Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!goofy.apple.com!chesley
From: chesley@goofy.apple.com (Harry Chesley)
Newsgroups: comp.sys.mac.hypercard
Subject: Re: playVideo videodisc command
Message-ID: <1869@internal.Apple.COM>
Date: 15 May 89 22:38:49 GMT
References: <9487@nlm-mcs.arpa>
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 40

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...