Tag Archives: highlights

ProRes Raw Over Exposure Magic Tricks – It’s all smoke and mirrors!

There are a lot of videos circulating on the web right now showing what appears to be some kind of magic trick where someone has shot over exposed, recorded the over exposed images using ProRes Raw and then as if by magic made some adjustments to the footage and it goes from being almost nothing but a white out of over exposure to a perfectly exposed image.

This isn’t magic, this isn’t raw suddenly giving you more over exposure range than you have with log, this is nothing more than a quirk of the way FCP-X handles ProRes Raw material.

Before going any further – this isn’t a put-down of raw or ProRes raw. It’s really great to be able to take raw sensor data and record that with only minimal processing. There are a lot of benefits to shooting with raw (see my earlier post showing all the extra data that 12 bit raw can give). But a magic ability to let you over expose by seemingly crazy amounts isn’t something raw does any better than log.

Currently to work with ProRes Raw you have to go through FCP-X. FCP-X applies a default sequence of transforms to the Raw footage to get it from raw data to a viewable image. These all expect the footage to be exposed exactly as per the camera manufacturers recommendations, with no leeway. Inside FCP-X it’s either exposed exactly right, or it isn’t.

The default decode settings include a heavy highlight roll-off. Apple call it “Tone Mapping”. Fancy words used to make it sound special but it’s really no different to a LUT or the transforms and processes that take place in other raw decoders. Like a LUT it maps very specific values in the raw data  to very specific output brightness values. So if you shoot just a bit bright – as you would often do with log to improve the signal to noise ratio – The ProRes raw appears to be heavily over exposed. This is because anything bright ends up crushed into nothing but flat white by the default highlight roll off that is applied by default.

In reality the material is probably only marginally over exposed, maybe just one to 2 stops which is something we have become used to doing with log. When you view brightly exposed log, the log itself doesn’t look over exposed, but if you apply a narrow high contrast 709 LUT to it, it then the footage looks over exposed until you grade it or add an exposure compensated LUT.  This is what is happening by default inside FCP-X, a transform is being applied that makes brightly exposed footage look very bright and possibly over exposed – because thats the way it was shot!

This is why in FCP-X  it is typical to change the color library to WCG (Wide Color Gamut) as this changes the way FCP-X processes the raw, changing the Tone Mapping and most importantly getting rid of the highlight roll off. With no roll-off, highlights and any even slight over exposure will still blow out as you can’t show 14 stops on a conventional 6 stop TV or monitor. Anything beyond the first 6 stops will be lost, the image will look over exposed until you grade or adjust the material to control the brighter parts of the image and bring them back into a viewable range. When you are in WCG mode in FCP-X the there is no longer a highlight roll off crushing the highlights and now because they are not crushed they can be recovered, but there isn’t any more highlight range than you would have if you shot with log on the same camera!

None of this is some kind of Raw over exposure magic trick as is often portrayed. It’s simply not really understanding how the workflow works and appreciating that if you shoot bright – well it’s going to look bright – until you normalise it in post. We do this all the time with log via LUT’s and grading too! It can be a little more straight forward to recover highlights from Linear Raw footage as comes form an FS5 or FS7 compared to log. That’s because of the way log maintains a constant data level in each highlight stop and often normal grading and colour correction tools don’t deal with this correctly. The highlight range is there, but it can be tricky to normalise the log without log grading tools such as the log controls in DaVinci Resolve.

Another problem is the common use of LUT’s on log footage. The vast majority of LUT’s add a highlight roll off, if you try to grade the highlights after adding a LUT with a highlight roll off it’s going to be next to impossible to recover the highlights. You must do the highlight recovery before the LUT is added or use a LUT that has compensation for any over exposure. All of these things can give the impression that log has less highlight range than the raw from the same camera. This is not normally the case, both will be the same as it’s the sensor that limits the range.

The difference in the highlight behaviour is in the workflows and very often both log and raw workflows are miss-understood. This can lead to owners and users of these cameras thinking that one process has more than the other, when in reality there is no difference, it’s appears to be different because the workflow works in a different way.

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Log and Raw Don’t have highlight a highlight roll off.

This just keeps coming up over and over. Almost all log gamma curves and the majority of raw recording formats don’t have a highlight roll-off. Any roll off that you might see is probably in the LUT’s that you are using.

The whole point of log and raw is to capture as much information about the scene that you are shooing as you can. Log normally achieves this by recording every stop above middle grey with a constant amount of data, so even the very brightest stop has the same amount of recording data as the ones below it – there is no roll off.

In conventional limited range recordings such as Rec-709, hypergamma, cinegamma etc, highlight roll-offs work by reducing the contrast in the highlights to make the amount of data needed to record the very brightest stops much smaller than used for the rest of the image. This allows 2 or 3 stops to be squeezed into a very small recording range, keeping most of the recording data available for a nice bright high contrast image. The reduction in contrast in the extreme highlights helps hide any highlight handling problems and makes it appear as though the sensors clipping point is reach in a more pleasing soft manner.

But you don’t want this in a log or raw recording as it makes grading much harder as the footage will contain different contrast ranges, each needing it’s own grading adjustments. Also by reducing contrast in the highlights you are reducing the data. It would be very difficult to un-pick a highlight roll off and if you did want to expand the data back out you will get issues such as banding.

S-log-levels Log and Raw Don't have highlight a highlight roll off.
Chart showing S-Log2 and S-Log3 plotted against f-stops and code values. Note how little data there is for each of the darker stops, the best data is above middle grey and there is no highlight roll-off. Note that current sensor only go to +6 stops over middle grey so S-Log2 and S-Log record to different peak levels.

S-Log2 and S-Log3 like almost all log gammas have no highlight roll-off. The only roll off is from middle grey and down. So if you underexpose you will start to roll away the data in your scenes mid range and that’s not good. Expose for the mid range, this is the most important part of any image. If your highlights are a bit clipped don’t worry about this too much. In post production you can add a roll off in the grade that will make any clipped highlights roll away gently. Adding a bit of highlight diffusion in post will also nicely mask any clipped highlights and make them look natural.