Math modifer not affecting liquid particles consistently.

atomician

I'm having trouble getting the math modifier to work as expected, as per attached file. I simply want to make liquid particles go up or down based on their color.

This is what I've done:

Gravity set to zero
Created a liquid emitter that spawns a particles in a capsule shape.
Particle color is set to red: rgb(255,0,0)
I then apply a Math Modifier to change the velocity of particles based on color:
- Target property is Velocity
- Target channel is XYZ (have also experimented with just Y)
- Source is the color property, channel R
- Operation is Add
- Multiplier is 0, 100, 0

I'm presuming that the R channel value of a solid red particle will be 255?

So as I understand it, this means the red particles would have their upwards velocity increased (every substep? - currently set to 20) according to this formula:

velocity = velocity + color.r * v

So we start off with zero velocity so I assume the formula looks like this:

substep 1:
velocity = 0 + 255*100  // final velocity is 25500 for Y axis

substep 2: 
velocity = 25500 + 255 *100 // final velocity is 51000

etc

So the particles should shoot up out of sight right? But instead what happens is they float slowly up to the origin of the project and then settle down and stop moving in the centre of the project. Please can I get some help as to why?

Dr. Sassi

Hi atomician

The add function would add per frame that amount to the velocity. Which could go to very extreme values, if applied to non non-fluid. Fluids work similar (to my understanding) like other dynamic options, they take over, like MoGraph has very little option to change the position of a clone (like overwriting the dynamically defined position) Test your setup with a Basic Emitter, there is works.
.

The color that is "red" in sRGB to "100%" is way less than 1.0; it is around 0.42. If you use ACES 2065 with its much larger color space, then a 100% red will give 1.0 for the color. Anything else will result in something smaller than 1.0. (0-255 are not numbers used internally; it is an option to allow previous integer-based color "thinking" to continue.)
.

In the moment a fluid is working, the overwrite Velocity change is, as far as I'm aware, not an option. There is a transition time "Ease In" where some externally introduced changes can take place, as your setup influences the first frame, and the next less, and typically stops completely at frame four. Fluid needs then Forces to change. This short little period takes you set up, and it is a smaller number, based on ~0.42 instead of 255
.

To get an idea of what can be changed, look at the
Simulation> Modifier> Liquid> Liquify
has to offer in the Set Property mode.
https://help.maxon.net/c4d/2025/en-us/Default.htm#html/OFPLIQUIFY-FP_LIQUIFY_GROUP.html#FP_LIQUIFY_MODE

My best wishes for your explorations

CINEVERSITY

Math modifer not affecting liquid particles consistently.