mask-mode

The mask-mode property can take multiple comma-separated <masking-mode> keyword values, including:

alpha

Indicates that the alpha (transparency) values of the mask image should be used.

luminance

Indicates that the luminance (brightness) values of the mask image should be used.

match-source

Indicates that the type of mask is determined by the source. This is the default property value.

• If the mask-image references an SVG <mask>, its mask-type property value is used, or it's mask-type attribute, if present. If neither is explicitly set, this value defaults to luminance.
• If the mask image source is an <image> or a <gradient>, the alpha values of the mask image are used.

A mask transfers its transparency, and depending on the mask type, it's luminance, to the element it is masking. If the mask is of type <image>, by default the alpha values of the mask image determine the visibility of each part of the masked element: where the mask is opaque, the corresponding part of the masked element is visible; where the mask is translucent, the element is as well, with those areas of the element being hidden. This is the default behavior for <image> masks when the mask-mode property is set to or defaults to match-source, and it is always the case when the mask-mode is explicitly set to alpha.

In the case of luminance masks, the visibility of the masked element depends on both the opacity of the mask and the lightness of the color of the opaque areas. White (100% luminance) opaque areas (alpha = 1) will be masked and visible, and black areas (0% luminance) transparent (alpha = 0) will be clipped. Areas with colors in between white and black and with partial opacity will be partially masked, reflecting the luminance and alpha transparency of each color making up the mask.

The opacity of a luminance mask is determined by the R, G, B, and A values of an rgb() color using the formula:

((0.2125 * R) + (0.7154 * G) + (0.0721 * B)) * A

For example, the color green is #008000 or rgb(0% 50% 0% / 1). In a luminance mask, any area masked by a solid green mask will be 35.77% opaque. If the mask-mode for this mask is set to alpha, since green is a fully opaque color, the masked area will be 100% opaque.

Each mask-mode value is a comma-separated list of values. When multiple values are present, each value corresponds to a mask layer in the same position in the mask-image property. The values define whether the associated mask images are treated as a luminance or an alpha mask.

In the case of match-source, whether luminance or alpha is used depends on the mask mode of the mask source. If the mask-image property values is a reference to an SVG <mask> element, the <mask> element's mask-type property value is used. If there is no CSS mask-type property set on the <mask> element, the value of the <mask> element's mask-type attribute is used, if present and supported. If neither is explicitly set, this value defaults to luminance; but only in the case of the <mask> element as the mask source. Otherwise, as noted before, if the mask image source is an <image>, rather than an SVG <mask>, the alpha values of the mask layer image is used.

mask-mode = 
  <masking-mode>#  

<masking-mode> = 
  alpha         |
  luminance     |
  match-source  

This example demonstrates the basic usage and different values of the mask-mode property.

HTML

We include three <div> elements, so we can demonstrate the three enumerated mask-mode keyword values.

<div class="alpha">ALPHA</div>
<div class="luminance">LUMINANCE</div>
<div class="matchSource">MATCH-SOURCE</div>

CSS

Each <div> is provided with the same background and masking image. The only difference between each <div> is the value of the mask-mode property:

div {
  background: blue linear-gradient(red, blue);
  mask-image: url(https://mdn.github.io/shared-assets/images/examples/mdn.svg);
}

.alpha {
  mask-mode: alpha;
}

.luminance {
  mask-mode: luminance;
}

.matchSource {
  mask-mode: match-source;
}

div {
  width: 227px;
  height: 200px;
  float: left;
  text-align: center;
  line-height: 65px;
  color: white;
  text-shadow: 1px 1px 2px black;
  font-family: sans-serif;
}

Results

Because the mask source is an <image> and not an SVG <mask>, the match-source value resolves to alpha.

• mask-type
• mask-image
• mask shorthand
• CSS masking module