<h4 id=out-of-gamut-premultiplied-rgba-value data-dfn-type=dfn>Out-of-Gamut Premultiplied RGBA Values

</h4>

An [=out-of-gamut premultiplied RGBA value=] is one where any of the R/G/B channel values

exceeds the alpha channel value.

<div class=example>

For example, the premultiplied sRGB RGBA value `[0.51, 0, 0, 0.5]`

represents the (unpremultiplied) color <code><a funcdef>color</a>(srgb 1.02 0 0 / 0.5)</code>.

</div>

Just like any color value outside the sRGB color gamut, any value is a well-defined point in the

extended color space (regardless of whether the point represents a real color).

When a canvas containing such a value is

[$get a copy of the image contents of a context|used as an image source$],

the color value must not be clamped in the image value given to the calling algorithm

regardless of the canvas's {{GPUCanvasConfiguration/format}}.

However, when a canvas containing such a value is displayed *on screen*, **if and only if** the

canvas has a range-limited format (like `unorm` formats, which have a range of 0&ndash;1),

the visible result for the entire canvas is undefined.

<div class=example>

For example, on a display with a color space of exactly Display P3,

an on-screen, solid-color {{PredefinedColorSpace/"srgb"}} canvas displays as follows:

<table class=data>

<thead>

<tr><th>{{GPUCanvasConfiguration/alphaMode}},<br>{{GPUCanvasConfiguration/format}}<th>Pixel Value<th>Result

<tbody>

<tr><td>{{GPUCanvasAlphaMode/"opaque"}},<br>{{GPUTextureFormat/"rgba16float"}}

<td rowspan=2>`[1.04, 0, 0, 1.0]`

<td rowspan=2><code><a funcdef>color</a>(display-p3 0.95 0.21 0.15 / 1.0)</code>,

<br>because the format is not range-limited.

<tr><td>{{GPUCanvasAlphaMode/"premultiplied"}},<br>{{GPUTextureFormat/"rgba16float"}}

<tr><td>{{GPUCanvasAlphaMode/"premultiplied"}},<br>{{GPUTextureFormat/"rgba16float"}}

<td>`[0.52, 0, 0, 0.5]`

<td><code><a funcdef>color</a>(display-p3 0.95 0.21 0.15 / 0.5)</code>,

<br>because the format is not range-limited.

<tr><td>{{GPUCanvasAlphaMode/"premultiplied"}},<br>{{GPUTextureFormat/"rgba8unorm"}}

<td>`[0.50, 0, 0, 0.5]`

<td><code><a funcdef>color</a>(display-p3 0.92 0.20 0.14 / 0.5)</code>,

<br>because the color is in-gamut for {{PredefinedColorSpace/"srgb"}}.

<tr><td>{{GPUCanvasAlphaMode/"premultiplied"}},<br>{{GPUTextureFormat/"rgba8unorm"}}

<td>`[0.52, 0, 0, 0.5]`

<td>(undefined result),

<br>because the format is range-limited

<br>and the color is out-of-gamut for {{PredefinedColorSpace/"srgb"}}.

</table>

The last result is undefined because the color could be:

- blended directly to the display buffer as

<code><a funcdef>color</a>(display-p3 0.95 0.21 0.15 / 0.5)</code>.

- blended through an intermediate sRGB buffer as `[0.5, 0, 0, 0.5]`,

then sent to the display buffer resulting in the appearance of

<code><a funcdef>color</a>(display-p3 0.92 0.2 0.14 / 0.5)</code>.

- displayed by some other process.

</div>

Note:

Implementations may defer compositing steps to operating system compositors,

which often have undefined behavior in these cases. Implementations may not be able to avoid

these undefined behaviors without significant power usage penalties.

If the source value has fewer than 4 RGBA channels, any missing green/blue/alpha channels are set to

[=out-of-gamut premultiplied RGBA value|out-of-gamut values=]. <!-- FIXME? -->

For example, 100% red at 50% alpha is `[0.5, 0, 0, 0.5]`.

When alpha is 0, no color is represented, regardless of the RGB values.

(Attempting to compute the color results in division by zero.)

In this mode, the canvas texture may represent [=out-of-gamut premultiplied RGBA values=].

If the canvas has a range-limited format and is displayed on screen, out-of-gamut values

have undefined display results. Refer to that section for details.