Protune Under Water: Contrast Crunch

Some time ago, I published an introductory blog post about Protune. In it, I explained that GoPro uses an S-shaped tonal curve for encoding the image the camera sensor sees into the three primary color channels RGB. The idea here is to extend the range at the far ends at the expense of the intermediate tonal range, in a way that Protune stays within the allowed tonal value range of 8 bits per channel H.264 video data. A good idea, it seems ... but unfortunately not always under water...

Protune: Always Better?

The tripping point that made me rethink the gospel of Protune came when I looked at the underwater footage shot with my new Canon HF G30. At first, I was slightly disappointed that the Canon exposure control did not make broad use of the available contrast. Luckily, some gearwheels inside my head started churning. And then it hit me that all the time I had believed in Protune without completely thinking through it and its limitations.

On the next dive, I arranged for shooting the same underwater scene in the same conditions simultaneously with a GoPro HD Hero 3 Black Edition and the Canon HF G30. The important question here is: what contrast range do I get in my footage for the typical underwater situations I'm filming in?

Of course, scene conditions vary widely. However, I'm often confronted with what I would consider difficult conditions. Not much light, a lot of debris and algae in the waters. So a lot of scattered light, and not much sight. A real test to any automatic exposure and contrast control in video cams ... especially as these automatic controls tend to be optimized for crisp surface imagery.

So I fixed one of my GoPro HD Hero 3s to my Ikelite dive housing for my Canon HF G30. Kind of canon viewfinder, me thinks. Then I shot the same scene with both cams. The GoPro HD Hero 3 Black Edition shot in 2.7K/25p in the usual wide perspective. The Canon shot in 1080p/25 with the zoom set into less than the half range.

Contrast Crunching Protune

And this is what I got: please see the image above. The main image comes from the GoPro HD Hero 3, which you can tell by its wiiide perspective. I graded it in post the usual way: Protune tonal curve decompression, contrast boost, saturation boost, and finally some sharpening. The reason is that raw Protune footage looks boringly dull. The important thing to note here is the histogram to the left: it shows the contrast range of the raw and ungraded Protune footage. This histogram thus reflect the amount of tonal information that's in the source. Nothing more there.

Unfortunately, this histogram gives no reason to GoPro to be proud of its cam. While exposure is fair, contrast ... well ... sucks. This situation is actually made even worse by Protune because exposure control puts the contrast range used into the middle range. And this is where Protune on purpose reduces resolution. For a scene with stark contrasts, this is fine. But for our purposes in difficult lighting conditions under water Protune actually does us no good. As you see in the histogram, the range towards the ends is totally unused.

The thing to learn is that in difficult low contrast situations Protune actually leads to loss of valuable contrast detail. Detail that would give use more room in post without causing posterisation. So, under water I would be better off most of the time not using Protune ... but unfortunately there is still the non-working auto white balance. If only GoPro would make the auto white balance optional without enforcing Protune. But, hey, did GoPro ever care about divers? Why else do they contruct and sell a camera that has severe thermal problems in anything except cold water? And which fogs at any chance? Whatever...

Protune is beneficial whenever the scene you are shooting is well lit and has a lot of contrast. The tonal curve bending done by Protune then allows to preserve some detail until post. As soon as contrast sucks, Protune may very well suck too, as it looses valuable detail in the mid range.

Contrast Cannon?

And now back to tge Canon HF G30. I shot the same scene at the same time, now in 1080p. I've scaled the full HF G30 image down and placed it into the upper right corner of the image above. The optical zoom of the HF G30 frames Eelfritt perfectly. Again, I graded also this image in post, but this time I needed just to boost contrast less and only slightly reduce green.

Now look at the right histogram that shows the contrast range of the footage fresh from the Canon HF G30. The automatic exposure control also does a fair job, seemingly better than GoPro's. Yet not stellar. There is more contrast in the video data left. And since the Canon HF G30 does no tonal curve compression in the mid range as GoPro does with Protune, there is more detail in the raw footage.

This will make grading more easy as this raw footage does not require boosting contrast to the same considerable extend that the GoPro footage from the HD Hero 3 does. Thus, much less danger of posterisation effects with the Canon HF G30, something that I noticed on many occasions with the HD Hero 3.

Let us finally take a look at Eelfritt in full frame, as shot from the same point as the HD Hero 3, but this time using the fine optics of the HF G30. Of course, comparing the 20× optical zoom of the Canon HF G30 to the HD Hero 3 and 3+ is like comparing oranges to peas.