Differences between version 20 and predecessor to the previous major change of DVDVideoTerminology.
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| Newer page: | version 20 | Last edited on Saturday, January 9, 2010 2:40:22 pm | by LawrenceDoliveiro | Revert |
| Older page: | version 19 | Last edited on Saturday, January 9, 2010 2:05:43 pm | by LawrenceDoliveiro | Revert |
@@ -24,8 +24,10 @@
A [subpicture stream|http://www.mpucoder.com/DVD/spu.html] actually consists of a series of drawing commands. These commands do ''not'' make up a programming language--there are no variables or conditionals etc. They are simply interpreted in linear sequence. Commands do things like "set colours", "set contrast (transparency)" and "draw pixels into this rectangular area". Thus, transparent areas of the subpicture are defined simply by not drawing into them.
Subpicture pixels are _run-length compressed_, and the complete subpicture image _must_ fit into less than 65536 bytes. Any image that cannot be compressed to fit within this restriction (in the worst case, an image could take up to 103680 bytes for PAL or 86400 bytes for NTSC) is simply not allowed.
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+Alternative subpicture streams can not only be used for subtitles in alternative languages, they can also be alternative ways of presenting the same information depending on how a widescreen (16:9) movie is presented on a 4:3 screen. In this situation, the player may either “letterbox” the movie (show it scaled to fit the width of the screen, centred with unused black areas above and below), or do “pan and scan” (scaled to fit the full height of the screen, with parts chopped off at the sides to follow dynamically-changing offset information encoded in the MPEG video stream), and each option can have its own version of a subpicture stream. In all cases, the subpicture coordinates fill the visible screen, not necessarily the video area. In particular in the letterboxed case, the subpicture coordinates still fill the 4:3 screen, so they can appear in the black areas.
!More on Menus
Menus are implemented as subpictures superimposed on top of a video background. There is a single subpicture defining the appearance of all menu buttons, with a set of 4 colours appropriate to their default (non-selected) state. Two additional sets of 4 colours each, to be used in the highlighted and selected (when the user presses the "OK" or "Enter" button on the remote with a menu button selected) states respectively, are specified in PCI packets in the navigation stream. The actual area of each button is defined as a rectangle in a field in the PCI packet; this is what selects what portion of the subpicture to show in which colours when a button is in a particular state.
