Difference Between LineLoop and Line in Three.js

In Three.js, rendering linear geometry is primarily achieved using the Line, LineLoop, and LineSegments classes. While both Line and LineLoop utilize a sequence of vertices to draw paths, they differ fundamentally in how they handle the final connection of a geometric shape. This article explains the technical differences between Line and LineLoop, how they impact your code structure, and their underlying WebGL rendering behavior.

The Core Difference in Vertex Connection

The primary distinction between Line and LineLoop is how the rendering engine handles the relationship between the first and last vertices in the geometry array.

• Line (Open Path): A standard Line object connects vertices in sequential order (\(p_0 \rightarrow p_1 \rightarrow p_2 \rightarrow ... \rightarrow p_n\)). The path remains open, leaving a gap between the final vertex (\(p_n\)) and the starting vertex (\(p_0\)).
• LineLoop (Closed Path): A LineLoop connects vertices in the same sequential order but automatically draws a final segment from the very last vertex back to the first vertex (\(p_n \rightarrow p_0\)). This guarantees a closed geometric loop.

Code Structure and Vertex Duplication

When using a standard Line object to create a closed shape like a square or a triangle, you must manually duplicate the first vertex and append it to the end of your geometry’s position attribute.

For example, to draw a closed triangle with Line: 1. Define Vertex A, B, and C. 2. Push positions into the geometry: [A, B, C, A].

If you do not add the final A vertex, the triangle will remain open between C and A.

With LineLoop, vertex duplication is unnecessary: 1. Define Vertex A, B, and C. 2. Push positions into the geometry: [A, B, C]. 3. Three.js automatically connects C back to A.

By eliminating the need to duplicate the starting vertex, LineLoop reduces data redundancy, making your code cleaner and easier to manage, especially when programmatically generating complex closed polygons.

WebGL Rendering and Performance

Under the hood, this behavioral difference translates directly to different WebGL rendering primitives:

• Line maps to the WebGL primitive gl.LINE_STRIP, which draws a line from the first point to the last.
• LineLoop maps to the WebGL primitive gl.LINE_LOOP, which instructs the GPU to draw the sequential lines and automatically close the loop.

Because LineLoop offloads the closing segment to the GPU via the native WebGL primitive, it is slightly more efficient. It saves GPU memory by requiring fewer vertex attributes to be uploaded to the buffer, which is highly beneficial when rendering many complex closed shapes or updating dynamic geometries in real-time.