ADVANCED GROOMING TIPS | EXPORTING TO UNREAL

Should you see gaps where the hair doesn’t grow evenly on the head, adjust the influence radius until full and use density for skin attribute override if required.

Going inside the Hairgen, place a clump node and connect the skin input. The clump node is a very powerful and versatile tool in Houdini's grooming kit. Using it you can achieve a wide range of effects and looks. In this case, we will be making use of the curling feature in an unintended way to simplify a usually complex process. 

The primary approach in this technique for braid generation is creating a base curve shape that will be chained onto a path curve. Step by step instructions:

• Add a Line, set the Points to 9, and leave the rest of the parameters at default. Group point 1-3 as A and 5-7 as B;
• Translate A by 0.15 in X and copy the parameter. Paste a relative reference in a new transform below in the X channel and add a *-1 to the end i.e:(ch("../width/tx")*-1). The first transform acts as the width control;
• Group point 1 and point 5 as C and point 3 and 7 as D;
• Translate C by 0.35 in Z and copy the parameter. Paste a relative reference in a new transform below in the Z channel and add a *-1 to the end i.e: (ch("../thickness/tz")*-1). The transform above acts as the thickness/height control;
• Translate point 6 In X by the width relative reference*1.1 i.e: (ch("../width/tx")*1.1) and then translate point 2 by the inverse i.e: (ch("../width/tx")*-1.1). Note these values can be adjusted to increase or decrease the outer peaks of the braid;
• Pull two chains off of the Peaks Inverse transform and transform the left chain by 0.625 in Y and the right chain by 0.137 in Y;
• Copy and transform each chain by 2 copies with a transform of -1 in the Y. This creates an offset Reference flag for the two chains in conjunction with the Peaks Transform root to see the positioning and offset of the braid segments;
• Blast points 4-11 on the Left Chain and 7-14 on the right which brings the offset curves back into 1 high length to match the root;
• Sort the point order in Y;
• Polypath to combine the primitives and merge the chains. Add any other post deformational transforms here;
• Merge the previous merge with Peaks Inverse Transform Node. This is the basis of the braid geometry;
• For the far-right chain, the input add an object merge the input curve comes in here;
• Add a poly frame for corrective and a second poly frame for changing the tangent to N. This allows us to use N as forward direction. 
  
  

Place a for each connected piece loop, inside the loop place a poly frame, this can be used to force adjustment should need to arise, for this example un-tick all boxes.

• Off to the side create a circle with the uniform scale of 0.004, this will be the control for the thickness of each braid piece;
• Scatter points across the surface of the circle forcing the total count to 250, this is the density of the final Hair that is generated and can be adjusted later to the desired levels;
• Add a connectivity node-set to the pint, this will generate a class attribute per point;
• Add a poly frame with Bitangent name as N;
• Connect this to a copy to curves node with the copy to curves second input connected to the poly frame in for each loop we created above. The copy to Curve parameters are:
•  Target Up Vector Y-axis;
• Add an attribute called class applying to points by adding it at the bottom under the attributes drop-down tab.
• Place an add node, select polygons tab, and set to add by group and add to attribute, setting the attribute name to the class. This will connect a curve between the points we scattered earlier;
• Add an attribute noise to the output for each loop (do not do this if you plan to deform this mesh using this setup in Houdini, you would need to point deform it as a rest position);
• Add a final resample to act as a final Resolution control to control the resolution of the final product, in this example set to Maximum segments, 250;
• Add a color node set to random from the attribute and set class as the attribute name, this will generate a random color per strand to better visualize the result;
• Finally, add an attribute create and make it a float value called width, set this to 0.0001.
  

As we can see, the final result is a full braid loop. You can apply the exact same shader to standard hair grooms. The system is flexible and can easily be made into a tool, one could add different base curve profiles for various braid types, to build the setup into a fully packed HDA to generate ropes and braid curves for various needs and requirements.

Feel free to add any other elements to the setup we have so far like eyebrows or accent curls, or even more braids using the techniques shown above. 

In the attribute promote, find the width attribute and promote it from a point class to a vertex class then change the new name to "groom_width" and select "delete original". This is so that Unreal Engine reads this as the hair width when it imports the alembic file. You can find a full list of other groom attributes that can be read natively by Unreal Engine here.

I have selected UE 4.27 for the purpose of easy MetaHuman workflow, however, the same would apply to UE5. Next place an Alembic ROP and using the default settings write out the static hair alembic to your desired file location.

Lastly, to adjust the groom material, you can open the groom material found in the properties menu on the right of the Unreal viewport. Double clock the material to bring up its Blueprint and make any adjustments to your preference. In this example, I simply adjusted the color to more closely match what was present in Houdini.

Thank you for taking the time to check out this tutorial.