# How Menus Work - Part 2

### Timing and Force Action

A Menu doesn't just appear and stay on screen forever, unless you want it to. There are several controllable phases to presenting a menu. In order, these are:

- Render the underlying video
- Render the subpicture overlay
- Enable highlighting and user interaction
- Disable user interaction and optionally take a default action
- End the highlighting, subpicture overlay, and video presentation.

And even though we are specifically talking about menus in these two articles, all of this information applies to button-over-video.

#### Render the Video

This will happen first, and automatically. The only important thing about the audio/video is that it be long enough for the entire menu program. If need be the audio/video can be looped.

#### Render the Subpicture

Believe it or not, this can be done at the same time as the video, or later. If care is not taken, the subpicture can be rendered before the video. This is usually not desirable visually. The controlling factor is in the subpicture itself, SP_DCSQ_STM, the delay. For menus and button over video the command to render the subpicture should be FSTA_DSP, force subpicture start.

#### Enable Highlighting and User Interaction

In the PCI are several timing values that are solely for menus. The first is hli_s_ptm (packet offset 0x062), the highlight start time. This is normally zero, so that highlighting and user interaction are enabled immediately, but it can be set to a presentation time later than the video presentation time stamp (PTS) to delay showing the selected button and allowing user interaction.

#### Disable User Interaction and Optionally Take a Default Action

Two values in the PCI that affect this. First is btn_sl_e_ptm (packet offset 0x06a), button select end time, which controls when this happens. If left at zero user interaction remains active as long as the menu is displayed. This again is a presentation time, and must be later than hli_s_ptm, if specified, and the video presentation time stamp.
The other value is foac_btnn (packet offset 0x075), force action button. If this is non-zero it specifies a button number that will get pressed for the user at button select end time. In this way you can provide a time-out, taking a default path if the user is too slow. For multiple display mode menus this is the base button number, the number used in the first set of 12 or 18 buttons.

#### End Highlighting

The value controlling this in the PCI is hli_e_ptm (packet offset 0x066), highlight end time. Normally this will be zero for a menu, allowing highlighting to continue as long as the menu is displayed. It becomes important for button-over-video to prevent the highlighting from continuing on until the next button is presented. But you could use it in a normal menu. Once again, it is a presentation time, and must be later than the other times listed above.

#### End Subpicture Rendering

Another important aspect of button-over-video, it can be used in normal menus as well. This is also controlled by the subpicture commands. The normal sequence of events is to have 2 SP_DCSQ (command sequences) in the subpicture, each with its own delay. The first controls the delayed rendering, and uses the FSTA_DSP command, the second controls how long the subpicture is displayed, and uses the STP_DSP command.

#### End Audio/Video Rendering

Of course you don't do this for button-over-video. But for normal menus the video has a certain duration, it may be a short as one frame. The duration can be extended by looping or freezing (still) at the end. The two most common forms are a still (single I frame extended by way of cell still time) or motion video that loops.

#### The Navigation Timer - Another way out

Note: This applies only to menus in the title set (VTS), not to "normal" menus in VMG or VTSM.
The navigation timer can be used to go to another menu or video segment that is not selectable by any button. This gives you another way to take action if the user doesn't press any buttons. Using the navigation timer requires another PGC in the same title domain as the current one as a target. You cannot directly leave the domain via the navigation timer, but, of course, the target PGC can execute any legal command to jump you to another title within the same titleset or a dummy PGC in the VMG domain.
To use the navigation timer as a time-out action place the SetNVTMR command in the pre-commands for the menu PGC. The value set into the timer is the number of seconds to wait before automatically going to the specified PGC. The timer should be cancelled if a button is pressed by setting the value to zero (the PGCn in this case doesn't matter).
The SetNVTMR has direct and indirect (uses the value from another register) forms. In hex it looks like:
42 00 00 rr pp pp 00 00 - SetVNTMR to register rr, PGCn pppp
52 00 vv vv pp pp 00 00 - SetNVTMR to vvvv, PGCn pppp
note: values (such as vvvv and pppp) in DVDs are stored with the most significant byte first (lowest byte address). This is the opposite of the byte order used by the x86 (Intel) architecture.
The register rr may be any GPRM (0x00 - 0x0f) or SPRM (0x80 - 0x97).

| More articles |

DVD-Video info home | Copyright © 2003 - 2020 MPUCoder, all rights reserved.