Why It's SO HARD To Do CGI Skin! #VFXBreakdown
3,228Recreating a human using CGI has always been a complicated task. The human body is something we all know intimately... We know exactly how it moves, looks, and reacts, any slight variation we see is instantly recognizable and therefore breaks the illusion.
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The process of recreating human skin has come a long way since the 1992 film "Death becomes her" produced the first software to recreate human skin and the 1999 film Fight Club produced the first photorealistic close-up rendering of a human face, and then continued to evolve with Brad Pitt in Benjamin Button in 2008.
But even though the VFX industry has been perfecting 3D scans, Motion capture, Rigging, and Animation it has never quite managed to get a CG Human to look 100% real... That is, until Weta Digital and the 2019 film Alita: Battle Angel.
The main reason CG humans have failed to fool us in the past is that human skin is an incredibly difficult thing to replicate. Human Skin is highly detailed, complex, and ever-changing, and this is what has made it impossible to digitally reproduce convincingly, it isn't just a protective external layer on our bodies, like the paint on your car, it's our body's largest organ.
It moves when the muscles or fat beneath it move but it also moves of its own accord to adjust to temperature changes.
It has different thicknesses and textures depending on its location on your body and exposure to the elements.
It doesn't just have one color but a combination of different tones that depend not only upon your natural tone, but exposure to the sun and even your mood!
Displacements.
Our faces, like any part of our bodies, aren't 2 dimensional, they have depressions and elevations that are affected by light differently and therefore alter our skin tone in that area.
From a distance, you can see that the chin, nose, and cheekbones are elevated and the eye sockets and areas around the nostrils and lower cheeks are depressions.
Move in closer and you notice we have wrinkles that have their own depressions and elevations. Go even closer and you'll see our pores, which are also depressions, the center of a pore is a darker tone and all of them have different sizes and orientations, the orientation of the pores is vital for accurately simulating the flow lines of our face.
Dynamic Changes.
Dynamic changes are the changes our skin undergoes when we use the muscles underneath it, these changes cause our forehead to wrinkle, lips to pout, cheeks to rise or mouth to open, creating depressions and elevations on which light acts differently and therefore the skin's tone changes in that area.
In order to accurately capture these dynamic changes, Weta used two cameras on the actress Rosa Salazar's HMC or Helmet Cam Unit, not only did this enable them to capture Salazar's performance but also depth information of what was physically happening to her face as she performed.
Weta's pipeline also uses Deep Learning so in addition to all the FACS poses they recorded with Salazar, they wanted additional data to add to the training data.
Subsurface Light.
As we mentioned before, our skin isn't just like the paint on our car, it's not uniform, some areas are thicker than others, some are tougher, some areas are greasier and others have more hair and the light that hits these areas responds differently.
Light does just bounce off objects, a certain amount of light actually penetrates the object and then bounces around inside it.
If you shine a light or a laser at an object you will notice it bounces off, but you can also see a certain amount of spill around the point where the light impacts, this is called Subsurface scattering and it's different for almost every object.
Albedo.
Albedo is the term they use for the map of the tones of your skin. imagine your face with a neutral expression, at a comfortable room temperature and with no bright lights or shadows cast on it, it wouldn't all just be one color, your forehead may be paler, your cheeks redder, this is your neutral Albedo or tonal map, it's individual to each person and
it's not a constant.
when we get angry, extra blood flows to our forehead and cheeks making these zones redder, when we are happy the blood flow to different areas changing our facial tone map depending on our mood.
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