Failed to iter over sparse accessor

[wsw0108]

sample model: https://github.com/cx20/gltf-test/tree/master/tutorialModels/SimpleSparseAccessor

use gltf;

fn main() {
    let mut args = std::env::args();
    let (document, buffers, _) = gltf::import(args.nth(1).unwrap()).unwrap();
    for mesh in document.meshes() {
        for primitive in mesh.primitives() {
            let reader = primitive.reader(|buffer: gltf::Buffer| Some(&buffers[buffer.index()]));
            let positions: Vec<[f32; 3]> = reader.read_positions().unwrap().map(|p| p).collect();
            println!("{:?}", positions);
        }
    }
}

[aloucks]

Could you elaborate on what you expected to happen and what actually happened?

Were any positions printed?
Were some, but not all, positions printed?
Did it panic?

[wsw0108]

     Running `target\debug\gltf-issue-313.exe ..\m3t\models\tutorialModels\SimpleSparseAccessor\glTF\SimpleSparseAccessor.gltf`
thread 'main' panicked at 'attempt to subtract with overflow', C:\Users\wsw\scoop\persist\rustup\.cargo\registry\src\mirrors.ustc.edu.cn-61ef6e0cd06fb9b8\gltf-0.16.0\src\accessor\util.rs:149:20

[aloucks]

gltf/src/accessor/util.rs

Lines 148 to 152 in 47acbef

	fn size_hint(&self) -> (usize, Option<usize>) {
	let hint = self.values.len() - self.counter as usize;
	(hint, Some(hint))
	}
	}

Does it work as expected if you change line 149 to:

let hint = self.indices.len() - self.counter as usize; 

[wsw0108]

No, it does not work.

The hint shoule be accessor.count() - self.counter according the gltf spec and trait size_hint.

[derwiath]

We encountered this issue on one of our assets. I dug into it and wrote a fix for it:

• Fix size hint and values read with sparse iterator #411

I added some unit tests based on the repro-code @wsw0108 posted above. Thanks for that 👍

At the same time I also discovered that the sparse accessor iterator could not read accessors where the bufferView field was unset. I took the opportunity to fix this issue as well.