garl@nlm-mcs.arpa (Gary Letourneau) (05/14/89)
I have a question concerning the video command playVideo.
A typical call of playVideo is:
playVideo here, stopFrame
where here < stopFrame. This works fine. However, if I try the
same command with stopFrame < here (in order to play backwards),
it just plays forward. Why did it not play backwards? Is the
keyword 'here' only allowed for playing forward?
How can I play backwards from the current frame? I tried the
following:
playVideo videoframe(), startFrame
This worked, but it did not give a smooth motion.
Again how I play backwards from the current frame? By the way,
I am using user XCMD version 2.2d9, and serial port XCMD version
2.5d1. Unfortunately, at the time I am writing this message, I do
not have available the version number of the videodisc driver
for a Pioneer 4200 that I am using.
Gary Letourneau
letourneau@mcs.nlm.nih.govchesley@goofy.apple.com (Harry Chesley) (05/16/89)
In article <9487@nlm-mcs.arpa> garl@nlm-mcs.arpa (Gary Letourneau) writes: > How can I play backwards from the current frame? I tried the > following: > > playVideo videoframe(), startFrame > > This worked, but it did not give a smooth motion. There are two ways to play backward from the current frame. The first is the one shown above. It queries the player for the current frame number (videoFrame()) and then plays from there to the frame number given. But since the player continues to play after the query, it results in a jump back to the frame returned by videoFrame (i.e., isn't smooth). The other way is to say: playVideo here,1 This will play from the current frame to the beginning of the disc, and will be smooth. Why can't you say something like this? playVideo here,1200 Because the XCMDs don't have any idea what frame the player is currently playing, so it can't know whether 1200 is before or after the current frame, and therefore it can't know whether to play forward or backward. To find out, it would have to query the player, which is just what you're doing with the videoFrame version. By specifying frame number 1 (or 0), it knows that it's playing to the start of the disc and therefore must be playing backward. In some players, you can actually give it a player control sequence to play from the current frame to a particular frame number and it figures out whether to go forward or backward. But other players need to be told explicitly whether it's forward or backward. So if the XCMDs used that feature, they wouldn't work on all players... I suppose I've said more than enough about this. It's just that I spent a lot of time thinking about it while writing the XCMDs, so...
ns@cat.cmu.edu (Nicholas Spies) (05/23/89)
Speaking of the videodisc XCMDs, has anyone at Apple (or elsewhere) written a genuine videodisc <driver> (DRVR) that could be used for _any_ Mac application that wants to talk to videodiscs. I would image that one would be able to choose the player type in the Control Panel, that the driver would present a common interface for those commands that apply to all players but would permit you to invoke _any_ special functions on special players, and that the driver would handle requests to the disc player on an asynchronis manner so timeouts are handled gracefully. The current videodisc XCMDs are a wonderful step in the right direction, but why doesn't Apple follow its own advise and package this functionality so any Mac application can make use of it... -- Nicholas Spies ns@cat.cmu.edu.arpa Center for Design of Educational Computing Carnegie Mellon University --