The parameters can be specified in any order.

<angle>

Specifies the direction in which the line segment extends from the offset starting position. The angle 0deg lies on the y-axis pointing up, and positive angles increase in the clockwise direction.

<size>

Specifies the length of the line segment, which is the distance between offset-distance 0% and 100%, relative to the containing box. It is an optional parameter (closest-side is used if no <size> is specified) and accepts one of the following keyword values:

closest-side: Distance between the ray's starting point and the closest side of the containing block of the element. If the ray's starting point lies on an edge of the containing block, the length of the line segment is zero. If the ray's starting point is outside the containing block, the edge of the containing block is considered to extend to infinity.

closest-corner: Distance between the ray's starting point and the closest corner in the element's containing block. If the ray's starting point lies on a corner of the containing block, the length of the line segment is zero.

farthest-side: Distance between the ray's starting point and the farthest side of the containing block of the element. If the ray's starting point is outside the containing block, the edge of the containing block is considered to extend to infinity.

farthest-corner: Distance between the ray's starting point and the farthest corner in the element's containing block.

sides: Distance between the ray's starting point and the point where the line segment intersects the containing block's boundary. If the starting point is on or outside the containing block's boundary, the length of the line segment is zero.

contain

Reduces the length of the line segment so that the element stays within the containing block even at offset-distance: 100%. Specifically, the segment's length is reduced by half the width or half the height of the element's border box, whichever is greater, but never going less than zero. It is an optional parameter.

at <position>

Specifies the point where the ray begins and where the element is placed in its containing block. This is an optional parameter. If omitted, the value used is the offset-position value of the element. If omitted and the element doesn't have an offset-position value, the value used for ray's starting position is offset-position: auto, which places the element at the top left corner of the element's box.

The ray() function allows the positioning of an element along a path using polar coordinates, rather than using the standard rectangular coordinates used by the translate() function or using animation to move an element along a defined path. The ray() function helps to create 2D spatial transitions.

The element is initially positioned by moving the element's offset-anchor point to the element's offset starting position. The default offset starting position of a ray is the default offset-position value: auto. If offset-position is explicitly specified as auto or omitted and allowed to default, the offset starting position is the top left corner (or 0 0) of the element's box. With offset-position: normal, the starting position of the ray is 50%, 50% of the element's containing block.

The <coord-box> value of the offset-path property provides the reference box for the ray path.

This example shows how to work with an element's anchor point, how the element gets oriented at the specified ray angle, and how to change a ray's starting point.

CSS

.box {
  background-color: palegreen;
  border-top: 4px solid black;
  position: absolute;
  opacity: 20%;
}

.box1 {
  offset-path: ray(0deg closest-side);
}

.box2 {
  offset-anchor: 0 0;
  offset-path: ray(0deg closest-side);
}

.box3 {
  offset-anchor: 0 0;
  offset-path: ray(150deg closest-side);
}

.box4 {
  offset-rotate: 0deg;
  offset-anchor: 0 0;
  offset-position: 20% 30%;
  offset-path: ray(closest-side 150deg);
}

.box5 {
  offset-rotate: 0deg;
  offset-path: ray(100deg closest-side at bottom right);
}

Similar to transform-origin, the default anchor point is at the center of an element. This anchor point can be modified using the offset-anchor property.

In this example, various offset-anchor and offset-path: ray() values are applied to a box. Results are displayed side-by-side for comparison. The element's containing block is depicted with a dashed border. One border of the box is highlighted to demonstrate different ray starting points and box orientations. After a box is positioned at the ray's starting point, it is oriented in the direction of the specified ray angle. If offset-position is not specified, the default offset starting position of a ray is the top-left corner of the element's containing block.

Result

• box1 gets initially positioned such that its anchor point (at the center) is at the offset starting position (top-left corner of the box). box1 is also rotated to orient it towards the 0deg angle of the ray. This will now be the starting point of the path. You can observe the change in position and rotation of the box by comparing it to the faded box0 in the background.
• The anchor point of box2 is changed to the top-left corner (0px 0px) using the offset-anchor property, and as a result, the element's anchor point and the offset starting position coincide. The ray angle is applied to the element at this starting point. The box is rotated to match the 0deg angle along y-axis and pointing up.
• With other settings the same as in box2, a greater positive angle of 150deg is applied in box3. Starting from the top-left corner, the box is rotated in a clockwise direction to reach the specified angle of 150deg.
• box3 and box4 have the same offset-anchor and offset-path values. In box4, however, an offset-rotate of 0deg is applied to the element. As a result, the element will remain rotated at this specific angle all along the ray's path, and the element will not rotate in the direction of the path. Notice in box4 that the ray path is at 150deg, but the box orientation will not change along the path because of offset-rotate. Also note that the offset-path property of box4 does not specify a starting <position>, so the ray's starting position is derived from the element's offset-position, which in this case is top 20% left 30%.
• In box5, the offset-path property specifies the at <position> value, which places the box at the bottom and right edge of the element's containing block.

In this example, the first shape is shown as a reference for its position and orientation. A ray motion path is applied on the other shapes.

CSS

body {
  display: grid;
  grid-template-columns: 200px 100px;
  gap: 40px;
  margin-left: 40px;
}

.container {
   transform-style: preserve-3d;
   width: 150px;
   height: 100px;
   border: 2px dotted green;
}

.shape {
  width: 40px;
  height: 40px;
  background: #2bc4a2;
  margin: 5px;
  text-align: center;
  line-height: 40px;
  clip-path: polygon(0% 0%, 70% 0%, 100% 50%, 70% 100%, 0% 100%, 30% 50%);
  animation: move 5000ms infinite alternate ease-in-out;
}

.shape2 {
  offset-path: ray(120deg sides contain);
}

.shape3 {
  offset-rotate: 0deg;
  offset-path: ray(120deg sides contain);
}

.shape4 {
  offset-path: ray(120deg closest-corner);
}

.shape5 {
  offset-path: ray(120deg farthest-corner);
}

@keyframes move {
  0%, 20% {
    offset-distance: 0%;
  }
  80%, 100% {
    offset-distance: 100%;
  }

Result

In the first two samples where offset-path is applied, notice the orientation of the shape without offset-rotate and with offset-rotate. The last two offset-path samples show the impact of corner <size> values: closest-corner and farthest-corner. The closest-corner value creates a very short offset-path because the shape is already at the corner. The farthest-corner value creates the longest offset-path, going from the top-left corner of the containing block to the bottom-right corner.

• offset-distance
• offset-path
• offset-rotate