Motion Path Module Level 1

1. Introduction

This section is not normative.

The transform property and its related properties allow a box to be arbitrarily repositioned (and rotated, scaled, etc) relative to its laid out position, without disrupting the layout of any other elements on the page. These positions can be animated or transitioned with CSS, but only in relatively simple ways: moving a box in a straight line from its starting position to its ending position.

This specification introduces the offset property, which takes a number of functions introducing paths through space, and a distance to travel along that path, and transforms (and possibly rotates) the element to match that point along the path.

This allows a number of powerful new transform possibilities, such as positioning using polar coordinates (with the ray() function) rather than the standard rectangular coordinates used by the translate() function.

It also allows an element to be animated along the defined path, making it easy to define complex and beautiful 2d spatial transitions.

For example, the following picture shows a curving path (indicated with dotted lines), and an airplane graphic positioned at various points along the path. The plane faces in the direction of the path at each position on the path.

1.1. Module interactions

This specification defines additional types of transforms (see [css-transforms-1]) that can be applied to an element.

As described in CSS Transforms 2 § 6 Current Transformation Matrix, the transforms defined by this document are layered after the individual transform properties (translate/rotate/scale, defined in [css-transforms-2]) and before the transform property (defined in [css-transforms-1]).

1.2. Values

This specification follows the CSS property definition conventions from [CSS21]. The <basic-shape> type is defined in CSS Shapes Module Level 1 [CSS-SHAPES]. The <coord-box> tpe is defined in CSS Box Model Module Level 3 [CSS-BOX-3]. Value types not defined in these specifications are defined in CSS Values and Units Module Level 3 [CSS3VAL].

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept CSS-wide keywords such as initial and inherit as their property value [CSS3VAL]. For readability it has not been repeated explicitly.

2. Motion Paths

2.1. Defining A Path: The offset-path property

Name:	offset-path
Value:	none \| <offset-path> \|\| <coord-box>
Initial:	none
Applies to:	transformable elements
Inherited:	no
Percentages:	n/a
Computed value:	as specified
Canonical order:	per grammar
Animation type:	by computed value
Media:	visual

Specifies the offset path, a geometrical path the box gets positioned on.

<offset-path> = <ray()> | <url()> | <basic-shape>

Values have the following meanings:

none

The element does not have an offset transform.

<offset-path> || <coord-box>

The element has an offset transform, defined by some offset path. See § 2.5.1 Calculating the path transform for details on how to calculate the offset transform.

All the usual effects of having a transform apply (such as creating a stacking context, etc.) See CSS Transforms 1 § 2 The Transform Rendering Model for details.

If <offset-path> is omitted, it defaults to inset(0 round X), where X is the value of border-radius on the element that establishes the containing block for this element. If <coord-box> is omitted, it defaults to border-box.

See the specific values (below) for the interpretation of each component.

<ray()>

The offset path is a line extending from the initial position at some angle. See § 2.1.1 The ray() Function for details.

The <coord-box> provides the [=/reference box=] for the ray.

<url>

A URL reference to an SVG shape element. The offset path is the referenced element’s equivalent path. [SVG2]

If the URL does not reference a shape element (because it references a different element, or resolves to a non-SVG document, or doesn’t resolve at all, etc) this behaves as path("m 0 0") (a <basic-shape>) instead.

The <coord-box> defines the viewport and user coordinate system for the shape element, with the origin (the 0,0 point) at the top left corner, and units being 1px in size.

<basic-shape>

The offset path is the equivalent path of the <basic-shape> function.

If circle() or ellipse() is used, and an explicit center position is not given, they default to using the position specified by offset-position, rather than their standard default.

The <coord-box> provides the [=/reference box=] for the <basic-shape>.

<coord-box>

Defines the box that the <offset-path> sizes into.

In CSS contexts, the boxes being referenced are from the element that establishes the containing block for this element.

In SVG contexts, all values behave as view-box.

2.1.1. The ray() Function

The ray() function defines an offset path as a straight line emerging from a point at some defined angle:

ray() = ray( <angle> && <ray-size>? && contain? )

<ray-size> = closest-side | closest-corner | farthest-side | farthest-corner | sides

Its arguments are:

<angle>

The offset path is a line segment that starts from the initial position and proceeds in the direction defined by the specified <angle>. As with gradient functions, <angle> values are interpreted as bearing angles, with 0deg pointing up and positive angles representing clockwise rotation.

This is also the initial direction.

<ray-size>

Specifies the length of the offset path (the distance between the offset-distance: 0% and offset-distance: 100% points) relative to the containing box.

If no <ray-size> is specified it defaults to closest-side.

Note: For sides, the distance depends on the <angle> specified; for all other values, the distance is constant regardless of the <angle>.

Individual keywords are:

closest-side

The distance from the initial position to whichever side of the containing block is closest.

closest-corner

The distance from the initial position to whichever corner of the containing block is closest.

farthest-side

The distance from the initial position to whichever side of the containing block is farthest.

farthest-corner

The distance from the initial position to whichever corner of the containing block is farthest.

sides

The distance from the initial position to the point where the offset path intersects the containing block’s boundary.

If the initial position is on the containing block’s boundary, or outside its bounds entirely, the distance is zero.

Note: For closest-side and closest-corner, if the initial position is on an edge/corner, that’s the closest one. (In other words, the distance is zero.)

Note: For closest-side and farthest-side, if the initial position is outside the containing block entirely, the edges of the containing block are considered to extend out to infinity.

contain

The used value of offset-distance is clamped so that the box is entirely contained within the path.

If no offset-distance would lead to the box being enclosed by the path, the path size is minimally increased so that such an offset-distance exists.

Not clear what this actually means. See Issue 363.

Here are some examples. The first example shows that some parts of boxes are outside of the offset path.

<style>
    body {
        transform-style: preserve-3d;
        width: 200px;
        height: 200px;
    }
    .box {
        width: 50px;
        height: 50px;
        offset-position: 50% 50%;
        offset-distance: 100%;
        offset-rotate: 0deg;
    }
    #redBox {
        background-color: red;
        offset-path: ray(45deg closest-side);
    }
    #blueBox {
        background-color: blue;
        offset-path: ray(180deg closest-side);
    }
</style>
<body>
    <div class="box" id="redBox"></div>
    <div class="box" id="blueBox"></div>
</body>

An image of boxes positioned without contain — offset-path without contain

In the second example, contain is given to the offset-path value of each box to avoid overflowing.

<style>
    body {
        transform-style: preserve-3d;
        width: 200px;
        height: 200px;
    }
    .box {
        width: 50px;
        height: 50px;
        offset-position: 50% 50%;
        offset-distance: 100%;
        offset-rotate: 0deg;
    }
    #redBox {
        background-color: red;
        offset-path: ray(45deg closest-side contain);
    }
    #blueBox {
        background-color: blue;
        offset-path: ray(180deg closest-side contain);
    }
</style>
<body>
    <div class="box" id="redBox"></div>
    <div class="box" id="blueBox"></div>
</body>

An image of boxes positioned with contain — offset-path with contain

In the third example, the path size is increased so that the box can be contained. The used offset distance is negative.

<style>
    body {
        transform-style: preserve-3d;
        width: 250px;
        height: 250px;
    }
    .box {
        width: 60%;
        height: 10%;

        offset-position: 20% 20%;
        offset-distance: 0%;
        offset-rotate: 0deg;
        offset-anchor: 200% -300%;
    }
    #blueBox {
        background-color: blue;
        offset-path: ray(-90deg closest-side contain);
    }
</style>
<body>
    <div class="box" id="blueBox"></div>
</body>

An image of an increased path size — offset-path with path size increased

In the fourth example, the initial position is outside the containing block.

<style>
    #container {
        transform-style: preserve-3d;
        width: 200px;
        height: 200px;
    }
    .box {
        width: 20%;
        height: 20%;
        offset-position: 140% 70%;
        offset-distance: 100%;
    }
    #redBox {
        background-color: red;
        offset-path: ray(-90deg sides);
    }
    #blueBox {
        background-color: blue;
        offset-path: ray(180deg closest-side);
    }
</style>
<div id="container">
    <div class="box" id="redBox"></div>
    <div class="box" id="blueBox"></div>
</div>

An image with initial position outside the containing block — Initial position outside the containing block

2.1.2. Examples Of <basic-shape> Positioning

This example uses a circle with implicit center position.

<style>
    body {
        width: 323px;
        height: 131px;
        margin: 0px;
        border: 2px solid black;
        padding: 8px;
        transform-style: preserve-3d;
    }
    .item {
        width:  90px;
        height: 40px;
        background-color: violet;
    }
    #middle {
        offset-position: auto;
        offset-path: circle(60%) margin-box;
        offset-distance: 25%;
        offset-anchor: left top;
    }
</style>
<body>
    <div class="item"></div>
    <div class="item" id="middle"></div>
    <div class="item"></div>
</body>

Normal flow determining circle center — The circle center is determined by normal flow.

2.1.3. Examples of <coord-box> Positioning

This example shows how <coord-box> offset path works in combination with border-radius.

<style>
    body {
        width: 500px;
        height: 300px;
        border-radius: 80px;
        border: dashed aqua;
        margin: 0;
    }
    #blueBox {
        width: 40px;
        height: 20px;
        background-color: blue;
        offset-path: margin-box;
    }
</style>
<body>
    <div id="blueBox"></div>
</body>

An image of example for geometry-box with border-radius — The initial position is the left end of the top horizontal line.

2.2. Position on the path: The offset-distance property

Name:	offset-distance
Value:	<length-percentage>
Initial:	0
Applies to:	transformable elements
Inherited:	no
Percentages:	refer to the total path length
Computed value:	a computed <length-percentage> value
Canonical order:	per grammar
Animation type:	by computed value
Media:	visual

Specifies the position of the box as a distance along the offset path.

<length-percentage>

Specifies the distance from the initial position of the offset path to the position of the box’s anchor point.

Percentages are relative to the length of the offset path-- that is, the distance between the initial position and the end position of the offset path.

2.2.1. Calculating the computed distance along a path

Processing the distance along an offset path operates differently depending upon the nature of the offset path:

References to <angle> offset paths with contain are unclosed intervals.
References to <angle> offset paths without contain are unbounded rays.
All basic CSS shapes are closed loops.
Offset paths (including references to SVG Paths) are closed loops only if the final command in the path list is a closepath command ("z" or "Z"), otherwise they are unclosed intervals.
References to SVG circles, ellipses, images, polygons and rects are closed loops.
References to SVG lines and polylines are unclosed intervals.

To determine the used offset distance for a given offset path and offset distance:

Let the total length be the total length of offset path with all sub-paths.
Convert offset distance to pixels, with 100% being converted to total length.
If offset path is an unbounded ray:

Let used offset distance be equal to offset distance.

Otherwise if offset path is an <angle> path with contain:

Let used offset distance be equal to offset distance, clamped so that the box lies entirely within the path.

If offset path is any other unclosed interval:

Let used offset distance be equal to offset distance clamped by 0 and the total length of the path.

Otherwise offset path is a closed loop:

Let used offset distance be equal to offset distance modulo the total length of the path. If the total length of the path is 0, used offset distance is also 0.

Note: “Modulo” here uses the traditional mathematical definition, where the output is always non-negative.

This example shows boxes placed along an unclosed interval.

<style>
    .item {
        width: 100px;
        height: 40px;
        offset-position: 0% 0%;
        offset-path: path('m 0 0 h 200 v 150');
    }
    #box1 {
        background-color: red;
        offset-distance: -280%;
    }
    #box2 {
        background-color: green;
        offset-distance: 190%;
    }
</style>
<body>
    <div class="item" id="box1"></div>
    <div class="item" id="box2"></div>
</body>

An example of boxes placed along an unclosed interval

This example shows boxes placed along a closed interval.

<style>
    .item {
        width: 100px;
        height: 40px;
        offset-position: 0% 0%;
        offset-path: path('m 0 0 h 200 v 150 z');
    }
    #box1 {
        background-color: red;
        offset-distance: -280%;
    }
    #box2 {
        background-color: green;
        offset-distance: 190%;
    }
</style>
<body>
    <div class="item" id="box1"></div>
    <div class="item" id="box2"></div>
</body>

An example of boxes placed along a closed interval

This example shows a way to align boxes within the polar coordinate system using offset-path, offset-distance.

<style>
    body {
        transform-style: preserve-3d;
        width: 300px;
        height: 300px;
        border: dashed gray;
        border-radius: 50%;
    }
    .circleBox {
        position: absolute;
        left: 50%;
        top: 50%;
        width: 40px;
        height: 40px;
        background-color: red;
        border-radius: 50%;
    }
    #circle1 {
        offset-path: ray(0deg farthest-side);
        offset-distance: 50%;
    }
    #circle2 {
        offset-path: ray(90deg farthest-side);
        offset-distance: 20%;
    }
    #circle3 {
        offset-path: ray(225deg farthest-side);
        offset-distance: 100%;
    }
</style>
<body>
    <div class="circleBox" id="circle1"></div>
    <div class="circleBox" id="circle2"></div>
    <div class="circleBox" id="circle3"></div>
</body>

An image of three boxes positioned to polar coordinates — An example of positioning box in polar coordinates

2.3. Define the starting point of the path: The offset-position property

Name:	offset-position
Value:	auto \| <position>
Initial:	auto
Applies to:	transformable elements
Inherited:	no
Percentages:	Refer to the size of containing block
Computed value:	The auto keyword or a computed <position>
Canonical order:	per grammar
Animation type:	by computed value
Media:	visual

Specifies the initial position of the offset path, for offset paths that don’t predefine their initial position. (Currently, ray() is the only offset path that doesn’t predefine its initial position).

Currently, circle() and ellipse() offset paths use this position as their center, if the center position is omitted from their arguments. This is not an initial position, obviously. Whether this is desired is being discussed in Issue 504.

Values are defined as follows:

auto: The initial position is the top-left corner of the box.
<position>: The initial position is the result of using the <position> to position a 0x0 object area within the box’s containing block.

This example shows positioning a box with offset-position.

<style>
    #wrap {
        position: relative;
        width: 300px;
        height: 300px;
        border: 1px solid black;
    }

    #box {
        width: 100px;
        height: 100px;
        background-color: green;
        position: absolute;
        top: 100px;
        left: 80px;
        offset-position: auto;
        offset-anchor: center;
        offset-path: ray(45deg);
    }
</style>
<body>
    <div id="wrap">
        <div id="box"></div>
    </div>
</body>

An image of offset-position: auto — An example when auto is given to offset-position

This example shows the interaction with the transform property, and with an individual transform property (rotate). The motion path transform is a vertical translation moving (left, top) to offset-position.

<style>
    #wrap {
        transform-style: preserve-3d;
        width: 400px;
        height: 350px;
    }
    .item {
        position: absolute;
        left: 200px;
        top: 0px;
        offset-position: 200px 100px; /* translates by 0px,100px */
        offset-anchor: left top;
        transform-origin: left top;
        width: 130px;
        height: 80px;
        border-top-right-radius: 23px;
    }
    #box1 {
        background-color: tomato;
        offset-position: auto;
    }
    #box2 {
        background-color: green;
    }
    #box3 {
        background-color: navy;
        rotate: 90deg; /* applied before motion path transform */
    }
    #box4 {
        background-color: gold;
        transform: rotate(90deg); /* applied after motion path transform */
    }
</style>
<body>
    <div id="wrap">
        <div class="item" id="box1"></div>
        <div class="item" id="box2"></div>
        <div class="item" id="box3"></div>
        <div class="item" id="box4"></div>
    </div>
</body>

An example when motion path and other transforms interact

This example uses position static, so offset-position generates translations from the normal flow positions. By amplifying these translations using scale, the normal flow is rotated 180 degrees around the offset-position, and the boxes are exploded away from each other.

<style>
    #wrap {
        transform-style: preserve-3d;
        width: 500px;
        height: 250px;
        line-height: 0px;
    }
    span {
        position: static;
        display: inline-block;
        width: 100px;
        height: 50px;
        border-top-right-radius: 23px;
        scale: 2.5 2.5; /* applied before motion path transform */
        offset-position: center;
        transform: scale(0.4); /* applied after motion path transform */
    }
    #box1 {
        background-color: tomato;
    }
    #box2 {
        background-color: green;
    }
    #box3 {
        background-color: navy;
    }
    #box4 {
        background-color: gold;
    }
</style>
<body>
    <div id="wrap">
        <div>
            <span id="box1"></span><span id="box2"></span>
        </div>
        <div>
            <span id="box3"></span><span id="box4"></span>
        </div>
    </div>
</body>

An example when motion path and scale interact

In this example, each offset-position value is ignored as offset-path is a <geometry-box>, but the other offset properties combine to have an effect equivalent to that for offset-position 'right bottom'.

<style>
    #wrap {
        transform-style: preserve-3d;
        width: 540px;
        height: 420px;
    }
    .item {
        position: absolute;
        width: 90px;
        height: 70px;
        border-top-right-radius: 23px;
        scale: 0.8 0.8; /* applied before motion path transform */
        offset-path: padding-box;
        offset-distance: 50%;
        offset-rotate: 0deg;
        offset-anchor: right bottom;
        transform: scale(1.25); /* applied after motion path transform */
    }
    #box1 {
        background-color: tomato;
        position: static;
        offset-position: auto; /* ignored */
    }
    #box2 {
        background-color: green;
        right: 0px;
        top: 0px;
        offset-position: 23% 45%; /* ignored */
    }
    #box3 {
        background-color: navy;
        left: 0px;
        bottom: 0px;
        offset-position: 34% 56px; /* ignored */
    }
    #box4 {
        background-color: gold;
        right: 0px;
        bottom: 0px;
        offset-position: 45px 67px; /* ignored */
    }
</style>
<body>
    <div id="wrap">
        <div class="item" id="box1"></div>
        <div class="item" id="box2"></div>
        <div class="item" id="box3"></div>
        <div class="item" id="box4"></div>
    </div>
</body>

An example when offset-position is ignored

2.4. Define an anchor point: The offset-anchor property

Name:	offset-anchor
Value:	auto \| <position>
Initial:	auto
Applies to:	transformable elements
Inherited:	no
Percentages:	relative to the width and the height of the element’s reference box
Computed value:	the auto keyword or a computed <position>
Canonical order:	per grammar
Animation type:	by computed value
Media:	visual

Defines the box’s anchor point—the point that is positioned along the offset path.

Values have the following meanings:

auto

The anchor point is the same as the point indicated by transform-origin.

Specifically, the computed value of transform-origin is resolved as a <position> against the element’s reference box.

<position>

The anchor point is the result of resolving the <position> against the element’s reference box.

Which box this is resolved against is being discussed in Issue 503.

The following explains how to set the anchor point of the box.

#plane {
    offset-anchor: center;
}

The red dot in the middle of the shape indicates the anchor point of the shape.

Shape with its anchor point — A red dot in the middle of a plane shape indicates the shape’s anchor point.

This example shows an alignment of four boxes with different anchor points.

<style>
    body {
        transform-style: preserve-3d;
        width: 300px;
        height: 300px;
        border: 2px solid gray;
        border-radius: 50%;
    }
    .box {
        width: 50px;
        height: 50px;
        background-color: orange;
        offset-position: 50% 50%;
        offset-distance: 100%;
        offset-rotate: 0deg;
    }
    #item1 {
        offset-path: ray(45deg closest-side);
        offset-anchor: right top;
    }
    #item2 {
        offset-path: ray(135deg closest-side);
        offset-anchor: right bottom;
    }
    #item3 {
        offset-path: ray(225deg closest-side);
        offset-anchor: left bottom;
    }
    #item4 {
        offset-path: ray(315deg closest-side);
        offset-anchor: left top;
    }
</style>
<body>
    <div class="box" id="item1"></div>
    <div class="box" id="item2"></div>
    <div class="box" id="item3"></div>
    <div class="box" id="item4"></div>
</body>

This example shows boxes centered at their offset-position.

<style>
    body {
        width: 500px;
        height: 500px;
    }
    .box {
        background-color: mediumpurple;
        offset-path: none;
        offset-anchor: center;
    }
    #item1 {
        offset-position: 90% 20%;
        width: 60%;
        height: 20%;
    }
    #item2 {
        offset-position: 100% 100%;
        width: 30%;
        height: 10%;
    }
    #item3 {
        offset-position: 50% 100%;
        width: 20%;
        height: 60%;
    }
    #item4 {
        offset-position: 0% 100%;
        width: 30%;
        height: 90%;
    }
</style>
<body>
    <div class="box" id="item1"></div>
    <div class="box" id="item2"></div>
    <div class="box" id="item3"></div>
    <div class="box" id="item4"></div>
</body>

An example of offset-anchor: center — An example of 'offset-anchor: center'

This example shows how offset-anchor computes to their offset-position.

<style>
    body {
        width: 500px;
        height: 500px;
    }
    .box {
        background-color: mediumpurple;
        offset-path: none;
        offset-anchor: auto;
    }
    #item1 {
        offset-position: 90% 20%;
        width: 60%;
        height: 20%;
    }
    #item2 {
        offset-position: 100% 100%;
        width: 30%;
        height: 10%;
    }
    #item3 {
        offset-position: 50% 100%;
        width: 20%;
        height: 60%;
    }
    #item4 {
        offset-position: 0% 100%;
        width: 30%;
        height: 90%;
    }
</style>
<body>
    <div class="box" id="item1"></div>
    <div class="box" id="item2"></div>
    <div class="box" id="item3"></div>
    <div class="box" id="item4"></div>
</body>

An example of offset-anchor: auto — An example of 'offset-anchor: auto'

2.5. Rotation at point: The offset-rotate property

Name:	offset-rotate
Value:	[ auto \| reverse ] \|\| <angle>
Initial:	auto
Applies to:	transformable elements
Inherited:	no
Percentages:	n/a
Computed value:	computed <angle> value, optionally preceded by auto
Canonical order:	per grammar
Animation type:	by computed value
Media:	visual

Defines the orientation of the box while positioning along the offset path.

Values have the following meanings:

auto

Indicates that the object is rotated (over time if offset-distance is animated) by the angle of the direction (i.e., directional tangent vector) of the offset path, relative to the positive x-axis. If specified in combination with <angle>, the computed value of <angle> is added to the computed value of auto.

Note: For ray paths, the rotation implied by auto is 90 degrees less than the ray’s bearing <angle>.

reverse

Indicates that the object is rotated (over time if offset-distance is animated) by the angle of the direction (i.e., directional tangent vector) of the offset path, relative to the positive x-axis, plus 180 degrees. If specified in combination with <angle>, the computed value of <angle> is added to the computed value of reverse.

Note: This is the same as specifying auto 180deg.

<angle>

Indicates that the box has a constant clockwise rotation transformation applied to it by the specified rotation angle. See definitions of auto or reverse if specified in combination with either one of the keywords.

When the offset path is a zero length path, the value of offset-rotate is 0 degree, the direction of the positive x-axis.

If the offset path is composed of multiple line segments, the orientation at the connection between the segments is the direction of the "later" segment. (This matches SVG’s notion of direction of a path.)

Note: The rotation described here does not override or replace any rotation defined by the transform property.

The following examples use the shape of a plane. The red dot in the middle of the shape indicates the anchor point of the shape. When no offset properties are set, the shape is not translated or rotated along the path.

Path without offset — A black plane at the beginning of the path, with no offset properties set.

When the shape’s anchor point is placed at different positions along the path and offset-rotate is 0deg, the shape is not rotated.

Path without rotation — A black plane at different positions on a blue dotted path without rotation transforms.

If the offset-rotate property is set to auto, and the shape’s anchor point is placed at different positions along the path, the shape is rotated based on the gradient at the current position and faces the direction of the path at this position.

Path with auto rotation — A black plane at different positions on a blue dotted path, rotated in the direction of the path.

In this example, the offset-rotate property is set to reverse. The plane faces the opposite direction of the path at each position on the path.

Path with reverse auto rotation — A black plane at different positions on a blue dotted path, rotated in the opposite direction of the path.

The last example sets the offset-rotate property to -45deg. The shape is rotated anticlockwise by 45 degree once and keeps the rotation at each position on the path.

Path with fixed rotation — A black plane at different positions on a blue dotted path, rotated by a fixed amount of degree.

This example shows how auto or reverse work when specified in combination with <angle>. The computed value of <angle> is added to the computed value of auto or reverse.

<style>
    body {
        width: 300px;
        height: 300px;
        margin: 0px;
        border: solid gray;
        border-radius: 50%;
    }
    .circle {
        offset-position: 150px 150px;
        offset-distance: 86%;
        width: 42px;
        height: 42px;
        background-color: mediumpurple;
        border-radius: 50%;
        display: flex;
        align-items: center;
        justify-content: center;
    }
    #item1 {
        offset-path: ray(0deg closest-side);
        offset-rotate: auto 90deg;
    }
    #item2 {
        offset-path: ray(45deg closest-side);
        offset-rotate: auto 90deg;
    }
    #item3 {
        offset-path: ray(135deg closest-side);
        offset-rotate: auto -90deg;
    }
    #item4 {
        offset-path: ray(180deg closest-side);
        offset-rotate: auto -90deg;
    }
    #item5 {
        offset-path: ray(225deg closest-side);
        offset-rotate: reverse 90deg;
    }
    #item6 {
        offset-path: ray(-45deg closest-side);
        offset-rotate: reverse -90deg;
    }
</style>
<body>
    <div class="circle" id="item1">1</div>
    <div class="circle" id="item2">2</div>
    <div class="circle" id="item3">3</div>
    <div class="circle" id="item4">4</div>
    <div class="circle" id="item5">5</div>
    <div class="circle" id="item6">6</div>
</body>

An image of example for offset-rotate — The boxes are rotated by the value of auto with a fixed amount of degree.

2.5.1. Calculating the path transform

Create a supplemental transformation matrix T1 for the local coordinate system of the box.
Find the initial position of the offset path specified by offset-position as T2.
Let P be the point at the used offset distance along the offset path.
Find the translation of the box such that its anchor point is placed at P, and apply that to T2.
Post-multiply T2 by the rotation specified by offset-rotate.
Post-multiply T2 to T1.
Post-multiply T1 to the local coordinate system of the box.

Do we need to say how to get the position in more detail?

There needs to be a process for converting rotate() to an angle.

2.6. Offset shorthand: The offset property

Name:	offset
Value:	[ <'offset-position'>? [ <'offset-path'> [ <'offset-distance'> \|\| <'offset-rotate'> ]? ]? ]! [ / <'offset-anchor'> ]?
Initial:	see individual properties
Applies to:	transformable elements
Inherited:	see individual properties
Percentages:	see individual properties
Computed value:	see individual properties
Animation type:	see individual properties
Canonical order:	per grammar

This is a shorthand property for setting offset-position, offset-path, offset-distance, offset-rotate and offset-anchor. Omitted values are set to their initial values.

3. Equivalent Paths For <basic-shape>

The <basic-shape> definition given by [css-shapes] defines each function as producing a shape—a 2-dimensional figure with an outline, an inside, and an outside.

This specification instead uses <basic-shape> as producing a path—a line with a starting point, ending point, and direction, that happens to trace out a particular shape’s outline. The details of what makes up a path are defined by SVG. [SVG2]

The equivalent path for all the <basic-shape> values are:

<path()>

<shape()>

The path is the defined path. [SVG11]

<circle()>

<ellipse()>

The path is the outline of the circle/ellipse. It starts at the topmost point of the circle/ellipse, and then is composed of four circular arcs, each comprising a quarter of the circle/ellipse, proceeding clockwise, ending with a segment-completing close path operation.

SVG defines the equivalent path of circle/ellipse differently, starting from the rightmost point. Should we align them? [Issue #506]

rect()

inset()

xywh()

The path is the outline of the (possibly-rounded) rectangle, composed of four or eight segments (depending on whether rounded corners are specified or not), and ending with a segment-completing close path operation. It starts at the left end of the top straight edge, immediately to the right of any rounded corners, and continues to the right (clockwise).

<polygon()>

The path is the outline of the polygon, composed of straight line segments connecting each coordinate pair to the following coordinate pair, and finally connecting the last back to the first, with a segment-completing close path operation.

For all of these, the direction at any point along the path is defined by SVG; see SVG 2 § 9.4 Path directionality.

Note: This list should be in sync with the full set of <basic-shape> functions defined in [css-shapes]. If anything is missing, this should be considered a specification bug. This list might move to Shapes in the future, but for now is kept here as this spec is the only consumer of this information.

4. Privacy Considerations

This specification introduces no new privacy considerations.

5. Security Considerations

This specification introduces no new security considerations.

Changes

This section is non-normative.

The following changes were made since the 9 April 2015 First Public Working Draft.

Renamed motion-path to offset-path for integrating with polar-angle.
- Added the ray() to define an offset path as a line segment which direction is specified by <angle>.
- Added <size> and contain value for the ray().
Renamed motion-offset to offset-distance for integrating with polar-distance.
Renamed motion-rotation to offset-rotate.
Added offset-position to specify the initial position of the path by merging polar-origin from [CSS-ROUND-DISPLAY-1].
Added offset-anchor to specify the origin point of the element by merging polar-anchor from [CSS-ROUND-DISPLAY-1].
Renamed the shorthand property motion to offset.
Made offset-rotate specify the rotation transformation by auto or reverse in combination with <angle>.

Acknowledgments

Thanks to fantasai, Hyojin Song, and all the rest of the CSS WG members for their reviews, comments, and corrections.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the [email protected] mailing list.

Motion Path Module Level 1

Abstract

Status of this document

1. Introduction

1.1. Module interactions

1.2. Values

2. Motion Paths

2.1. Defining A Path: The offset-path property

2.1.1. The ray() Function

2.1.2. Examples Of <basic-shape> Positioning

2.1.3. Examples of <coord-box> Positioning

2.2. Position on the path: The offset-distance property

2.2.1. Calculating the computed distance along a path

2.3. Define the starting point of the path: The offset-position property

2.4. Define an anchor point: The offset-anchor property

2.5. Rotation at point: The offset-rotate property

2.5.1. Calculating the path transform

2.6. Offset shorthand: The offset property

3. Equivalent Paths For <basic-shape>

4. Privacy Considerations

5. Security Considerations

Changes

Acknowledgments

Conformance

Document conventions

Conformance classes

Partial implementations

Implementations of Unstable and Proprietary Features

Non-experimental implementations

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

Informative References

Property Index

Issues Index