<hue-interpolation-method>

A <hue-interpolation-method> value consists of the name of a hue interpolation algorithm followed by a literal token hue:

shorter hue
longer hue
increasing hue
decreasing hue

Any pair of hue angles correspond to two radii on the color wheel, which cut the circumference into two possible arcs for interpolation. Both arcs start at the first radius and end at the second radius, but one goes clockwise and the other goes counterclockwise.

For a pair of hue angles θ1 and θ2 normalized to the range [0deg, 360deg), there are four algorithms to determine which arc is used when interpolating from θ1 to θ2:

shorter

Use the shorter arc. When the two radii coincide, the arc degenerates to a single point. When both arcs have the same lengths:

• If θ1 < θ2, use the clockwise arc;
• If θ1 > θ2, use the counterclockwise arc.

θ1 = 45deg, θ2 = 135deg	θ1 = 135deg, θ2 = 45deg

longer

Use the longer arc. When the two radii coincide:

• If θ1 ≤ θ2, the arc becomes the full circumference with a clockwise orientation.
• If θ1 > θ2, the arc becomes the full circumference with a counterclockwise orientation.

When both arcs have the same lengths:

• If θ1 < θ2, use the clockwise arc;
• If θ1 > θ2, use the counterclockwise arc.

θ1 = 45deg, θ2 = 135deg	θ1 = 135deg, θ2 = 45deg

increasing

Use the clockwise arc. When the two radii coincide, the arc degenerates to a single point.

θ1 = 45deg, θ2 = 135deg	θ1 = 135deg, θ2 = 45deg

decreasing

Use the counterclockwise arc. When the two radii coincide, the arc degenerates to a single point.

θ1 = 45deg, θ2 = 135deg	θ1 = 135deg, θ2 = 45deg

As there are only two arcs to choose from, these algorithms are pairwise equivalent under certain circumstances. Specifically:

• If 0deg < θ2 - θ1 < 180deg or θ2 - θ1 < -180deg, shorter and increasing are equivalent, whereas longer and decreasing are equivalent.
• If -180deg < θ2 - θ1 < 0deg or θ2 - θ1 > 180deg, shorter and decreasing are equivalent, whereas longer and increasing are equivalent.

A notable feature of increasing and decreasing is that when the hue angle difference passes through 180deg during transition or animation, the arc will not flip to the other side like shorter and longer do.

<hue-interpolation-method> = 
  [ shorter | longer | increasing | decreasing ] hue  

The following example shows the effect of using different hue interpolation algorithms in a linear-gradient().

HTML

<div class="hsl">
  <p>HSL</p>
</div>
<div class="hsl-increasing">
  <p>HSL increasing</p>
</div>
<div class="hsl-decreasing">
  <p>HSL decreasing</p>
</div>
<div class="hsl-shorter">
  <p>HSL shorter</p>
</div>
<div class="hsl-longer">
  <p>HSL longer</p>
</div>
<div class="hsl-named">
  <p>HSL named</p>
</div>
<div class="hsl-named-longer">
  <p>HSL named (longer)</p>
</div>

CSS

.hsl {
  background: linear-gradient(
    to right in hsl,
    hsl(39deg 100% 50%),
    hsl(60deg 100% 50%)
  );
}
.hsl-increasing {
  background: linear-gradient(
    to right in hsl increasing hue,
    hsl(190deg 100% 50%),
    hsl(180deg 100% 50%)
  );
}
.hsl-decreasing {
  background: linear-gradient(
    to right in hsl decreasing hue,
    hsl(39deg 100% 50%),
    hsl(60deg 100% 50%)
  );
}
.hsl-shorter {
  background: linear-gradient(
    to right in hsl shorter hue,
    hsl(39deg 100% 50%),
    hsl(60deg 100% 50%)
  );
}
.hsl-longer {
  background: linear-gradient(
    to right in hsl longer hue,
    hsl(39deg 100% 50%),
    hsl(60deg 100% 50%)
  );
}
.hsl-named {
  background: linear-gradient(to right in hsl, orange, yellow);
}
.hsl-named-longer {
  background: linear-gradient(to right in hsl longer hue, orange, yellow);
}

Result

• <color-interpolation-method>
• <hue> data type