How do I distribute hair particles evenly using guiding curves? (Ponytail with Hair Dynamics)

Question

I tried increasing the steps under both the viewport and render tab, messing with the guiding curves, and amplification under the Children tab. No method I've tried so far has been able to simultaneously keep the ponytail form and cover my character's entire head with hair. In order to get the shape I wanted, I made the two different particles systems and connected both systems under one guiding curve.

Hair Dynamics are enabled for the ponytail and the main hair, with interpolated children.

Answer 1

Combine a Short Hairstyle with a Dynamic Ponytail

Since the main hair doesn't move, but only the ponytail, you can leave the main hair static and make only the ponytail dynamic (plus dynamic bangs, and another layer of fly-off hair if you want more realism.) It's like a hair extension and fairly easy to do.

^{Turnaround of the ponytail hairstyle}

Preparation

• create a little cylinder or "ring" (it's more a disc). It will be the emitter for the ponytail. I've added a Torus object (12 major, 8 minor segments), deleted the inner faces, and grid-filled the hole.

^{Ponytail hair emitter ("ring")}

• place this ponytail emitter ("ring") behind the head where you want to have the ponytail
• Important! Apply the scale to the "ring" and the head because a lot of the hair settings are relative to the emitter's size. You will get weird-looking results in the simulation and in Cycles renderings. Eevee is more error-tolerant.

Create and Comb the Main Hair

Note: The main hair is on a haircap/scalp emitter. The head is a separate object and is bald. All objects have real-world size.

• add a Hair Particle System to the haircap/scalp. 500 particles, 8 segments are fine. You can increase/reduce the number of segments in Particle Edit mode (menu Particle → Rekey; particles must be selected.)
• Comb the hair. Select the lower hair (everything at the level of the ears in front view, box select) and comb it up in the direction to the "ring". Tighten the strand of hair guides and try to pull them through the ring.
• at the front of the head, pull the hair gently back so the children can't accidentally disappear inside the head (bald spots!)
• when you're finished, cut all hair particles (guides) behind the ring.
• repeat the combing for the side parts, then for the back top, and finally for the bangs (if you want a parting)
• try to layer the hair parts
• don't forget you can hide combed parts of the hair so it gets not messed up by further edits. You also can select the roots (menu) and select more segments (keys) with Numbpad++ to easily select single hair guides and comb them.

Create the Ponytail

• select the "ring", in Edit Mode, select the inner faces, and assign them to a Vertex Group "Ponytail" (Ctrl+G). This is where the ponytail will grow.
• assign material for the hair ("Ponytail" group) and another material for the outside of the ring (inverted selection)
• add a Hair Particle System to the ring emitter, 500 particles, 26cm, 8 segments
• use the created Vertex Group "Ponytail" in Particle Properties → Vertext Groups → Density so the hair grows "through" the ring and not on the outside of the ring.
• enable Particle Properties → [X] Hair Dynamics, in section Hair Dynamics → Structure increase the random value (0.422). This widens the ponytail a little
• Note: the values for Vertex Mass and Stiffness have an effect on the stiffness but I left them at the default settings and just did a little weight painting.

^{Weight paint of the ponytail}

• in Particle Edit mode, weight paint the keys (black dots) a little, see screenshot. This spreads the ponytail somewhat, makes it fluffy and it doesn't look completely flat from the side.
• run the animation. In about 60 frames the ponytail should have fallen down nicely. It should neither sap off nor stick out.

^{Simulated hair dynamics of the ponytail}

Tweak the Hair Particle Settings

• Children, select Interpolated, amount: 80. For the ponytail only: Length: 0.959, Threshold: 0.306. It makes the end of the ponytail a little more shaggy.
• adjust the values for Clumping, Roughness, and Kink as you like.
• use a small number for Roughness → Random, like 0.021
• if the children of the main hair try to overgrow the ring emitter then try to increase Clumping → Clump a bit, e.g. 0.479
• Kink works well for the ponytail (used Wave in the example) but select a small Amplitude like 0.02 m so it doesn't "explode"
• for Hair Shape use values like Strand Shape: -0.9, Diameter Root: 0.03, Tip: 0.01, Diameter Scale: 0.01 (default). These values are important for Cycles if you don't want to have super fat hair particles.

Animation test: