This is sort of related to putting an actor's face on another, often in the case of the original actor being unavailable, including being deceased - the ultimate case of unavailability.
I suppose there are two ways to include an actor if they are unable to perform in the role. Given that the reason why you want a specific performer back is because this current production is a sequel or takes place in the same universe, you can always use archival or cut footage from a prior engagement. The problem with this approach is that it leaves the character seemingly in their own world: they don't react to anything and events just happen around them. Audiences pick up on this quickly and the movie magic is lost as a result.
The CG case is where we can "de-age" or bring an actor back to life, which I suppose is a sort of de-aging in itself. It involves mapping a digital prosthetic to the stand-in's face and remaping movements from the body double back onto the mask. In the case of de-aging, the body double is just the older version of the same actor. This is done with mo-cap typically and more and more special effects are being done within video game engines and development suites like Unreal and Unity, also contributing to the cutscene look of the final output.
In the past, film editors have been able to "live paint" as if using photoshop to retouch an image. The software follows the image between frames an applies the transformation. This additive method of altering the image is used in every single medium to high-production value piece of video media and we are used to the retouched look of it to the point where we don't notice it whatsoever. A skilled artist can do the de-aging here.
Hollywood is behind the high-tech world of AI-driven facial replacement by a few years even thought it all feels like old news to us. I suspect it will only get better and move from "uncanny valley" to "passable" within about five years and firmly into the world of "undetectable" within a decade. However, I believe that AAA video games will show us more realistic renditions a few years in advance of cinema because that industry's technology is driving film and TV and I believe will continue to do so for years to come.
I suppose there are two ways to include an actor if they are unable to perform in the role. Given that the reason why you want a specific performer back is because this current production is a sequel or takes place in the same universe, you can always use archival or cut footage from a prior engagement. The problem with this approach is that it leaves the character seemingly in their own world: they don't react to anything and events just happen around them. Audiences pick up on this quickly and the movie magic is lost as a result.
The CG case is where we can "de-age" or bring an actor back to life, which I suppose is a sort of de-aging in itself. It involves mapping a digital prosthetic to the stand-in's face and remaping movements from the body double back onto the mask. In the case of de-aging, the body double is just the older version of the same actor. This is done with mo-cap typically and more and more special effects are being done within video game engines and development suites like Unreal and Unity, also contributing to the cutscene look of the final output.
In the past, film editors have been able to "live paint" as if using photoshop to retouch an image. The software follows the image between frames an applies the transformation. This additive method of altering the image is used in every single medium to high-production value piece of video media and we are used to the retouched look of it to the point where we don't notice it whatsoever. A skilled artist can do the de-aging here.
Hollywood is behind the high-tech world of AI-driven facial replacement by a few years even thought it all feels like old news to us. I suspect it will only get better and move from "uncanny valley" to "passable" within about five years and firmly into the world of "undetectable" within a decade. However, I believe that AAA video games will show us more realistic renditions a few years in advance of cinema because that industry's technology is driving film and TV and I believe will continue to do so for years to come.