Add ImageData Float16Array support

[ccameron-chromium]

Add the ImageDataPixelFormat enum type, which can take on values of "rgba-unorm8" (the current default of 8-bit unsigned normalized RGBA) and "rgba-float16" (the new option for 16-bit floating point RGBA).

Add ImageDataPixelFormat to the ImageDataSettings dictionary used by ImageData constructors, to allow creation of ImageData objects that are backed by Float16Array.

Add ImageDataPixelFormat as an attribute to ImageData.

Add a new ImageDataArray type which is the union of Uint8ClampedArray and Float16Array. Change the ImageData data attribute to be this union type instead of Uint8ClampedArray.

Fix the Canvas Pixel ArrayBuffer section to correctly describe the pixel layout of an ImageBuffer (there was a bug in the spec specifying that an ImageData's pixel data starts at the beginning of its data attributes's backing ArrayBuffer, when what was intended was for it to start at the beginning of its data attribute).

Fixes #10856

• At least two implementers are interested (and none opposed):
  • Chromium (author)
  • Canvas Floating Point Color Types mozilla/standards-positions#708
  • Canvas Floating Point Color Types WebKit/standards-positions#91
• Tests are written and can be reviewed and commented upon at:
  • Staging at: https://chromium-review.googlesource.com/6372220
• Implementation bugs are filed:
  • Chromium: crbug.com/40277213
  • Gecko: (will file once PR number is created to link to in issue)
  • WebKit: (will file once PR number is created to link to in issue)
• MDN issue is filed:
  • (will file once PR is ready to land -- see feedback here)
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/canvas.html ( diff )
/infrastructure.html ( diff )

[Kaiido]

This isn't new, but aren't the arguments inverted here?
All the call-sites do Initialize imageData given sw, sh, settings set to [...], but here the algorithm expects number of rows as the first argument and then pixels per row.
Though maybe this should be fixed separately and I'm sure there is some modernization that could get in there.

[ccameron-chromium]

I swapped the ordering of the parameters, since that seems fairly urgent to not have wrong (when we do a modernization pass, we should also change the names to width and height).

[domfarolino]

Add ImageDataPixelFormat as an attribute to ImageData.

This PR also adds the pixelFormat readonly attribute to ImageData, but I'm not completely sure why. The explainer indicates that it is for feature detection, but couldn't you just as easily type-check the underlying data? Is this member just added as a convenience mechanism for that, or is it necessary for some other reason? My concern is that it isn't necessary, but that it does add complexity just to validate/synchronize the type of the input source with the pixelFormat descriptor in the settings object.

I could imagine this being important if you wanted to accept a Float16Array yet force it to be treated like a Uint8ClampedArray by passing in pixelFormat = "rgba-unorm8", but we don't seem to support that.