What is the purpose of the setDrawRange function in the Three.js library?

I've been diving into a three.js codebase.

While studying the code and the documentation, there's one thing that's puzzling me.

In this particular code snippet. http://jsfiddle.net/w67tzfhx/

there exists the following piece of code.

function init(){

    var geometry = new THREE.BufferGeometry();
    var positions = new Float32Array(MAX_POINTS*3);
        geometry.addAttribute('position',new THREE.BufferAttribute(positions,3))

        drawCount =2;
        geometry.setDrawRange(0,drawCount );

        mat =  new THREE.LineBasicMaterial( { color: 0xff0000, linewidth: 2 } );

        line= new THREE.Line(geometry, mat)

        scene.add(line)

        updatePositions();

}

What exactly is setDrawRange? And how does drawCount function in this context?

The official documentation explains it as:

.setDrawRange ( start : Integer, count : Integer ) : null Set the .drawRange property. For non-indexed BufferGeometry, count represents the number of vertices to render. For indexed BufferGeometry, count signifies the number of indices to render.

which I find somewhat confusing.

Would appreciate any insights into clarifying what this means. Many thanks in advance.

Answer №1

When it comes to Geometry, the number of vertices plays a crucial role. For instance, a cube consists of at least 36 vertices (6 for each side). This is because every side requires 2 triangles to form a rectangle and each triangle needs 3 vertices. To draw specific sets of vertices, such as the first or last 6, you would use the setDrawRange function accordingly.

Vertices in geometry can be indexed or non-indexed. Indexed vertices introduce a layer of indirection where certain indices refer to specific vertices. This system allows for more flexibility when working with complex geometries.

Although not commonly used, the setDrawRange function has practical applications. For example, if you need to draw multiple lines, you can utilize this function to specify which lines should be rendered by Three.js. By managing the draw range, you can optimize performance and streamline the rendering process.

For further insights into Geometry and its functionality, check out additional resources on custom Geometry and BufferGeometry here and here

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