Adds 3DTILES_binary_buffers extension #415
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…rectly handle STRING
| |**byteOffset**|`integer`|The offset relative to the start of the buffer in bytes.|☑️ Yes| | ||
| |**byteLength**|`integer`|The length of the bufferView in bytes.| ☑️ Yes| | ||
| |**elementCount**|`integer`|The number of elements in the buffer view.| ☑️ Yes| | ||
| |**elementByteOffsetsBufferView**|`integer`|The index of the bufferView containing byte offsets for each element. Must be defined for the STRING type.|No| |
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Can elementByteOffsetBufferView be used for properties containing arbitrary sized chunks of binary data? What would be the elementType in that case?
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Sorry, duplicate comment to #415 (comment). I thought github lost this one.
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Yes, a NONUNIFORM element type will be introduced in next draft to meet the requirements for EXT_3dtiles_feature_metadata.
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| *Note: The buffer views for the `BIT` component type must be padded with trailing `0`s to meet the nearest byte boundary.* |
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The padding requirements should be defined for the other component types. In general properties must start on a byte offset that's a multiple of the size of the component type.
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What's the reason behind this? As long as the byte boundary is respected, the data can be read using byte offsets and byte lengths. What is gained by having the offset be a multiple of the size of the component type?
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Talked offline about this, but it's for performance reasons. In JavaScript typed arrays need to be aligned to their component type within the underlying array buffer which means unaligned data needs to be cloned into a new typed array. Proper alignment also provides performance benefits in systems level languages.
Early on glTF had the same rationale: KhronosGroup/glTF#167
https://github.com/CesiumGS/cesium/blob/2a9ca28dcbb38eee7013505987516446a8eacb4f/Source/Scene/Batched3DModel3DTileContent.js#L370-L382 - this is where CesiumJS clones the typed array if it isn't aligned. 3D Tiles didn't always have byte alignment requirements which lead to slow paths like this.
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@sanjeetsuhag @lilleyse excited to see this. Just a reminder to please bump to me when ready for review. 😄 |
… of arbitrary size binary data
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This extension may need a way to indicate whether a string property is stringified JSON |
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A lot of the data alignment rules in this spec are now OBE since we're moving to Apache Arrow. I'll open a PR with the new, simplified version of the spec. |
This extension enables storage of binary buffers. Specification is available here.