Particle vertex data in shader?

[eppz]

Hi,
I have a particle like below, using Stretched Billboards.

(See gif here)

Now I’d like to access the Speed Multiplier value (actually the length of the particle) for each particle in shader. I’d be happy either with the vertex position data (white) or even a simple speed value (green).

Please help if you have some information here.
It would be quiet time consuming to trial-and-erroring this.

[eppz]

Wow, it was not that time consuming.
The length of the velocity, not surprisingly.

float speed = length(input.velocity.xy)