Threejs: How can one export with GLTFExporter an indexed geometry with draw range?

Question

I have a specific problem, I would like to export an indexed geomtry that has a drawrange. Using the GLTFExporter, after having faced the issue with typescript integration (known issue apparently), I had the bad luck to discover that this was not implemented in the exporter:

// @TODO Indexed buffer geometry with drawRange not supported yet

https://github.com/mrdoob/three.js/blob/master/examples/js/exporters/GLTFExporter.js line 564

Checking the commit history showed me that the last update was 3 months ago and I don't think this is gonna come any time soon. I tried to remove the index buffer and rewrite my position bufferattribute array base on my draw range but I must do something wrong because it does not work, it simply breaks my geometry. Would any of you have a work around for me or some explainations on how to proceed with my geometry?

Thank you in advance.

EDIT:

My current work-around is to "de-index" my geometry for the export and keep the drawRange, this case is handled by the exporter. It is not ideal, and it forces me to recreate a full new geometry with new BufferAttributes. But since this operation is only done for the export, I can even have this process happening in an asynchronous way. I wish there was a better way.

Answer 1

As mentionned in my edit, I bypassed my problem by de-indexing of my geometry, it is not the best solution, but since I only need it for this export, here is how I proceeded:

// original attributes
const vertices  = geometryTmp.getAttribute("position");
const normals  = geometryTmp.getAttribute("normal");
const uv  = geometryTmp.getAttribute("uv");

// new buffer arrays
let verticesTmp = new Float32Array(3 * geometryTmp.index.array.length);
let normalTmp = new Float32Array(3 * geometryTmp.index.array.length);
let uvTmp = new Float32Array(2 * geometryTmp.index.array.length);


let j = 0;
for(let i = 0; i < verticesTmp.length; i += 3) {
    let index = geometryTmp.index.array[j];
    verticesTmp[i] = vertices.getX(index);
    verticesTmp[i+1] = vertices.getY(index);
    verticesTmp[i+2] = vertices.getZ(index);

    normalTmp[i] = normals.getX(index);
    normalTmp[i+1] = normals.getY(index);
    normalTmp[i+2] = normals.getZ(index);
    j++;

}

j = 0;
for(let i = 0; i < uvTmp.length; i += 2) {
    let index = geometryTmp.index.array[j];
    uvTmp[i] = uv.getX(index);
    uvTmp[i+1] = uv.getY(index);
    j++;
}

let newGeomtry = new THREE.BufferGeometry();
newGeomtry.addAttribute( 'position', new THREE.BufferAttribute( verticesTmp, 3 ) );
newGeomtry.addAttribute( 'normal', new THREE.BufferAttribute( normalTmp, 3 ) );
newGeomtry.addAttribute( 'uv', new THREE.BufferAttribute( uvTmp, 2 ) );

newGeomtry.drawRange = geometryTmp.drawRange;
mesh.geometry = newGeomtry;  

// After I do that to all the meshes I need, them to a new THREE.Scene that will be given to the exporter with truncateDrawRange = true

I hope it helps someone too.