Byte offset greater than byte length

[milosrs]

I'm trying to read data from scene.bin file into memory. I am achieving this with this piece of code.

`

		if (bufferInfo.buffersAccessors.buffer.uri.length() > 0 || bufferInfo.buffersAccessors.buffer.byteLength > 0) {
		Microsoft::glTF::Buffer buffer = bufferInfo.buffersAccessors.buffer;
		
		path += bufferInfo.buffersAccessors.buffer.uri;
		path = std::filesystem::absolute(path);
		buffer.uri = path.string();

		std::shared_ptr<StreamReader> streamReader = std::make_shared<StreamReader>(path);
		Microsoft::glTF::GLTFResourceReader reader(streamReader);

		stream = reader.ReadBinaryData<T>(buffer, bufferInfo.buffersAccessors.view);
	}`

Then I simply do this.

`

	const size_t bufferDataOffset = mainPart.accessor.byteOffset + mainPart.view.byteOffset;

	bufferData.erase(bufferData.begin(), bufferData.begin() + bufferDataOffset);

	bufferInfo.bufferData = bufferData;`

When I'm trying to read attribute TEXCOORD_0 of MeshPrimitive from scene.bin file, I always get a situation where ByteOfsset is greater than ByteLength. This results in a crash at bufferData.erase(). What should I do in these situations? Is my offset calculation even correct?

[chriche-ms]

If you just want to read MeshPrimitive attribute data then I would suggest using the ReadBinaryData overload that accepts an Accessor parameter. The code looks like:

std::string accessorId;

if (meshPrimitive.TryGetAttributeAccessorId(ACCESSOR_TEXCOORD_0, accessorId))
{
    const Accessor& accessor = document.accessors.Get(accessorId);
    const auto data = resourceReader.ReadBinaryData<T>(document, accessor);
}

Where data is a vector containing the accessor's data. This way you can let the glTF SDK do all the offset calculations for you.

Also, I recommend that you do any path manipulation in your StreamReader implementation and avoid modifying the Document whenever possible.

[agrittmsft]

You could also use MeshPrimitiveUtils::GetTexCoords(_0):

std::vector<float> GetTexCoords(const Document& doc, const GLTFResourceReader& reader, const Accessor& accessor);
std::vector<float> GetTexCoords_0(const Document& doc, const GLTFResourceReader& reader, const MeshPrimitive& meshPrimitive);

[milosrs]

Both work like a charm. Thanks!

[chriche-ms]

Original code snippet is mixing byte offsets with vector element indices. Solution is to either add bufferDataOffset / sizeof(T) or use higher level functionality such as MeshPrimitiveUtils::GetTexCoords