Search

The Official Capture Blog

On tone mapping, exposure and aperture brightness

One of the rendering advancements we made in Capture Nexum was the adaptation of and rendering in ACES AP1, also known as ACEScg. Rendering in ACEScg comes with a couple of major advantages over traditional sRGB rendering as it has a larger colour gamut than sRGB and comes with an HDR tone mapper. The larger colour gamut  makes it possible to accurately blend colours outside the sRGB spectrum  which is great news because  a lot of (up to around a third) of all the gels we use in entertainment cannot be represented in sRGB and many, especially the blue, emitters of LED fixtures are also too saturated for sRGB. Getting the colours right is only half the story though - a large part of the secret to a good looking visualization lies in intensities.

The human eye  and brain make quite a team. It lets you find your way in your bedroom at night as well as on the beach in full summer daylight and is even fairly good at telling details in darker areas in otherwise lit environments. Granted, it has its limitations - this is why we need sun shades for consoles on outside jobs and we crank the brightness of our cell phones to max when we're outdoor, so we can read it without squinting. And this exactly is our challenge in lighting visualization - while the iPhone 7 has a peek brightness of 700 nits, computer screens designed for sRGB only goes to 100 nits. Yet we need to visualize blindingly strong fixtures focused into the audience without making darker parts of the scene impossible to read.

In truth the equation is impossible, but fortunately we already have a convention in place since long, courtesy of cinematography and digital photography. It all comes down to the behaviour of film under different camera exposures and the fact that digital cameras of today all essentially emulate this behaviour since we have been trained to a particular look from watching movies, TV  and digital photos since we were kids. The exposure level of the camera dictates the "comfortable viewing brightness" and anything far below it turns out indistinguishably dark and anything far enough above it burns out to white (is "over exposed"). These rules that dictate how intensity affects colour when mapping high intensity range imagery to low intensity computer screens is what is called "tone mapping".

Exposure illustration - read more on https://photographylife.com/underexposure-and-overexposure-in-photography

In the image above you can see that it is only possible to tell details in the dark ventilation gratings under the windows in the over exposed part of the image, while it is only possible to tell details of the window panes in the under exposed parts of the image - there is no way of getting details in both at the same time. Also notice how the over and under exposed parts of the wall look almost colourless.

 Here is where we arrive at the point of this long excursion. Since we released Capture Nexum we have received a lot of feedback on fixture apertures looking white in visualization which may seem counter intuitive and makes it difficult to program colour effects and things like pixel mapping. However, if you run a Google image search on "concert lights" or take a look at pretty much any lighting design photo posted on Everything Stage Lighting on Facebook,  you will notice that the actual apertures of fixtures are very often over exposed and white, as otherwise the rest of the scene would likely have been pitch black. Now that Capture is using the tone mapping of ACES, the visualization results are near identical to those of digital  photography.

Illustration of over exposed apertures

Obviously this is both good an bad. On the positive side, things look "as they should", but on the other hand it can also be unpractical in some situations. When it gets really unpractical is when you have no haze or few illuminated surfaces that can give away the actual colour  of the lights and help your brain stitch it all together. In these situations what you can do is either decrease the brightness of the fixtures while programming and/or reduce the exposure setting of the view and make sure you have a bit of bloom effect enabled or even just the tiniest bit of smoke present.

One of the things we are working on for the next version of Capture is one or more alternative tone mappers with the ability to preserve colour information at the cost of perceived intensity realism. Sometimes realism is not what you need!

Tags