DBW Communication’s Stefano Rebecchi talks 3D techniques and challenges
Based in Rome, DBW Communication srl has been providing a wide variety of innovative services for broadcast since 2000. As its founding membership status of the 3D Stereoscopic Group would suggest, the company has been a pioneer of 3D filming, and in this special interview DBW Communication’s Stefano Rebecchi describes the challenges attendant to 3D capture.
Rebecchi explains: “All the video materials video created for stereoscopic 3D viewing are subject to a very strict quality control. The first correction is on the image: the two lenses are never completely identical and therefore produce differences in focus and depth of field. This produces a vision out-of-phase between the two eyes, potentially causing serious eye problems, at times.
“Other problems arise from the cameras that produce images slightly higher one from the other. A difference of only 1mm sometimes produces many discrepancies in the proportion from the height of the same subject in a long shot. It is the vertical parallax issue: the brain couples these two images that do not coincide, creating problems.
“Many other challenges are related to language, for example the ‘violation of the stereoscopic window’. In a 3D image the perimeter of the screen is a window through which we look at the scene. The ‘stereographer’ is the professional who determines the spatial positioning of objects: on the bottom of the screen or in the frontal plane, or they leave it and get closer to the viewer.
“But sometimes – for example, in a medium shot of a man where his face comes out of the natural boundary of the screen while the body remains inside ‘unnaturally’ – the optical perspective paradox creates a scene beyond our normal understanding of people and objects.
“A first control [mechanism] in a film is [that provided as] a whole by the stereographer. If any doubt remains, an objective ‘stereoscopic processor’ by Sony or 3ality comes into action. However, there are cases where the violation of the stereoscopic window is intentional and justified because it is ‘authorial style’.”
Camera workflows and management
“When shooting in 3D on TV, as in sports generally, there are cuts which correspond to the positions of the many cameras on the field, always double for each position,” continues Rebecchi. “DBW Communication is able to manage simultaneously up to 26 HDTV cameras for filming in 3D HD through the use of 13 stereoscopic motorised rigs. All functions of the rigs, as well as those of the cameras, are controlled in real-time by a series of C-Motion high precision motors.
“The cameras are connected via fiber optic to the studio and equipped with high-precision motors that receive control information from the director. Each camera has its own very expensive dedicated processor. A professional specialist – or ‘convergence puller’ – controls the processor and performs the continuous correction of convergence.
“Sometimes in sports, some 2D HD signals are converted in 3D and integrated with the rest of the production with a three-frame delay. The two factors that determine the 3D effect are the distance between the lenses – trying to repeat the distance between the eyes – and the point at which the eyes are crossed, which represents the convergence.
“In a typical 3D football game filming the feeling of depth is sometimes recreated so much higher than that the viewer sitting in the same position as the cameras would have.
“The lenses of the cameras are placed farthest away voluntarily on the rig at a distance greater than the actual distance between the pupils, and in some cases even more than 70cm. Therefore the information between the two virtual eyes differs a lot and the 3D effect is much emphasised.”