TypedArray Stride Parameter

Shu-yu Guo for Surma

Motivation: ImageData

// ctx is a CanvasRenderingContext2D

const imageData =

ctx.getImageData(0, 0,

ctx.canvas.width,

ctx.canvas.height);

// imageData.data is a Uint8ClampedArray

// containing data in the scheme

// [ r0, g0, b0, a0, r1, g1, b1, a1, ... ]

What If I Want to Access Just the Alpha Values?

I Mean, I can...

const getAlpha = (u8,idx) => u8[idx*4+3];

But Wouldn’t It Be Nice

const alphas =

new Uint8ClampedArray(

imageData.buffer,

3 * Uint8Array.BYTES_PER_ELEMENT,

imageData.width * imageData.height,

/* stride */ 4);

N-Dimensional Data in Many Places

• Bitmaps
• Vertices
• Tensors (used in a budding new field called “Machine Learning”)

Proposed API

• length is the strided length (e.g. 64 bytes viewed with a stride of 4 uint8s is 64/4=16)
• stride is a multiple of element size, throws otherwise
• stride is lower bounded by and defaulted to 1, throws otherwise

new TypedArray(buffer[, byteOffset[, length[, stride]]]);

Feature Detection

"stride" in new TypedArray()

No Options Bag

• Pros: Allow defining stride without defining offset and length.
• Cons: Too invasive.
• Strawperson: length=-1?

No Unaligned Access

• TAs are all aligned today
• Easier codegen
• Easier shared memory and atomics

Stage 1?