AM

3

C100 MkII - Blue Sky "Banding"

Hello fellow film makers!
Just have a quick question regarding my C100 that I can't find the answer to when googling.

I keep finding shots from outside, where the sky just looks horrific and really banded (if that's the right word). It looks low resolution and the blue has different "steps" to it.

Just wondering if anyone have any experience with this, has an answer to what it could be and if there is any fix to it?

Here is a couple of links to screen shots of footage that I shot. One is the raw footage, which was shot in C-Log, 320ISO, 50fps, Mp4 35mbps.
The other one is the same shot, just added some contrast to the image and sharpness, that unfortunately makes it look even uglier.

Links:
Raw: https://dl.dropboxusercontent.com/u/11946478/Raw-C-Log-Iso320.png

Contrast added: https://dl.dropboxusercontent.com/u/11946478/Contrast%20and%20sharpness%...

Would love to hear what you guys think.

18 Comments

So It is great that you shot this in Clog. You may have a good shot at recovering this.
I can hazard a guess the banding is being introduced in the Clog->rec709 conversion.

Sky blue is one of the most challenging colors encountered in normal shooting. It is a large continuous mono-chromatic gradient.

The easiest post fix is to add grain in the canon log space. A small amount of grain can break up the visible artifacts and prevent the eye from getting color edges to focus on.

The next easiest will be to reduce the saturation or contrast of just the sky. I would guess this will work best after the log lut in rec 709, or your display of choice, space.

More complicated fixes:
Create your own custom canon log to rec 709 lut that doesn't stretch the blue color vector as far.

Track and blur the sky on shots where this is an issue. There are a few ways to do this and this can be effected in display space, rec 709.

In shooting the 50Mb codec felt a bit light for the quality of the sensor,talking from experience with the c300 mk1. You can try gradient diffusion for the sky to break it up, or an external recorder that will hit 4:4:4 color sampling and a higher bit rate. Though likely the lut is the real challenge as the original canon log to rec709 luts had a lot of contrast and make some issues like this stand out.

April 14, 2017 at 2:29PM

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Reply
Joseph Slomka
Color Scientist - Acquisition through archival
344

Hi Joseph! Thanks for a great answer. Just a question as colour grading is probably my least experienced area: How would you go about adding grain to the canon log? Do I need plug ins or would it work with stock files to add on top?

Morten Furre

April 14, 2017 at 8:06PM

Morten,
As far as adding grain goes there are many ways to do it. You can try the buit in function of any of the finishing software. If the problem is in the source footage, applying the gain to the source will help out, otherwise you can add it to the output.

Joseph Slomka

April 17, 2017 at 3:08PM

While Joseph says it is great you shot it in C-Log I tell you it is one of the causes the banding happened.
C-Log covers 14 stops?
Trying to put 14 stops into a 8 bit codec means you have 256 values in each color channel that need to be stretched over all those stops. That is leaving you with less than 20 values for 1 stop. Since blue skies have a subtle gradient in the blue channel, your codec's bitrate just can't display it and is forced to band at the edges when any channel changes its value by 1.
If your sky has a gradient with a range of 3 stops in the blue channel and it is covers half of the screen it has less than 60 blue values that need to cover 540 lines. On average that means blocks of 9 lines high. With 1 stop difference you get blocks of 27 lines.
The lower the 'contrast' of a 'solid' in the frame and the higher the sensor's latitude and the lower the bitrate of the codec, the more banding you get.

The difference between 8 bit and 10 bit is 2 bits. That is the difference between 256 and 1024 values for each channel. In a 10bit codec the banding will be at least 4 times smaller (if only 1 channel in involved). If the color gradient involves all 3 channels, the amount of banding can be 64x smaller.

Adding noise can indeed help, because it 'blurs' the edges of the banding.

April 16, 2017 at 8:28AM

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WalterBrokx
Director, DOP, Writer, Editor, Producer
8565

Thanks Walter!
I was actually thinking of doing some tests comparing C-Log and Wide-DR around this issue, because I've never noticed it this bad when I've done shoots for clients that has been outside. Reason being is because most of it has been fast turnaround projects, so I've used wideDR instead.

Might have to try it and see how I go.

Morten Furre

April 16, 2017 at 8:56AM

FYI: The Canon C100 mk1 and C100 mk2 cameras capture 12 F-Stops of dynamic range, but you still get banding when recording internal 8-bit files.

Adding noise to footage you've already shot might be enough to fix things, and you might want to look into shooting with an external recorder to up the bit-rate and to record in 4:2:2 color format. ( instead of 4:2:0 )

Guy McLoughlin

April 16, 2017 at 8:32PM

Morten,
Camera tests are always a great idea. Bit rate and compression issues don't help this problem. The further you stretch the capture images on the final display the more banding you will see.

The advantages of shooting in a log format, and one as mild as Clog I over weigh the general disadvantages of shooting in a gamma encoded format.

Just quickly looking at the Clog curve it gives you ~4 stops of exposure over neutral grey and expect about 4 below that many below.

In terms of encoding the Log format does not encode stops of data as Walter explained. For the darkest stops where the signal is mostly noise, very few bits are used to encode. The camera effectively is designed to only start encoding data 3 stops below 18% grey. Only 11 code values are used in 8 bit to describe stops -4 to 0, that is only 44 in 10 bits. You would need to recorded 12 bit uncompressed(which this camera doesn't have ) to have significant image data in the deepest blacks in clog.

The Clog curve is designed to more heavily weight the sections around mid tone and highlight, with the brightest stops, getting the most encoding range.
Stops +11 to +12 get 39 of the possible 255 code values in 8 bit.

Joseph Slomka

April 17, 2017 at 3:37PM

It's because you're shooting at iso320. You're losing latitude and information in your highlights. Shoot at the native iso850 and use the built in ND filters to get correct exposure. I shoot with a c100 mark ii and I never go below the native iso and I've never had any banding issues.

April 16, 2017 at 10:33PM, Edited April 16, 10:39PM

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Reggie Brown
Cinematographer
332

Good point. The camera will let you select a low iso, but it will just effectively reduce your dynamic range when you shoot bright images.

Joseph Slomka

April 17, 2017 at 3:38PM

Here's a related article that's worth reading. It'll give you a better understanding of using native iso's.
I tested it on the c100 mark ii and it stands true for that camera. I'd suggest testing it on yours too!

https://www.cinema5d.com/exposing-with-native-iso/

April 17, 2017 at 12:01AM, Edited April 17, 12:04AM

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Reggie Brown
Cinematographer
332

Been shooting with C100 mkii for 2 years now...totally agree with the above, never shoot below 850 iso. You'll find that you'll have no choice but to stop a lens down in bright light even with the 6 stops of ND, but it's worth it. You'll retain a lot more information at 850 iso than anything below that.

That said, I still occasionally find banding in the sky with my footage if I'm heavy handed with it. I've become fairly adept at shooting with it so that I rarely get banding, but there are instances where I'm outside and then come to tweak the contrast of the footage and bam, the banding is there. The c100 really is squeezing a lot into it's puny codec, but when exposed in the sweet spot it rarely matters because the image is beautiful, but if you're slightly over exposed and bring it down, or you want to slap some really heavy contrast on there (such as if I shoot something with sky/ocean in there and want it to be dramatic) then you'll see banding appear. It's worth noting though that whilst 10 bit 422 would certainly help with this, it isn't a cure all. I also have a BMPCC and find that in similar situations there is no banding at all in either RAW or ProRes, but instead there can be some nasty and colourful noise in the shadows with that kind of heavy grade so I end up having to work that harder than the C100 footage.

So I guess all I'm saying is that in any case you'll need to treat an image in the correct way to get what you want. As someone said above, a little film grain can help break up the banding in a pinch, but try messing around with different exposures in the same situation. Good luck dude!

April 17, 2017 at 5:27AM

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Liam Martin
DP, editor, part time director
474

Thank you Liam! I did have a suspicion that the 320ISO was an issue as well, but wasn't sure. Great to get some insight from other C100 users!

Morten Furre

April 17, 2017 at 10:49PM, Edited April 17, 10:49PM

Yep. I made this mistake for many months of shooting with my C100 MkII after switching from Canon 5D mkIII where I was used to the "best" ISO being the lowest ISO. On the C100 MkII, the "best" (native) ISO is 850 (hence the brackets around it when you select it [850]). Try that next time!

From there, use the ND filters or f-stop to adjust for outdoor lighting.

April 17, 2017 at 3:54PM, Edited April 17, 3:55PM

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Thanks Jack!

Morten Furre

April 17, 2017 at 10:49PM

Also, upon reviewing your image. This banding is actually related to the compression and bitrate on the C100 (which in my opinion isn't the best). I use an external recorder which inevitably removes this type of banding in most cases. To avoid the banding while recording internally, be sure to use the native ISO, but also don't underexpose. It tends to occur more in underexposed areas. Use Waveform and make sure your important (brighter) areas are around 60 in c-log.

April 17, 2017 at 3:57PM, Edited April 17, 4:00PM

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I've lent out my C300 to a number of people who are used to shooting on DSLR's, and they get disappointing results, because their thinking is "lower ISO, better Image Quality". This is very much not true for the C-line. I never, ever go below 850, except on the very rare occasion I have an INTENTIONALLY dark scene with low contrast, where no highlights are close to clipping. It might seem counter-intuitive, but the low ISO settings are only for dark scenes, where you want to push your latitude into the shadows. Notice I say dark, and not "low-light", because the scene usually needs to be brighter to your eye than it looks on the monitor, and most of the image lying between 10-50 IRE on your waveform monitor.

April 18, 2017 at 10:37PM

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I haven't shot on an inter-frame codec in a long time, but back when I used to shoot on a canon dslr, and I know this may seem weird, before color grading I would convert all of the clips into pro res 422. This significantly removed the banding I would get in the gradient shadows.

Also I've used the noise removal software in Resolve to soften banding on iphone clips. The motion noise removal in resolve blends together like colors and significantly takes care of this problem.

This answer isn't supported by science or anything but it has helped me in the past.

Best of luck.

April 19, 2017 at 4:59PM, Edited April 19, 4:59PM

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Josef Lorenzo
Director
242

Thanks Josef! I'll try the noise removal software in Resolve for sure!

Morten Furre

May 2, 2017 at 4:44PM

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