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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Purple / Magenta Highlights

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Daniel Stone

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Hey guys,

I've heard this is a common problem with the RED cameras, so I did a search and haven't found a lot of useful information.

The issue:

We just filmed a car commercial with the Epic; most shots were RPPs with ND, Pola and a Hot Mirror. The footage looks beautiful except for the fact that everything near white had a purple/magenta color (the sky, overexposed highlights on the car, etc.). Both my DP and DIT said it's a "RED thing" and happens with all RED cameras.

I would love to know what you guys are doing to fix these 'purple highlights'. We've always just corrected it with secondaries in coloring but we've gotten to the point where we're shying away from the RED cameras because we know they're going to require this extra step. Is there something obvious we're missing? A fix in Redcine?

Thanks for any input!
 
it's usually not a RED thing but rather an infraRED thing or an Filter-Cast thing... but I agree - I often see this on RED footage ;)
other then in post it can be fixed with good filters (ND only where needed and not stronger then needed! Use Hot Mirror or combined IRND filters as needed) - I think stacking up too much filters (we never stack more then 2) can introduce side-effects, also keep an eye on your monitoring on set - you usually see this on your monitoring already (a bit of a desaturated look, no strong colors, no strong green, blacks appear not to be black)! As soon as you see it on your monitoring it's "baked in" and if it affects the footage too much it's very hard to fix in post!

how strong was your ND filter? 0.3 is usually fine, but everything above 0.6 and (depending on the light situation) you can notice it - usually weak but gets stronger with higher ND levels (it's really depending on weather, location, temperature, light-reflections)... Talking about ND only here (without Hot Mirror or combined filter).
 
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Daniel post your single frame here for download R3d.
You have a try to enable DRX in redcine-x?
 
It's not a filter issue, the issue is your not clipping in all channels evenly. Use the DRX slider in Redcine-X or Resolve and you will see those highlights go to white. DRX aligns all the channels for you so the channels clip evenly and go white. It's an easy fix. Try out the DRX slider and see how that works for you, and tell your DIT to use the slider next time to fix it.
 
hmm, if the colors are totally fine otherwise and only "clipping" areas or the sky are affected Matt should be right...
a frame grab would help to see what's the problem here. If your blacks are black and your non-clipping colors are "right" it's not an IR/filter issue! :)
 
Anyone who says "it's a RED THING" in a blanket fashion is usually pointing out their own lack of knowledge.

All it means is they see a lot of people making the same mistake they are making. Kind of embarrassing isn't it?
 
DRX fixes this completely. Key thing is not to over-expose though!

BTW - it's not a RED thing. I first noticed this on Canon DSLR RAW files in over-exposed regions.

Graeme
 
Matt Ryan said:
It's not a filter issue, the issue is your not clipping in all channels evenly.
Click to expand...
That's true, but you can also fix this in post with a good color-correction system. Even film does not clip evenly in all colors, because of the way the different emulsion layers are set up. I've never encountered a case where I wasn't able to balance out the levels in some way -- sometimes with some work -- so that everything clipped at equal, balanced levels.
 
Hey guys,

Thanks so much for all of the awesome info!

We have a prep day coming up with the Epic so we're going to do some testing. I'm also going to try the DRX slider.
 
I have seen this issue in other cameras, but it seems by far most prevalent in RED footage. I'm always amazed how often it does not get corrected and ends up in final deliverables. And when I see it in videos I know it was shot on RED. It always seemed odd to me that RED can't just have the camera deliver white whites in-camera without any DRX adjustments (if I recall correctly DRX can have a significant effect on saturation and other things). Is there some technical reason all of RED's gamma curves are designed to clip green at a lower luma than the other channels?

I remember once doing some quick tests and finding that a +30 green adjustment seems to balance out the issue at all ISOs. So this implies that you could potentially just always shoot with a full minus green filter on the camera and +30 green tint adjustment to avoid the issue completely. Would that work, Graeme? Why do all RED cameras have unbalanced white levels? Is there some important reason for this design?
 
Noah, when we were originally developing the R1, other RAW cameras had this issue with differential highlight clipping. Early on I came up with the DRX algorithm that fixes things, although rather than have it permanently on, it went on a user control.

Your comment on gamma curves doesn't make sense.

All cameras will lack balance in R=G=B because light is un-balanced and silicon is un-balanced (and even if it was, altering the light would un-balance things again).

Graeme
 
Noah Yuan-Vogel said:
…if I recall correctly DRX can have a significant effect on saturation and other things
Click to expand...

DRX has changed somewhat in recent builds, and the effect is much more limited to the highlights than it used to be.
 
Noah Yuan-Vogel said:
I have seen this issue in other cameras, but it seems by far most prevalent in RED footage. I'm always amazed how often it does not get corrected and ends up in final deliverables. And when I see it in videos I know it was shot on RED.
Click to expand...
Naaaa, many cameras will clip into a non-white color, depending on how it's been set up. And this is fairly trivial to fix in color correction. I generally try to be a very even-tempered guy and I believe in giving a client (especially a DP) what they want, but I'm pretty stubborn on the issue of clipped images staying white. They look much too weird to me if they're clipping yellow or green.
 
Graeme Nattress said:
Noah, when we were originally developing the R1, other RAW cameras had this issue with differential highlight clipping. Early on I came up with the DRX algorithm that fixes things, although rather than have it permanently on, it went on a user control.

Your comment on gamma curves doesn't make sense.

All cameras will lack balance in R=G=B because light is un-balanced and silicon is un-balanced (and even if it was, altering the light would un-balance things again).

Graeme
Click to expand...

Thanks for your response, I guess what I meant about the gamma curves was just that aren't the white point of each of the color channels defined by the gamma curve anyway, so couldn't the curve just be designed so white clip looked white? Or is there a particular problem with that or reason not to design it in that way? Are you saying it happens because the green channel clips before any other channel and there is no way to fix it with a gamma curve in camera? Perhaps a user can fix it in camera also just by adjusting the clip point of the green channel in a user curve since that is available in camera? I'm just trying to understand, as I'm not sure the exact reason or root problem is completely clear to most people, including myself. I'd understand if you didn't want to give away your color science secrets, though.

Also are you saying a minus green filter wouldn't balance the clip point and fix the issue?
 
No, white points and colorimetry are nothing to do with gamma curves at all. The correct way to fix this issue is with the DRX control. Different channels will clip first depending on the nature of the light source and hence DRX takes such differences into account and will also ensure you don't loose detail / dynamic range through doing so.

Graeme
 
DRX is very effective, simple and quick tool.

Many post-productions don't fix magenta highlights issue and it's present on high budget commercials, music videos and Hollywood blockbusters.
 
Filip Orlandic said:
Many post-productions don't fix magenta highlights issue and it's present on high budget commercials, music videos and Hollywood blockbusters.
Click to expand...
I must be watching the wrong shows! None of the stuff I see on normal American network TV has anything but white highlights when they clip. My usual objection is that they clip too often, ​but that's me. They're not magenta (or the opposite, green).
 
Marc Wielage said:
I must be watching the wrong shows! None of the stuff I see on normal American network TV has anything but white highlights when they clip. My usual objection is that they clip too often, ​but that's me. They're not magenta (or the opposite, green).
Click to expand...

MTV channel is full of magenta highlights and features such as Jack the Giant Slayer and many others. I don't wanna name them.
I see that as a production and post-production fault.
 
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