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c++jsonparsingrenderinggltf

Properties of the stride in a glTF file?

The Khronos docs define the stride as:

When a buffer view is used for vertex attribute data, it may have a byteStride property. This property defines the stride in bytes between each vertex.

I am somewhat confused as, many of the examples I have tried already (3 of them) had a stride of 0 so I merely ignored the attribute until now. Those examples render just fine.

I was inferring the "stride" from the type. e.g. if the type was a vec3 and the component type was float, I loaded every 12 bytes as one element. Some things that I am not entirely sure about reading the spec are,

When stride is non 0, does this mean the data could be interleaved? When the stride is non 0, can the data be non continuous (e.g. padding bytes)? In other words, can you run into situations where the buffer is not interleaved but the total size of sizeof(type_component) * element_count is not a divisor of the total subsection of memory to be read?

Solution

  • Yes, accessors (in glTF) are like vertex attributes in OpenGL/WebGL, and are allowed to interleave. The stride is on the bufferView, to force accessors that share that bufferView to all have the same stride. A value of zero means "tightly packed".

    Note that you may interleave components of different sizes, such as vec3 (POSITION) with vec2 (TEXCOORD_0), so a stride might be the sum of different sizes.

    Here's a diagram from the Data Interleaving section of the glTF tutorial. It's a little small here but you can click for a larger view. In this example, there are two accessors, one for POSITION and one for NORMAL, sharing a single BufferView.

    Data Interleaving