Refactor the Arrow Mesh3D type to use zero-copy Buffers

[jleibs]

For the arkit scene example this reduces visit time from 7ms to 20us.

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[Wumpf]

nice!!

[Wumpf]

c is not already an array/slice that can be passed directly?

[jleibs]

I think we're basically doing an endianess swap here.

ColorRGBA stores the color as 0xRRGGBBAA, but to_array converts it to [r,g,b,a].

    pub fn to_array(&self) -> [u8; 4] {
        [
            (self.0 >> 24) as u8,
            (self.0 >> 16) as u8,
            (self.0 >> 8) as u8,
            self.0 as u8,
        ]
    }

[emilk]

it's a matter of turning a u32 into a Rgba32Unmul. We could add a Rgba32Unmul::from_u32 helper but then we would need to document exactly how that works etc…

[emilk]

350x speedup - not bad

[emilk]

Hum, why doesn't Buffer<ColorRGBA> work?

[jleibs]

In short because deserialization support for buffer is implemented as:

// Blanket implementation for Buffer
impl<T> ArrowDeserialize for Buffer<T>
where
    T: ArrowDeserialize + NativeType + ArrowEnableVecForType,
    for<'b> &'b <T as ArrowDeserialize>::ArrayType: IntoIterator,
{
    type ArrayType = ListArray<i32>;

    #[inline]
    fn arrow_deserialize(
        v: <&Self::ArrayType as IntoIterator>::Item,
    ) -> Option<<Self as ArrowField>::Type> {
        v.map(|t| {
            t.as_any()
                .downcast_ref::<PrimitiveArray<T>>()
                .unwrap()
                .values()
                .clone()
        })
    }
}

Similar to our workaround for BinaryBuffer, we could probably do something like implement a struct ColorRGBABuffer(Buffer<ColorRGBA>), to specialize it.

I was hoping I could just make ColorRGBA act as a native type, but arrow2::NativeType is sealed: https://github.com/jorgecarleitao/arrow2/blob/main/src/types/native.rs

[emilk]

we should impl IntoIter for &BinaryBuffer… but whatever