Controlling Z-depth is important in maintaining the stereoscopic effect.
The emergence of stereoscopic 3-D TV as a practical option for broadcasters has opened up a whole new set of issues in relation to best practices for channel branding graphics. Key issues include where to place the 3-D graphics in terms of the perspective, or Z-depth, for optimal viewing and how to control the Z-depth of graphics during playout to compensate for changes to the program perspective.
Getting the Z-depth right
To illustrate this Z-depth issue and some of the associated challenges, it's worth considering a simple case of logo insertion for a stereoscopic 3-D program with a changing 3-D perspective. (See Figure 1.) When the program has a near (negative) depth, with the 3-D effect appearing to come out of the screen toward the viewer, there is a requirement to have the logo positioned in front of the action to maintain a natural perspective (Case 1). During a sequence with a flat perspective, or a far depth (with the perspective effect going into the distance), the branding graphics need to be just in front of the action (Cases 2 and 3). If the Z-depth positioning of the graphic is incorrect at any point, the channel branding may lose its 3-D effect or, worse, the presence of the logo or graphic may interfere with the 3-D effect of the program itself.
Adjusting the graphics Z-depth
The Z-depth of a channel branding graphic can be changed to suit a program sequence by adjusting the horizontal separation of the left and right branding graphics, which are required to create a stereoscopic 3-D logo. This method of controlling the Z-depth of elements is often called horizontal image translation (HIT). By separating the right and left images of the graphics in one direction, the graphics will appear to come out of the screen. Conversely, when the left and right branding graphics are moved horizontally relative to each other in the other direction, they will appear to move into the screen. (See Figure 2.)
Hence, one way to address a logo's Z-depth problem is to have multiple versions of the logo, each with different left and right horizontal separations. This is not difficult to accomplish, as stereoscopic 3-D graphics can be created using standard graphics tools such as Adobe's Premiere and After Effects, which are much easier to use than traditional 3-D news graphics (nonstereoscopic).
However, an obvious problem with this approach is that it demands more complex media management to cope with all the different depth positions required for a logo. Therefore, this approach has not been widely adopted, and the focus has been toward controlling the Z-depth by dynamically changing the separation of a single pair of left and right branding images. Moving the Z-position of the graphics is relatively simple; it's more difficult to decide when to move them.
Controlling the Z-depth for live content
There are multiple options for automated and manual control of the Z-depth of channel branding graphics. One factor that influences the approach is the type of content to be played out, in terms of whether it is live or prerecorded.
A significant number of the initial applications for stereoscopic 3-D TV are likely to be live events, such as sports. With this type of programming, playout automation can be used for driving graphics, such as bugs, on and off for different segments. However, it can't easily be used for Z-depth control because of the unpredictable nature of live programming.
In this case, it's often best to supplement the automated control of branding with manual Z-depth control, using a branding control panel with depth presets. (See Figure 3.) By using presets, the operator can quickly and easily rectify graphics Z-depth issues using smooth depth adjustment transitions.
In the future, it's anticipated that advances in 3-D metadata playout will enable more sophisticated automated control. The channel branding processors will be able to read Z-depth metadata, probably in a similar manner to reading AFD metadata, and automatically adjust the position of the channel branding to optimize the presentation. (See Figure 4.)
Another related automated control option is dynamic measurement of the Z-depth by the channel branding processor, or an associated signal processor, and performing on-the-fly adjustment of the branding graphics according to the depth data. This may represent a good back-up solution in the absence of Z-depth metadata. However, both of these advanced automated control techniques are still in the formative stages and are not fully proven to date.
Z-depth control of recorded content
Broadcasters that play out channels of recorded content, such as 3-D movies, have additional options for Z-depth control. One simple approach is prerecording of the channel branding with the content ahead of playout on-air. However, this is not very practical because the show content includes the branding graphics, which limits the reuse of the same copy of the content on different channels or with different branding.
Another alternative is to have the automation system control the Z-position of the logo inserter by recalling the position presets using either a serial command or general purpose input. This is more flexible but requires an extra piece of information that has to be entered in the traffic or automation system, which is not possible or practical for many operators.
A third option is to have the content creator specify the position as the content is edited or reviewed, and to enter this information as metadata in the program. This is broadly similar to the approach currently used for presenting closed captions in 3-D Blu-ray productions. An operator manually adjusts the branding Z-depth using a fader while rerecording the content. This is a time-consuming process, but it offers the advantages of simplicity and consistent quality control. Standards committees are working diligently on Z-depth metadata standards to enable this simplified playout model.
In summary, we're still very much in the early adoption phase of 3-D TV, with the playout equipment still being developed and the associated workflow processes still being refined. There are some obvious parallels between the current state of channel branding for 3-D TV and the early phase of HDTV with its associated aspect ratio control issues. However, equipment vendors, broadcasters and organizations such as SMPTE are now working together closely to overcome the current obstacles, and more elegant and more efficient solutions to 3-D TV branding are already on the horizon.
Michel Proulx is CTO of Miranda Technologies.