How do the 5 Combine Modes work (Add, Subtract, Merge High, Merge Low, Multiply) and when do I use each one?

Overview

Combine Mode is one of the most important concepts in 3D modeling. When components overlap, their Combine Mode decides how they interact. Wrong combine modes cause most "my model looks wrong" problems. The modes are Add, Subtract, Merge High (shown as Merge), Merge Low (shown as Low) and Multiply.

The Component Tree works bottom-up: the lowest component is combined with the modeling plane first, then each component above it is combined with the cumulative result of everything below it. The first level and first component only combine with the modeling plane. Combine Modes can also be set on Levels, which then govern how everything on that Level interacts with the Level below.

As the official documentation puts it, if the second shape's area is completely within the original's you will probably be adding or subtracting, and if the shapes only partly overlap you will probably use Merge or, very occasionally, Low.

---

The 5 Combine Modes

1. Add

The height of the upper component is added on top of whatever is below it at every point.

What it looks like: a 0.3 inch dome Added on a 0.5 inch flat background is 0.8 inch tall in the dome area.

When to use:

• The upper shape sits completely inside the lower shape.
• Stacking independent elements with no edge transition needed.

When NOT to use:

• Shapes that overlap at their edges. You get a sharp ridge where the added height begins.
• Organic blending between overlapping shapes.

Example: a maple leaf fully inside a dome sign face. Adding its height raises the leaf on top of the dome.

---

2. Subtract

The height of the upper component is removed from the accumulated height below it. Where the subtracted shape goes into the background, the result becomes negative regions.

What it looks like: a dome background with a Subtract flat plane on top carves a depression into the dome.

When to use:

• Recesses, pockets, cutouts and depressions within a raised model.
• Making the inside of a letter lower than its surroundings, or recessed lettering in a background.
• Creating the recessed half of an inlay or mold.
• Subtract works best when the removed shape sits completely inside the original shape. Where it only partly overlaps you get an uneven transition.

Example: the creases that define muscles on an animal relief can be made with Subtract shapes created from the recessed areas.

---

3. Merge High (most used for organic work)

In overlapping areas, the higher component wins. Areas that do not overlap stay as they were. This is effectively a Boolean union.

What it looks like: two overlapping domes with Merge look like one fused shape with no ridge at the intersection. Wherever one component is taller it dominates, and there is no additive stacking.

When to use:

• Organic shapes that partly overlap: leaves, flowers, animals, scrollwork.
• Clipart on a background where edges would otherwise create harsh transitions (for example a grapevine over a textured background: the vine shows where it is taller, the texture shows in the gaps).
• A good default for most organic 3D work.

The key rule: use Merge when shapes partly overlap. Use Add when one shape is fully inside the other.

Example: a rope border, banner and two figures that overlap. The rope is lower than everything, the banner is higher than the figures, and Merge leaves whichever is higher visible in each overlap.

---

4. Merge Low (rarely used)

In overlapping areas, the lower component wins. Areas that do not overlap stay as they were. This is effectively a Boolean intersection of the low points. Low is only available when combining components, not in the modeling tools, so set it on an existing component using Component Properties or the right-click menu.

When to use: recessing a shape into a raised shape.

Classic example: a keyboard key. Combine a flat-topped button component with a curved top-face component that carries the letter A, with both set to Merge Low. The letter then follows the curved top surface instead of sticking up above it.

---

5. Multiply

Where there is an overlap, the result of all the previous components in the list is multiplied by the heights in this component. Where either has zero height, the result goes to zero. Multiply is a fifth mode that you set from the right-click menu after a component has been created.

When to use:

• A texture that should follow an underlying shape and fade toward its edges.
• Fading a background texture out at the borders of a relief.

Example: a texture component set to Multiply on top of a dome relief. The texture is strongest at the top of the dome and fades toward the edges. The exact result depends on both height values at once, so judge it in the 3D view and test on a small piece first.

---

How the Component Tree Processes Components

1. The bottom component combines with the modeling plane.
2. Each component above it is combined with the accumulated result of everything below it.
3. The final result is the Composite Model you see in the 3D view.

Order matters:

• A Subtract component must be above the component it subtracts from.
• A Merge component above a background merges with that background.
• Moving a component up or down in the tree (drag it, or use the blue arrow buttons at the top of the tree) changes the result.

Levels: the contents of a Level are combined first, then the Levels are combined together. A Level has its own combine mode, so the whole Level can be treated as one unit. A Group also has its own icon among the combine mode icons in the tree.

---

Changing a Component's Combine Mode

1. Double-click the component in the Component Tree to open Component Properties, and change the mode there.
2. Right-click the component to use its pop-up menu, which offers the Combine Type.
3. In the 3D view, select the component and use the Additional Settings option below it, which lets you adjust the Combine mode, Fade, Tilt and Appearance.

In Aspire, the combine mode can also be chosen in the Create Shape form while you create a component.

---

Quick Decision Guide

---

Common Mistakes

• Using Add when shapes have overlapping edges. This creates visible ridges. Switch to Merge.
• Using Merge when a shape is fully inside another: if the inner shape is shorter than the outer one it disappears. Use Add to keep it visible.
• Wrong Component Tree order. A Subtract only works on what is below it in the tree.
• Using Multiply on solid shapes. Anywhere one component has zero height the result is zero.

---

For a worked example of Multiply, see the entry on fading a texture into a dish or bowl shape. For how Levels, Mirror Mode and Level Clipping work, see the entry on Component Levels.