How do I mask one part inside another?

Invert-cutting a part into another part is one of the most common rigging techniques used in Harmony. This article will cover the best practices using a pupil cut into an eye as an example.

If you want to read more about the type of cutters in Harmony, see What is the difference between a default and an inverted Cutter?

Using the invert-cutting technique

Here are some ways the invert-cutting technique can be used in a rig:

• For cutting a pupil into an eye (like in this article).
• For cutting multiple features into something else, like teeth and tongue into a mouth, or face features into a head.
• For cutting a pattern/logo into clothing, or a tattoo into a body part.

Step 1: Separate the layers

1. Separate the artwork of the part that needs to be cut and the part that will be cutting onto their own layers. In the case of an eye it would be the Eye and the Pupil.
2. If the part that is cutting has line art, the line and the colour will also need to be separated onto different art layers. This is to ensure we can place the Eye’s line art above the Pupil. This is what the eye’s art layers look like:

Step 2: Set up the cutter system

1. In the Node view, find the layers and organize them so that there is space between them and the composite.

   

2. Using the Enter key dropdown, or the Node Library view, bring in the following nodes:
   • Cutter
   • Line-Art
   • Colour-Art
3. Put the Line-Art and Colour-Art nodes under the Eye layer and attach a connection to each one. Remove the main connection from the Eye layer so that the Pupil is the only thing attached.

4. Slide the Cutter node into the Pupil’s connection by holding Alt/Option and moving the Cutter over the cable until it snaps in. Make sure the cable is running through the right-most port and out the bottom to the composite. This means we are defining the Pupil as the image that will be cut or masked.

5. Pull two cables from under the Colour-Art and connect them as follows:
   • One directly from the Colour-Art to the right of the Pupil on the composite. This ensures we see the Colour-Art layer under the Pupil.
   • One into the Matte (left) port of the Cutter. This will define the Colour-Art layer of the Eye as the artwork that needs to cut the Pupil, and the cable will become a dashed line.

6. With the current setup, the Eye’s fill is hiding the pupil instead of invert-cutting it.
7. Invert the cutter by doing one of the following:
   • Double-click the mask icon on the Cutter node so it changes from a black background to a white one .
   • Open the Cutter node’s settings and tick the ‘Inverted’ checkbox, the close the window. The mask icon should now have a white background.

Step 3: Reattach the line art

1. At this point, the Pupil will be cut into the Eye’s Colour-Art layer, however the line art of the Eye is not visible.

2. Pull a cable from underneath the Line-Art node and attach it to the left (above) the Pupil layer on the composite. This will make sure the line covers the pupil and the eye.

3. Your part is now invert-cut with the line art on top.

Q&A 

Question: 

Why does my invert-cut part overlap half of the line art?

Answers: 

1. Make sure that the Line-Art node is attached above the cut part on the composite.
2. Make sure the cut part is not above the part connected to the Line-Art layer in Z-depth. Check the Z position of the part’s peg.

Question: 

Why is there a thin white line between the parts in the Render view?

Answer: 

These lines are caused by anti-aliasing and can be fixed by connecting cutters correctly. Make sure you follow the above instructions properly and read the article Why is there a thin line between my layers in the Render view? for more information.