Coloring Points vs Primitives in Houdini

Primitive Number 4 is colored red in Houdini

When a color is assigned to a primitive, that side of the geometry will display the respective color. Primitives have points that make up the primitive.

The screenshot on the right shows a cube where primitive 4 is colored red resulting in the top face of the cube being red.

The points 0, 1, 2, & 3 make up primitive 4.

So what happens if a color is assigned to the points instead of the primitives?

Will it display the same result?

All points of the top face is colored red in Houdini

The screenshot on the right shows a cube where the points 0, 1, 2, & 3 are colored red, but the result is different!

What is the difference between assigning color to points versus assigning color to primitives?

In the screenshot on the right where points are being colored instead of the primitive itself shows the red bleeding through to other primitives. That’s because the points 0, 1, 2, & 3 are shared between other primitives.

Primitive ID Numbers & Point ID Numbers in Houdini

When we have a solid geometry, let’s take a cube as an example. A cube consists of 6 primitives and 8 unique points. All 8 points are being shared by multiple edges to form the solid cube. In the screenshot on the right, the cube is displaying the primitive numbers in light purple and the point numbers are displayed as cyan (light blue).

Point Number 1 is being shared by 3 primitives. Point number 1 is being shared by primitive number 0, 1, and 4.

In the color node when points are used to color the geometry, Houdini can’t tell which side to color, because point 1 is part of multiple primitives, its a shared point. This results in red bleeding over to other primitives in the above screenshot.

When coloring primitives, it’s crystal clear which part of the geometry to color, because the primitive is unique.