OBJ loader doesn't handle negative vertex indices

[avinxshKD]

Increasing access

Increasing access
Allows properly loading OBJ files that use negative indices (standard OBJ format feature).

Most appropriate sub-area of p5.js?

• WebGL

Feature enhancement details
The OBJ loader has a TODO about not supporting negative vertex indices, which are actually pretty common in OBJ files.

In src/webgl/loading.js around line 615:
// TODO: Faces can technically use negative numbers to refer to the

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• WebGL
• Build process
• Unit testing
• Internationalization
• Friendly errors
• Other (specify if possible)

Feature enhancement details

In the OBJ format, negative numbers count backwards from the most recent vertex. So -1 means the last vertex that was defined, -2 means second-to-last, etc. A lot of modeling software uses this.

Right now p5.js either ignores these or handles them incorrectly, which can mess up how models look when loaded.

Should be fixable by converting negative indices to positive ones based on the current vertex count (like vertexCount + negativeIndex + 1).

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[avinxshKD]

@davepagurek @perminder-17 Pls do let me know if im mistaken or did not taken care of smthg, thanks

[davepagurek]

Good catch, that's in the spec on Wikipedia too, TIL! https://en.wikipedia.org/wiki/Wavefront_.obj_file#Vertex_indices Possibly in implementation we can switch to using .at(-1) (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at) instead of square bracket array indices to help with that.

[avinxshKD]

hey @davepagurek Oh nice, didn't know about .at() ,, that's cleaner than what I was thinking.
Though just to clarify the implementation . We're parsing these as strings from the file first, right? So it be something like:

let index = parseInt(faceData);
if (index < 0) {
  vertex = vertices.at(index);  // .at() handles the negative indexing
} else {
  vertex = vertices[index - 1];  // OBJ uses 1-based indexing
}

Or do you think we should convert negatives to positive indices upfront during parsing? Like vertexCount + negativeIndex before we even access the aray?

Just want to make sure I'm handling the 1-based OBJ indexing correctly too (since -1 should refer to the last vertex, but positive 1 refers to the first vertex, not index 0).

Let me know which approach you prefer, thanks

[davepagurek]

So a -1 in an obj refers to the last index right? that happens to work the same as with .at() so I think just passing those (once parsed into numbers) to .at() works well, your code example looks good to me!

[avinxshKD]

Got it, makes sense So were parsing the indices as-is from the file (keeping negatives), and then .at() handles both the negative indexing and the off-by-one conversion automatically since we're working with 0-based arrays. Clean solution.

I'll update the implementation to use .at() for vertex lookups instead of manual bracket indexing. That should handle both negative refs and the 1-based → 0-based conversion in one go.

Thanks for the clarity @davepagurek

[avinxshKD]

@davepagurek hey pls have a look when you get chance, implemented the .at() approach we discussed #8507

[davepagurek]

Reopening this until it's also merged into dev-2.0 🙂

[avinxshKD]

@davepagurek Hey here applied the same for 2.0 #8512. Pls do let me know if any adjustments needed.

Also, is there a preferred place to track upcoming or priority work for the project?