Conventionally, web pages have been shown through a rectangular screen such as PC, tablet, and smart phone. The window content area in a web browser is a rectangle. Each HTML element follows the W3C box model and thus is also a rectangle.
Nowadays, devices come in varied shapes of the displays.
It needs to consider the shape of the display when implementing web pages on devices.
However, current web standards lack some features to support the devices as follows:
The 'device-radius' media feature is added to Media Queries.
Current user agents are not capable of detecting the shape of a display so that authors cannot apply various layouts for a round display. To resolve the issue, 'device-radius' informs the web page of the property regarding the shape of the display.
To apply the shape of a display to content area, we extend the 'shape-inside' property of CSS Shapes. The position of the element which is overflowed from the display is adjusted inside the display when using this property even if the authors don’t know the exact shape of the display.
We also add the 'border-boundary' property to CSS Borders. The borders of the element can be drawn along the edge of the display even if the element is overflowed.
For the better web design suitable for a round display, we introduce polar positioning.
In conventional positioning method, the Cartesian coordinates, elements are positioned by offsets in x-axis and y-axis. But, we suggest a method to position elements with specifying the distance and angle from the origin point of coordinates.
This module provides features such as:
* Detecting the rounded display
* Aligning contents in the display’s shape
* Drawing borders along the display’s edge
* Positioning elements in polar coordinates
Note: To define a 'discrete' type media feature, the feature is evaluated in a boolean context.
The 'shape' media feature describes the general shape of the targeted display area of the output device. It accepts the following values:
Note: There are cases when the UA may know the shape even in the absence of OS APIs. For example, when rendering to PDF, the shape is known to be a rectangle, so 'shape: rect' evaluates to ''true'' while ''shape: round'' to ''false''
Note: For other shapes, such as polygons, we need to extend the media features with additional parameters. The current features have limitations to support the diversity beyond round shapes. How can we express star-shaped polygons? (e.g. SVG syntax, etc.) Of course, there is a trade-off between simplicity and expressiveness.
> (i.e. circle() or ellipse()), allowing web authors to conveniently align contents with the display edge.
When a containing block is placed on one end of the display and the containing block has 'shape-inside: display', the descendant blocks of the containing block are basically put on the overlapping region between the containing block and the display area. The overlapping region's shape is mostly complicated shape, so it's difficult to define the shape using previous method like basic-shape. The figure 4 describes these circumstances as follows.
What if content overflows? Clipping or scrolling?
Drawing borders around the display border
The 'border-boundary' property
We add the 'border-boundary' property to set a boundary constraint that affects the borders of an element.
Name: border-boundary
Applies to: all elements
Value: none | parent | display
Initial: none
Inherited: yes
When the 'border-boundary' property on an element is set to 'parent', additional borders of the element could be drawn where the element's area and the borders of its parent are met. When it is set to 'display', additional borders could be drawn where the element's area and the borders of screen are met. The default value is 'none', imposing no boundary constraint on the borders.
The example below shows how the 'border-boundary' property works on drawing borders. The result is shown in Figure 5B.
<style>
#container {
border-boundary: display;
}
#redBox {
border: 5px red solid;
}
#greenBox {
border: 5px green solid;
}
#blueBox {
border: 5px blue solid;
}
</style>
<div id="container">
<div id="redBox"></div>
<div id="greenBox"></div>
<div id="blueBox"></div>
</div>
Note: If the value of 'border-boundary' is parent or display, border lines of the element are actually just a visual effect. It triggers a layout for rendering in a general way, but in the above cases (border-boundary: parent|display), the layout doesn't occur and it only draws the border lines inward from the containing block's borders. With this situation, the borders might hide contents around the display edge.
Content positioning using polar coordinate system
Polar coordinate system is a two-dimensional coordinate system that describes the position of a point in a plane with a distance from a reference point and an angle from a reference direction. Elements could be placed along a circle or concentric circles, and the polar coordinate system is useful to handle such cases.
This section introduces polar positioning to support layout of elements in the polar coordinate system where the position of an element is determined by a distance from the center point within the containing element and an angle from the Y-axis.
The conventional coordinates used in web is the Cartesian coordinate system, but using 'polar-distance' or 'polar-angle' enables positioning an element in the polar coordinate system. The 'polar-angle' and 'polar-distance' properties specify the angular value and distance of an element from the origin in polar coordinates.
This example shows a way to align elements within the polar coordinate system.
<body>
<div id="circle1" style="position: absolute; polar-angle: 0deg; polar-distance: 50%"></div>
<div id="circle2" style="position: absolute; polar-angle: 90deg; polar-distance: 20%"></div>
<div id="circle3" style="position: absolute; polar-angle: 225deg; polar-distance: 100%"></div>
</body>
An example of polar positioning
In conventional coordinate system used for positioning an element in a containing block, the offset of the element is applied depending on the edges of the element’s containing block.
In comparison, the default origin of polar coordinates is positioned at the center point of a containing block. The position for the origin point can be set by 'polar-origin'.
The 'polar-angle' property
The 'polar-angle' property specifies the angle from the Y-axis.
Name: polar-angle
Applies to: all elements
Value: <>
Initial: 0
Inherited: no
Percentages: N/A
Animatable: as angle
Note: In polar coordinate system, a pole is the reference point and points are described as been a certain distance from it, as also a certain angle from the polar axis. In mathematical theory, the polar axis is commonly defined as the positive direction of the x-axis, but we consider the polar axis as the positive direction of the y-axis position as other CSS specifications usually do. Therefore, when the 'polar-angle' value of an element is 0, the element is positioned on the y-axis. If the angle value of an element increases in the positive direction from 0, the element moves clockwise. The method to determine a direction using 'polar-angle' works the same way in [[css-values-3]], <> value.
The 'polar-distance' property
The 'polar-distance' property determines how far elements are positioned from the origin of polar coordinates.
Name: polar-distance
Applies to: all elements
Value: [ <> | <> <>? ] && contain?
Initial: 0
Inherited: no
Percentages: relative to distance from the origin of polar coordinates to edge of containing block
Animatable: as length, percentage, or calc
The 'polar-distance' specifies the distance between the origin of polar coordinates and the anchor point of the element.
Values have the following meanings:
- <length>
-
Gives a fixed length between the origin of polar coordinates and the anchor point of element.
- <percentage>
-
Is relative to the distance between the origin of polar coordinates and the point of contact which is made by the edge of containing block and the gradient-line from the origin of polar coordinates. The value of the gradient of the line is polar angle value.
- <size>
-
Decides a point used for measuring the distance from the origin point.
It is defined as
<size> = [ closest-side | closest-corner | farthest-side | farthest-corner ]
If omitted it defaults to closest-side.
When the <size> is used with <percentage>, the calculated value of 'polar-distance' is constant regardless of the value of 'polar-angle'.
- closest-side
-
The distance is measured between the origin and the closest side of the box from it.
- closest-corner
-
The distance is measured between the origin and the closest corner of the box from it.
- farthest-side
-
The distance is measured between the origin and the farthest side of the box from it.
- farthest-corner
-
The distance is measured between the origin and the farthest corner of the box from it.
Note: When the origin of polar coordinates is at one of the corners of the containing block, the closest side takes the edge which meets with the origin. Even if the offset given by <> or <> changes, the position of the element specified with 'closest-side' is the same.
- contain
-
Adjust 'polar-distance' value of the positioned element which overflows shape of the containing block.
The main purpose of this value is avoiding overflow when positioning elements.
When overflowing occurs, the distance between the point of origin of polar coordinates and the anchor point of the element is reduced,
until there are 2 contact points or less between edge of shape of containing block and content block of the element.
When elements are defined like below,
<div style="position: absolute; width: 40px; height: 30px;">
<div id = “circle1” style="position: absolute; width: 4px; height: 4px;"></div>
<div id = “circle2” style="position: absolute; width: 4px; height: 4px;"></div>
</div>
Specifying different <size> value for the same <percentage> value makes a difference in the positions of the elements.
The style below is for the (A),
<style>
#circle1 {
polar-origin: 10px 15px;
polar-angle: 0deg;
polar-distance: 100% closest-side;
}
#circle2 {
polar-origin: 10px 15px;
polar-angle: 90deg;
polar-distance: 100% closest-side;
}
</style>
And the next definition is for the (B)
<style>
#circle1 {
polar-origin: 10px 15px;
polar-angle: 0deg;
polar-distance: 100% closest-corner;
}
#circle2 {
polar-origin: 10px 15px;
polar-angle: 90deg;
polar-distance: 100% closest-corner;
}
</style>
(A) With closest-side
(B) With closest-corner
Using <size> with <percentage> for polar-distance
Here are some examples.
The first example shows positioning elements with polar-distance not using extent keyword value. Some parts of elements are outside the boundary of the containing block's shape:
<body>
<div style="position: absolute; polar-angle: 45deg; polar-distance: 100%"></div>
<div style="position: absolute; polar-angle: 180deg; polar-distance: 100%"></div>
</body>
In the second example, 'contain', the extent keyword value is added to the polar-distance value of each element to avoid overflowing.
<body>
<div style="position: absolute; polar-angle: 45deg; polar-distance: 100% contain"></div>
<div style="position: absolute; polar-angle: 180deg; polar-distance: 100% contain"></div>
</body>
The 'polar-origin' property
The 'polar-origin' property establishes the point of origin for coordinate system. It sets the horizontal and vertical representative point at which the offset values of the element is applied.
Name: polar-origin
Applies to: all elements
Value: auto | <>
Initial: auto
Inherited: no
Percentages: Refer to the size of containing block
Animatable: as length, percentage, or calc
Values are defined as follows:
- auto
-
Computes to center if 'polar-origin' is used in polar coordinates. But it computes as the box model layout in Cartesian coordinates.
In polar coordinates, there are many use cases specifying the position of the origin at the center point of the containing block. Therefore an element is positioned to the horizontally and vertically center in the area of the containing block when auto value is given to the 'polar-origin'.
But if it is used in conventional coordinate system, the origin of coordinate system is at the upper left corner of the containing block by default. For example, 'top' and 'left' are used for positioning the element, auto value makes the same result of giving 'top left' as the value of 'polar-origin'.
- <>
-
Values of <> are defined like below:
- <percentage>
-
A percentage for the horizontal offset is relative to the width of the containing block. A percentage for the vertical offset is relative to height of the containing block.
- <length>
-
A length value gives a fixed length as the offset. The value for the horizontal and vertical offset represent an offset from the top left corner of the containing block.
- top
-
Computes to 0% for the vertical position.
- right
-
Computes to 100% for the horizontal position.
- bottom
-
Computes to 100% for the vertical position.
- left
-
Computes to 0% for the horizontal position.
- center
-
Computes to 50% (left 50%) for the horizontal position if the horizontal position is not otherwise specified, or 50% (top 50%) for the vertical position if it is.
This example shows how auto value of polar-origin is calculated differently depending on which coordinate system elements are positioned in.
#item1 {
polar-origin: auto;
polar-distance: 10px;
polar-angle: 90deg;
}
"polar-origin: auto" is the same result with "polar-origin: center"
#item2 {
polar-origin: auto;
left: 10px;
}
"polar-origin: auto" is the same result with "polar-origin: top left"
As it’s available to specify the point of origin for polar coordinates, properties such as 'margin', 'padding' would work different from the conventional coordinate system. We need different approaches to define those properties which are related to layout.
When 'polar-origin' is not auto, it needs to be decided whether 'top', 'right', 'bottom', and 'left' properties are ignored or interpreted somehow.
The 'polar-anchor' property
The 'polar-anchor' property sets an anchor point of the element. The anchor point specifies a position which is a representative point of the element. The anchor point could be set as any point within a content area of the element rather than being positioned to the upper left corner of the element by CSS box model.
The value of 'polar-distance' is the distance between an anchor point and the origin point of coordinates.
Name: polar-anchor
Applies to: all elements
Value: <>
Initial: 50% 50%
Inherited: no
Percentages: relative to width and height of an element
Animatable: as <>
Meanings of some keywords in <> are different from those used in 'polar-origin' property.
- <percentage>
-
A percentage for the horizontal offset is relative to width of content box area of the element. A percentage for the vertical offset is relative to height of content box area of the element. For example, with a value pair of '100%, 0%', an anchor point is on the upper right corner of the element.
- <length>
-
A length value gives a length offset from the upper left corner of the element's content area.
Only with 'polar-angle' and 'polar-distance' to position elements, adjusting values of those properties for avoiding elements sticking out of the containing block is required. When the appropriate anchor point is given, there is no need to adjust 'polar-distance' value to avoid overflowing when positioning elements in the containing block.
This example shows an alignment of four elements with different anchor points positioned in a containing block.
<style>
#item1 {
position: absolute;
polar-angle: 45deg;
polar-distance: 100%;
polar-anchor: right top;
}
#item2 {
position: absolute;
polar-angle: 135deg;
polar-distance: 100%;
polar-anchor: right bottom;
}
#item3 {
position: absolute;
polar-angle: 225deg;
polar-distance: 100%;
polar-anchor: left bottom;
}
#item4 {
position: absolute;
polar-angle: 315deg;
polar-distance: 100%;
polar-anchor: left top;
}
</style>
<body>
<div id="item1"></div>
<div id="item2"></div>
<div id="item3"></div>
<div id="item4"></div>
</body>
An example of 'polar-anchor'
Is 'auto' needed for value of 'polar-anchor'?
The alternative naming without the polar- prefix for 'polar-origin' and 'polar-anchor' is needed. When it becomes to use polar positioning regardless of the value of the position property, 'polar-origin' and 'polar-anchor' would be used independently from polar positioning.
How can the margin be applied to an element when polar-origin: 50% 50% and polar-anchor: 50% 50%?
Which would be a base point for applying it between the upper left corner or center of a containing block?
With the former, the position of the element changes when the value of the margin changes, while with the latter, the margin value doesn't affect the position of the element.
Can 'polar-anchor' be replaced by 'margin-top' and 'margin-left'?
There are several differences between 'polar-anchor' and 'margin-left' & 'margin-top'.
In case of 'margin-left' & 'margin-top', percentages resolve to the width of the element itself.
but in case of 'polar-anchor', percentage resolves to the width and height of a containing block of the element.
When elements are positioned in polar coordinates, there are many usecases which show the elements rotated toward the origin of polar coordinates. In such cases, degree of rotation has to be specified in the 2d rotation function for each element. But when using the keyword value instead of the <> value, the calculation of accurate rotation degree may be omitted.
The two-dimensional rotation function 'rotate(<angle>)' used in polar coordinates is extended as follows:
rotate()
= rotate( <> | polar-angle | polar-angle-reverse)
Values have the following meanings:
- polar-angle
-
Computes to the value of 'polar-angle' property.
- polar-angle-reverse
-
Computes to the value of the 'polar-angle' property plus 180 degrees.
Note: polar-angle and polar-angle-reverse keywords resolve to angle values when determining the computed value of transform property.
It makes possible to define an animation such as transforming between rotate(0deg) and rotate(polar-angle).
If there is an animation using 'polar-angle' property, polar-angle value and polar-angle-reverse value track the changes in the value of the 'polar-angle' property.
This example shows how the keyword value works in the 2d rotation function.
<style>
#item1 {
position: absolute;
polar-angle: 0deg;
polar-distance: 90%;
transform: rotate(polar-angle);
}
#item2 {
position: absolute;
polar-angle: 45deg;
polar-distance: 90%;
transform: rotate(polar-angle);
}
#item3 {
position: absolute;
polar-angle: 135deg;
polar-distance: 90%;
transform: rotate(polar-angle-reverse);
}
#item4 {
position: absolute;
polar-angle: 180deg;
polar-distance: 90%;
transform: rotate(polar-angle-reverse);
}
#item5 {
position: absolute;
polar-angle: 225deg;
polar-distance: 90%;
transform: rotate(polar-angle-reverse);
}
#item6 {
position: absolute;
polar-angle: -45deg;
polar-distance: 90%;
transform: rotate(polar-angle);
}
</style>
<body>
<div id="item1">1</div>
<div id="item2">2</div>
<div id="item3">3</div>
<div id="item4">4</div>
<div id="item5">5</div>
<div id="item6">6</div>
</body>
An example of the extension for 2d rotate function
Use Cases
Use cases are described on
these.
Changes
- Added 'polar-anchor'
- Added 'contain' for 'polar-distance' for avoiding the overflow issue.
- Added 'polar-origin'
- Added extension for 2d rotation function for polar coordinates
- Changed naming of keyword values for 2d rotation function
- Removed
polar value of 'position'. Polar positioning applies to absolute/fixed/sticky/relative positioned elements
- Added auto value to 'polar-origin' and make it as initial value
- Added <size> keywords to 'polar-distance'
- Added the Acknowledgements, Use cases, and Changes
Acknowledgements
This specification is made possible by input from Dong-Young Lee, Soonbo Han,
Florian Rivoal, Joone Hur,
Kang-Soo Seo, Sangjo Park, Woojun Jung, Chisoon Jeong, Yunbum Sung,
Alan Stearns, Brad Kemper, and the CSS Working Group members.
Thanks also to Adenilson Cavalcanti for editorial input.
