Channel Variables and uses?

[LaidlawFX]

Hello,

I came across the channel variables doc and was wondering if anybody could give me some short worded examples of how you would use them. Like the $CH was cool to use for an adjunct to relative/absolute referencing to modify the string, by replacing tx, ty, tz. The segment stuff has no context for me, nor the slope stuff.

Help Channel Expression Variables

Bad linkage, Type Global Expression Variables in sesi's help

Here is the list I'm most curious about.

Channel Variables

$IV In value (value at start of segment).

$OV Out value.

$IM In slope

$OM Out slope

$IA In acceleration

$OA Out acceleration

$LT Local time - not including stretch or offset

$IT Start time of segment

$OT End time of segment

$LIT Local start time of segment

$LOT Local end time of segment

$PREV_IT Previous segment start time

$NEXT_OT Next segment end time

Edited October 18, 2011 by LaidlawFX

[Macha]

Interesting. I never noticed these things!

About segments, is it related to how Houdini treats keys? As far as I know Houdini treats keys and channel curves in a special way. It is all about whats between the keys. So if you set a key and then write an expression in it then whatever follows that key will use that expression. For example, make a few keys. Choose one of then and exchange the default expression linear() to cos($FF). You see? Everything following the key (the segment) is set to that expression. Magic! The next key we can set the expression to something else, etc. Those are the segments.

Those globals must have some use for this!

Edited October 18, 2011 by Macha

[edward]

The "segment" is the portion of the fcurve between keys. Houdini's channels are segment-based as opposed to key-based. At each end of a segment, there is a "half-key", which defines the value, slope, etc. of the key on that particular side of the segment. The "in" values refer to the half-key values to the left of the segment, while the "out" values refer to the half-key values to the right of the segment.

As an example use of these local variables, try typing this expression on a parameter that has no animation: "($T - $IT) * $IM + $IV" (without the quotes). Now scope the expression in the channel editor then manipulate the slope on the one key that was created at frame 1. This creates a "linear" segment that is controlled by the slope to left of the segment.

[LaidlawFX]

Houdini is like pandora's box, once it is open it keeps on pouring more stuff out...

Thanks guys, would never have found that out on my own.

Do you know any more useful common functions like those expressions? A couple more examples would help me contextualize using them a little bit better.

[Krion]

Is it possible to get the channel variable from another channel?

A function or something whose arguments would look like chanvar("../../geo1/tx", IT)? (i.e. get beginningtime of the current segment from that channel)

 

Thanks.

Edited September 1, 2021 by Krion