Integrating chromakey video - by Martin Blazeby
Prior to recording the masked performances of actors during the workshop phase of the project, the ancient theatre context settings were carefully selected and the relevant 3D models of these spaces were prepared. These ranged from temporary stages, small performance spaces, medium sized roofed odea and large sized open theatres. All of these 3D models had been created for previous research based projects but needed to some modifications to coincide with the chromakey video output. This mainly entailed the creation of virtual cameras intended to simulate spectators viewpoints situated at specific locations from within each 3D theatre.
Working in the chromakey studio, the 3D theatres were setup and previewed to the actors so that each recorded performance could be tailored specifically for each individual theatre setting. The chromakey video camera was then positioned within the studio designed to mimic the viewpoints of the virtual cameras within the 3D theatres. The physical performance area of the recording studio was also plotted and modelled in each 3D theatre, along with the height of the actor.
A calibration still image was taken of the actor in the performance area using the studio video camera; this was then compared by overlaying a calibration still image taken from the virtual spectators’ position within the 3D theatre model. The two calibration images needed to match as precisely as possible with each other, for this to succeed, the angle and projection of the studio camera had to coincide with the angle and projection of the virtual camera; this then ensures an exact alignment between the two perspectives and exact proportions relating to the actors' height.
This calibration process sometimes took several attempts for a like for like comparison, resulting in fine positioning of the studio camera and virtual camera which was necessary to achieve a good match before any footage was recorded. However, due to limitations of the studio camera position and studio size (the camera has a maximum recording distance of around 8 metres from that of the actor), it was not possible to match accurately that of a virtual camera in a large theatre environment such as the Theatre of Pompey. The maximum distance between the actor and camera viewpoint in the Theatre of Pompey is in the region of 70 metres, therefore it was impossible to match precisely the two perspectives, but still possible to calculate the exact actors' height for calibration purposes.
This initial calibration setup was a vital process, as the chromakey footage of the masked performance needed to fit as precisely as possible with the dimensions of the virtual theatre environment. Files produced during the calibration process were deemed just as important as the resultant chromakey footage as they create vital documentation as part of the paradata process and proof of scholarly accuracy.
The resultant masked performance video footage then is subjected to a post production editing phase using chromakey software. The green studio background is removed (keyed-out) and replaced with the virtual theatre spectators' view as a still backdrop; the masked actor is then scaled in size according to the calibration template created for each theatre. The overall effect of combining the virtual camera within the 3D model and the real camera used during the chromakey recording sessions provides a pseudo realistic image of the actor upon the stage as viewed by a virtual spectator at a specific place within the theatre.
Illustrated case studies for integrating chromakey video into virtual theatre environments:
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