Transparency between / outside two nulls

[marchermitte]

Hello!
I'm having a though time figuring that one out. (I'm not a node expert, far from It!)
I'm animating a tentacle. I have blades running along It. I have to use transparency to limit the blade opacity inside the boundary of the tentacle. (I'm doing It like that becouse I need hi-speed moving blades, which I can't get with the traditional "tank thread trick") Basicly I want to use two nulls parented to the bone to limit the blades opacity between those two nulls. Of course, the tentacle won't have the same surface and therefore won't be affected.
The problems I'm having are:
-The transparency doesn't extend before the first base null.
-the mophed blade keep the original end null position but offset It when at 100% morph. They use a relative position rather than absolute.
I tried a more classical technic with a black and white transparency map projected in the x axis. It works as long as no deformation happen but then keep the frame 0 size and position whatever the current deformation is.
I hope It makes sense...

[marchermitte]

Ok, I would have loved to know how to do this with nodes (limit transparency outside the distance delimited by two nulls). I found a simpler way with shift's boolean texture.

[Medi8or]

Do you have to use nulls?

As I understand it, you move the blades with a morph, so how about using morph amount to move the keys in your gradient node?

Something like nodes below.
"Morph map (2)" has an expression that gets morph amount. The expression is just "[tentacle.MorphGroup.sawMorph]".