Make TypeArray deref into slices?

[DexterHill0]

Summary

Maybe I'm missing something, but is there any way to take a typed array (say, UInt8Array) and turn it into a &[u8], or the equivalent? I found issue #5 but that was a very long time ago and I'm not sure if its still applicable.

In my use case I load a large image as an ArrayBuffer from an HTTP request, convert it into a UInt8Array and pass that to the WASM. Inside the WASM is a function that takes a &[u8] and it would be convinient, and more efficient, to be able to take a reference to the UInt8Array rather than first allocate an enourmous vector, then take a slice of that.

[daxpedda]

Unfortunately this is technically not possible. An ArrayBuffer represents external memory that can't be represented in pointers to the internal memory in Wasm (I don't believe the multi-memory proposal can fill that gap as well).

The solution is to accept a Uint8Array instead of an &[u8] to avoid having to copy everything into the Wasm memory, as long as that fits your requirements.

[DexterHill0]

That's unfortunate but makes sense. Not sure the solution is possible since the image library doesn't accept anything but a u8 slice. Am I correct in saying that if I wrapped the UInt8Array in an ImageData, passed that to the WASM, and then called .data() on the WASM side, that it would also have to allocated in WASM memory? I assume it does as .data() returns an owned vec but it's worth asking.

[daxpedda]

I assume the UInt8Array is pointing to external memory?

Currently ImageData::data() returns a Vec<u8>, so that definitely makes a copy. You would have to introduce custom bindings returning a Uint8ClampedArray.

It is however possible to construct ImagaData with internal memory through Clamped. Same problem though: as ImageData::data() returns a Vec, allocation happens even though it's theoretically all pointing to Wasm memory. Not entirely sure if wasm-bindgen is able to generate bindings representing that though.

[DexterHill0]

I assume the UInt8Array is pointing to external memory?
Yes, the array points to external memory. The array is constructed in JS and then passed to WASM.

Should have been clearer in my reply when I said "wrapped the UInt8Array in an ImageData". By this I mean wrap it in JS and pass the ImageData to the wasm. So create it new ImageData(new UInt8ClampedArray(response.arraybuffer)) and then pass that value down to the WASM. But I think your point still applies that it would have to make a copy in the WASM.

[daxpedda]

Well you could still use a custom binding to ImageData::data() to returns a Uint8ClampedArray and do something with that without copying the data to Wasm memory. But if you want a Vec or [u8] you have to copy.

[DexterHill0]

Makes sense. Thanks for the help!