In WebGPU, nearly all of the data you provide to it needs to be layed out in memory to match what you define in your shaders. This is a big contrast to JavaScript and TypeScript where memory layout issues rarely come up.
In WGSL when you write your shaders, it’s common to define struct
s.
Structs are kind of like JavaScript objects, you declare members of
a struct, similar to properties of a JavaScript object. But, on top
of giving each property a name, you also have to give it a type.
AND, when providing the data it’s up to you to compute where
in a buffer that particular member of the struct will appear.
In WGSL v1, there are 4 base types
f32
(a 32bit floating point number)i32
(a 32bit integer)u32
(a 32bit unsigned integer)f16
(a 16bit floating point number) ^{[1]}A byte is 8 bits so a 32 bit value takes 4 bytes and a 16 bit value takes 2 bytes.
If we declare a struct like this
struct OurStruct { velocity: f32, acceleration: f32, frameCount: u32, };
A visual representation of that structure might look something like this
Each square block is a byte. Above you can see our data takes 12 bytes.
velocity
takes the first 4 bytes. acceleration
takes the next 4,
and frameCount
takes the last 4.
To pass data to the shader we need to prepare data to match the
memory layout OurStruct
. To do that we need to make an ArrayBuffer
of 12 bytes, then setup TypedArray
views of the correct type so we
can fill it out.
const kOurStructSizeBytes = 4 + // velocity 4 + // acceleration 4 ; // frameCount const ourStructData = new ArrayBuffer(kOurStructSizeBytes); const ourStructValuesAsF32 = new Float32Array(ourStructData); const ourStructValuesAsU32 = new Uint32Array(ourStructData);
Above, ourStructData
is an ArrayBuffer
which is a chunk of memory.
To look at the contents of this memory we an create views of it.
ourStructValuesAsF32
is a view of the memory as 32bit floating point
values. ourStructValuesAsU32
is a view of the same memory as
32bit unsigned integer values.
Now that we have a buffer and 2 views we can set the data in the structure.
const kVelocityOffset = 0; const kAccelerationOffset = 1; const kFrameCountOffset = 2; ourStructValuesAsF32[kVelocityOffset] = 1.2; ourStructValuesAsF32[kAccelerationOffset] = 3.4; ourStructValuesAsU32[kFrameCountOffset] = 56; // an integer value
TypedArrays
Note, like many things in programming there are multiple ways we could
do this. TypedArray
s have a constructor that takes various forms. For example
new Float32Array(12)
This version makes a new ArrayBuffer
, in this case of 12 * 4 bytes. It then creates the Float32Array
to view it.
new Float32Array([4, 5, 6])
This version makes a new ArrayBuffer
, in this case of 3 * 4 bytes. It then creates the Float32Array
to view it. And it sets the initial values
to 4, 5, 6.
Note you can also pass another TypedArray
. For example
new Float32Array(someUint8ArrayOf6Values)
will make a new ArrayBuffer
of size 6 * 4, then create a Float32Array
to view it,
then copy the values from the existing view
into the new Float32Array
. The values are copied by number, not in binary.
In other words, they are copied like this
srcArray.forEach((v, i) => dstArray[i] = v);
What does “copied by value” mean? Take this example
const f32s = new Float32Array([0.8, 0.9, 1.0, 1.1, 1.2]); const u32s = new Uint32Array(f32s); console.log(u32s); // produces 0, 0, 1, 1, 1
The reason is you can’t put values like 0.8 and 1.2 into a Uint32Array
new Float32Array(someArrayBuffer)
This is the case we used before. A new Float32Array
view is made on an
existing buffer.
new Float32Array(someArrayBuffer, byteOffset)
This makes a new Float32Array
on an existing buffer but starts
the view at byteOffset
new Float32Array(someArrayBuffer, byteOffset, length)
This makes a new Float32Array
on an existing buffer. The view
starts at byteOffset
and is length
units long. So if we passed 3
for length the view would be 3 float32 values long (12 bytes) of
someArrayBuffer
Using this last form we could change the code above to this
const kOurStructSizeBytes = 4 + // velocity 4 + // acceleration 4 ; // frameCount const ourStructData = new ArrayBuffer(kOurStructSizeBytes); const velocityView = new Float32Array(ourStructData, 0, 1); const accelerationView = new Float32Array(ourStructData, 4, 1); const frameCountView = new Uint32Array(ourStructData, 8, 1); velocityView[0] = 1.2; accelerationView[0] = 3.4; frameCountView[0] = 56;
Further, every TypedArray
has the following properties
length
: number of unitsbyteLength
: size in bytesbyteOffset
: offset in the TypedArray
’s ArrayBuffer
buffer
: the ArrayBuffer
this TypedArray
is viewingAnd TypedArray
s have various methods, many are similar to Array
but
one that is not is subarray
. It creates a new TypedArray
view
of the same type. Its parameters are subarray(begin, end)
were
end
is not included. So someTypedArray.subarray(5, 10)
makes
a new TypedArray
of the same ArrayBuffer
of elements 5 to 9
of someTypedArray
.
So we could change the code above to this
const kOurStructSizeFloat32Units = 1 + // velocity 1 + // acceleration 1 ; // frameCount const ourStructDataAsF32 = new Float32Array(kOurStructSizeFloat32Units); const ourStructDataAsU32 = new Uint32Array(ourStructDataAsF32.buffer); const velocityView = ourStructDataAsF32.subarray(0, 1); const accelerationView = ourStructDataAsF32.subarray(1, 2); const frameCountView = ourStructDataAsU32.subarray(2, 3); velocityView[0] = 1.2; accelerationView[0] = 3.4; frameCountView[0] = 56;
ArrayBuffer
Having a view of the same arrayBuffer means exactly that. For example
const v1 = new Float32Array(5); const v2 = v1.subarray(3, 5); // view the last 2 floats of v1 v2[0] = 123; v2[1] = 456; console.log(v1); // shows 0, 0, 0, 123, 456
Similarly if we have different typed views
const f32 = new Float32Array([1, 1000, -1000]) const u32 = new Uint32Array(f32.buffer); console.log(Array.from(u32).map(v => v.toString(16).padStart(8, '0'))); // shows '3f800000', '447a0000', 'c47a0000'
The values above are the 32bit hex representations of the floating point values for 1, 1000, -1000
map
issuesBe aware, the map
function of a TypedArray
makes a new typed array of the same type!
const f32a = new Float32Array(1, 2, 3); const f32b = f32a.map(v => v * 2); // Ok const f32c = f32a.map(v => `${v} doubled = ${v *2}`); // BAD! // you can't put a string in a Float32Array
If you need to map a typedarray into some other type you’ll either need to loop over the array yourself
or else convert it to a JavaScript array which you can do with Array.from
. Taking the example above
const f32d = Array.from(f32a).map(v => `${v} doubled = ${v *2}`); // Ok
WGSL has types made from the 4 base types. They are:
type | description | short name |
---|---|---|
vec2<f32> | a type with 2 f32 s | vec2f |
vec2<u32> | a type with 2 u32 s | vec2u |
vec2<i32> | a type with 2 i32 s | vec2i |
vec2<f16> | a type with 2 f16 s | vec2h |
vec3<f32> | a type with 3 f32 s | vec3f |
vec3<u32> | a type with 3 u32 s | vec3u |
vec3<i32> | a type with 3 i32 s | vec3i |
vec3<f16> | a type with 3 f16 s | vec3h |
vec4<f32> | a type with 4 f32 s | vec4f |
vec4<u32> | a type with 4 u32 s | vec4u |
vec4<i32> | a type with 4 i32 s | vec4i |
vec4<f16> | a type with 4 f16 s | vec4h |
mat2x2<f32> | a matrix of 2 vec2<f32> s | mat2x2f |
mat2x2<u32> | a matrix of 2 vec2<u32> s | mat2x2u |
mat2x2<i32> | a matrix of 2 vec2<i32> s | mat2x2i |
mat2x2<f16> | a matrix of 2 vec2<f16> s | mat2x2h |
mat2x3<f32> | a matrix of 2 vec3<f32> s | mat2x3f |
mat2x3<u32> | a matrix of 2 vec3<u32> s | mat2x3u |
mat2x3<i32> | a matrix of 2 vec3<i32> s | mat2x3i |
mat2x3<f16> | a matrix of 2 vec3<f16> s | mat2x3h |
mat2x4<f32> | a matrix of 2 vec4<f32> s | mat2x4f |
mat2x4<u32> | a matrix of 2 vec4<u32> s | mat2x4u |
mat2x4<i32> | a matrix of 2 vec4<i32> s | mat2x4i |
mat2x4<f16> | a matrix of 2 vec4<f16> s | mat2x4h |
mat3x2<f32> | a matrix of 3 vec2<f32> s | mat3x2f |
mat3x2<u32> | a matrix of 3 vec2<u32> s | mat3x2u |
mat3x2<i32> | a matrix of 3 vec2<i32> s | mat3x2i |
mat3x2<f16> | a matrix of 3 vec2<f16> s | mat3x2h |
mat3x3<f32> | a matrix of 3 vec3<f32> s | mat3x3f |
mat3x3<u32> | a matrix of 3 vec3<u32> s | mat3x3u |
mat3x3<i32> | a matrix of 3 vec3<i32> s | mat3x3i |
mat3x3<f16> | a matrix of 3 vec3<f16> s | mat3x3h |
mat3x4<f32> | a matrix of 3 vec4<f32> s | mat3x4f |
mat3x4<u32> | a matrix of 3 vec4<u32> s | mat3x4u |
mat3x4<i32> | a matrix of 3 vec4<i32> s | mat3x4i |
mat3x4<f16> | a matrix of 3 vec4<f16> s | mat3x4h |
mat4x2<f32> | a matrix of 4 vec2<f32> s | mat4x2f |
mat4x2<u32> | a matrix of 4 vec2<u32> s | mat4x2u |
mat4x2<i32> | a matrix of 4 vec2<i32> s | mat4x2i |
mat4x2<f16> | a matrix of 4 vec2<f16> s | mat4x2h |
mat4x3<f32> | a matrix of 4 vec3<f32> s | mat4x3f |
mat4x3<u32> | a matrix of 4 vec3<u32> s | mat4x3u |
mat4x3<i32> | a matrix of 4 vec3<i32> s | mat4x3i |
mat4x3<f16> | a matrix of 4 vec3<f16> s | mat4x3h |
mat4x4<f32> | a matrix of 4 vec4<f32> s | mat4x4f |
mat4x4<u32> | a matrix of 4 vec4<u32> s | mat4x4u |
mat4x4<i32> | a matrix of 4 vec4<i32> s | mat4x4i |
mat4x4<f16> | a matrix of 4 vec4<f16> s | mat4x4h |
Given that a vec3f
is a type with 3 f32
s and
mat4x4f
is an 4x4 matrix of f32
s, so it’s 16 f32
s,
what do think the following struct looks like in memory?
struct Ex2 { scale: f32, offset: vec3f, projection: mat4x4f, };
Ready?
What’s up with that? It turns out every type has alignment requirements. For a given type it must be aligned to a multiple of a certain number of bytes.
Here are the sizes and alignments of the various types.
But wait, there’s MORE!
What do you think the layout of this struct will be?
struct Ex3 { transform: mat3x3f, directions: array<vec3f, 4>, };
The array<type, count>
syntax defines an array of type
with count
elements.
Here’s you go…
If you look in the alignment table you’ll see vec3<f32>
has
an alignment of 16 bytes. That means each vec3<f32>
, whether
it’s in a matrix or an array ends up having an extra space.
Here’s another one
struct Ex4a { velocity: vec3f, }; struct Ex4 { orientation: vec3f, size: f32, direction: array<vec3f, 1>, scale: f32, info: Ex4a, friction: f32, };
Why did size
end up at byte offset 12, just after orientation but scale
and
friction
got bumped offsets 32 and 64
That’s because arrays and structs have their own own special alignment rules so
even though the array is a single vec3f
and the Ex4a
struct is also a single
vec3f
they get aligned according to different rules.
type | align | size |
---|---|---|
struct S with members M_{1}...M_{N} | max(AlignOfMember(S,1), ... , AlignOfMember(S,N)) | roundUp(AlignOf(S), justPastLastMember)
where justPastLastMember = OffsetOfMember(S,N) + SizeOfMember(S,N) |
array<E, N> | AlignOf(E) | N × roundUp(AlignOf(E), SizeOf(E)) |
You can read the rules in more detail here in the WGSL spec.
Computing sizes and offsets of data in WGSL is probably the largest pain point of WebGPU. You are required to compute these offsets yourself and keep them up to date. If you add a member somewhere in the middle of a struct in your shaders you need to go back to your JavaScript and update all the offsets. Get a single byte or length wrong and the data you pass to the shader will be wrong. You won’t get an error, but your shader will likely do the wrong thing because it’s looking at bad data. Your model won’t draw or your computation will produce bad results.
Fortunately there are libraries to help with this.
Here’s one: webgpu-utils
You give it your WGSL code and it gives an API do all of this for you. This way you can change your structs and, more often than not, things will just work.
For example, using that last example we can pass it to webgpu-utils
like this
import { makeShaderDataDefinitions, makeStructuredView, } from 'https://greggman.github.io/webgpu-utils/dist/0.x/webgpu-utils-1.x.module.js'; const code = ` struct Ex4a { velocity: vec3f, }; struct Ex4 { orientation: vec3f, size: f32, direction: array<vec3f, 1>, scale: f32, info: Ex4a, friction: f32, }; @group(0) @binding(0) var<uniform> myUniforms: Ex4; ... `; const defs = makeShaderDataDefinitions(code); const myUniformValues = makeStructuredView(defs.uniforms.myUniforms); // Set some values via set myUniformValues.set({ orientation: [1, 0, -1], size: 2, direction: [0, 1, 0], scale: 1.5, info: { velocity: [2, 3, 4], }, friction: 0.1, }); // now pass myUniformValues.arrayBuffer to WebGPU when needed.
Whether you use this particular library or a different one or none at all is up to you. For me, I would often spent 20-30-60 minutes trying to figure out why something was not working only to find that I manually computed an offset or size wrong so for my own work I’d rather use a library and avoid that pain.
If you do want to do it manually though, here’s a page that will compute the offsets for you
f16
support is an optional feature ↩︎