Add CanvasColorType, with unorm8 (the default) and float16 support

source

@@ -65366,12 +65366,15 @@ typedef (<span>HTMLOrSVGImageElement</span> or
 
 enum <dfn enum>PredefinedColorSpace</dfn> { "<span data-x="dom-PredefinedColorSpace-srgb">srgb</span>", "<span data-x="dom-PredefinedColorSpace-display-p3">display-p3</span>" };
 
+enum <dfn enum>CanvasColorType</dfn> { "<span data-x="dom-CanvasColorType-unorm8">unorm8</span>", "<span data-x="dom-CanvasColorType-float16">float16</span>" };
+
 enum <dfn enum>CanvasFillRule</dfn> { "<span data-x="dom-context-2d-fillRule-nonzero">nonzero</span>", "<span data-x="dom-context-2d-fillRule-evenodd">evenodd</span>" };
 
 dictionary <dfn dictionary>CanvasRenderingContext2DSettings</dfn> {
   boolean <span data-x="dom-CanvasRenderingContext2DSettings-alpha">alpha</span> = true;
   boolean <span data-x="dom-CanvasRenderingContext2DSettings-desynchronized">desynchronized</span> = false;
   <span>PredefinedColorSpace</span> <span data-x="dom-CanvasRenderingContext2DSettings-colorSpace">colorSpace</span> = "<span data-x="dom-PredefinedColorSpace-srgb">srgb</span>";
+  <span>CanvasColorType</span> <span data-x="dom-CanvasRenderingContext2DSettings-colorType">colorType</span> = "<span data-x="dom-CanvasColorType-unorm8">unorm8</span>";
   boolean <span data-x="dom-CanvasRenderingContext2DSettings-willReadFrequently">willReadFrequently</span> = false;
 };
 
@@ -65663,6 +65666,10 @@ interface <dfn interface>Path2D</dfn> {
     specifies the <span data-x="concept-canvas-color-space">color space</span> of the rendering
     context.</p>
 
+    <p>The <code data-x="dom-CanvasRenderingContext2DSettings-colorType">colorType</code> member
+    specifies the <span data-x="concept-canvas-color-type">color type</span> of the rendering
+    context.</p>
+
     <p>If the <code
     data-x="dom-CanvasRenderingContext2DSettings-willReadFrequently">willReadFrequently</code>
     member is true, then the context is marked for <span
@@ -65687,6 +65694,10 @@ interface <dfn interface>Path2D</dfn> {
      a string indicating the context's <span data-x="concept-canvas-color-space">color
      space</span>.</li>
 
+     <li><code data-x="dom-CanvasRenderingContext2DSettings-colorType">colorType</code> member is
+     a string indicating the context's <span data-x="concept-canvas-color-type">color
+     type</span>.</li>
+
      <li><code data-x="dom-CanvasRenderingContext2DSettings-willReadFrequently">willReadFrequently</code>
      member is true if the context is marked for <span data-x="concept-canvas-will-read-frequently">readback
      optimization</span>.</li>
@@ -65820,6 +65831,19 @@ context.fillRect(100,0,50,50); // only this square remains</code></pre>
 
   <hr>
 
+  <p>The <code>CanvasColorType</code> enumeration is used to specify the <span
+  data-x="concept-canvas-color-type">color type</span> of the canvas's backing store.</p>
+
+  <p>The "<dfn enum-value for="CanvasColorType"><code
+  data-x="dom-CanvasColorType-unorm8">unorm8</code></dfn>" value indicates that the type
+  for all color channels is 8-bit unsigned normalized.</p>
+
+  <p>The "<dfn enum-value for="CanvasColorType"><code
+  data-x="dom-CanvasColorType-float16">float16</code></dfn>" value indicates that the type
+  for all color channels is 16-bit floating point.</p>
+
+  <hr>
+
   <p>The <code>CanvasFillRule</code> enumeration is used to select the <dfn>fill rule</dfn>
   algorithm by which to determine if a point is inside or outside a path.</p>
 
@@ -65962,6 +65986,12 @@ context.fillRect(100,0,50,50); // only this square remains</code></pre>
   data-x="concept-canvas-color-space">color space</span> indicates the color space for the
   <span>output bitmap</span>.</p>
 
+  <p>The <code>CanvasSettings</code> object also has a <dfn
+  data-x="concept-canvas-color-type">color type</dfn> setting of type
+  <code>CanvasColorType</code>. The <code>CanvasSettings</code> object's <span
+  data-x="concept-canvas-color-type">color type</span> indicates the data type of the
+  color and alpha components of the pixels of the <span>output bitmap</span>.</p>
+
   <p>To <dfn data-x="canvas-setting-init-bitmap">initialize a <code>CanvasSettings</code> output
   bitmap</dfn>, given a <code>CanvasSettings</code> <var>context</var> and a
   <code>CanvasRenderingContext2DSettings</code> <var>settings</var>:</p>
@@ -65980,6 +66010,10 @@ context.fillRect(100,0,50,50); // only this square remains</code></pre>
    <var>settings</var>["<dfn dict-member for="CanvasRenderingContext2DSettings"><code
    data-x="dom-CanvasRenderingContext2DSettings-colorSpace">colorSpace</code></dfn>"].</p></li>
 
+   <li><p>Set <var>context</var>'s <span data-x="concept-canvas-color-type">color type</span> to
+   <var>settings</var>["<dfn dict-member for="CanvasRenderingContext2DSettings"><code
+   data-x="dom-CanvasRenderingContext2DSettings-colorType">colorType</code></dfn>"].</p></li>
+
    <li><p>Set <var>context</var>'s <span data-x="concept-canvas-will-read-frequently">will read
    frequently</span> to <var>settings</var>["<dfn dict-member
    for="CanvasRenderingContext2DSettings"><code
@@ -65994,6 +66028,8 @@ context.fillRect(100,0,50,50); // only this square remains</code></pre>
   <span>this</span>'s <span data-x="concept-canvas-desynchronized">desynchronized</span>, "<code
   data-x="dom-CanvasRenderingContext2DSettings-colorSpace">colorSpace</code>" → <span>this</span>'s
   <span data-x="concept-canvas-color-space">color space</span>, "<code
+  data-x="dom-CanvasRenderingContext2DSettings-colorType">colorType</code>" → <span>this</span>'s
+  <span data-x="concept-canvas-color-type">color type</span>, "<code
   data-x="dom-CanvasRenderingContext2DSettings-willReadFrequently">willReadFrequently</code>" →
   <span>this</span>'s <span data-x="concept-canvas-will-read-frequently">will read frequently</span>
   ]».</p>
@@ -71832,6 +71868,16 @@ interface <dfn interface>OffscreenCanvasRenderingContext2D</dfn> {
   data-x="dom-canvas-toBlob">toBlob()</code> method to the canvas, given the appropriate dimensions,
   has no visible effect beyond, at most, clipping colors of the canvas to a more narrow gamut.</p>
 
+  <p>For image types that support multiple bit depths, the serialized image must use the bit depth
+  that best preserves content of the underlying bitmap.</p>
+
+  <p class="example">For example, when serializing a 2D context that has
+  <span data-x="concept-canvas-color-type">color type</span> of
+  <span data-x="dom-CanvasColorType-float16">float16</span> to <var>type</var>
+  "<code>image/png</code>", the resulting image would have 16 bits per sample.
+  This serialization will still lose significant detail (all values less than 0.5/65535
+  would be clamped to 0, and all values greater than 1 would be clamped to 1).</p>
+
   <p>If <var>type</var> is an image format that supports variable quality (such as
   "<code>image/jpeg</code>"), <var>quality</var> is given, and <var>type</var> is not
   "<code>image/png</code>", then, if <var>quality</var> <span data-x="js-Number">is a Number</span>
@@ -71971,9 +72017,9 @@ interface <dfn interface>OffscreenCanvasRenderingContext2D</dfn> {
   <p>As certain colors can only be represented under premultiplied alpha (for instance, additive
   colors), and others can only be represented under non-premultiplied alpha (for instance,
   "invisible" colors which hold certain red, green, and blue values even with no opacity); and
-  division and multiplication on 8-bit integers (which is how canvas's colors are currently stored)
-  entails a loss of precision, converting between premultiplied and non-premultiplied alpha is a
-  lossy operation on colors that are not fully opaque.</p>
+  division and multiplication using finite precision entails a loss of accuracy, converting between
+  premultiplied and non-premultiplied alpha is a lossy operation on colors that are not fully
+  opaque.</p>
 
   <p>A <code>CanvasRenderingContext2D</code>'s <span>output bitmap</span> and an
   <code>OffscreenCanvasRenderingContext2D</code>'s <span>output bitmap</span> must use premultiplied