core/stylesheets/compass/typography/_units.scss
// @private Default font-size for all browsers
$browser-default-font-size: 16px;
// Base font size in pixels, if not already defined.
// Should be the same as the font-size of the html element.
$base-font-size: 16px !default;
// Whether to output fallback values in px when outputting rems.
$rem-with-px-fallback: true !default;
// Convert any CSS <length> or <percentage> value to any another.
//
// @param $length
// A css <length> or <percentage> value
//
// @param $to-unit
// String matching a css unit keyword, e.g. 'em', '%', etc.
//
// @param $from-context
// When converting from relative units, the absolute length (in px) to
// which $length refers (e.g. for $lengths in em units, would normally be the
// font-size of the current element).
//
// @param $to-context
// For converting to relative units, the absolute length in px to which the
// output value will refer. Defaults to the same as $from-context, since it is
// rarely needed.
@function convert-length(
$length,
$to-unit,
$from-context: $base-font-size,
$to-context: $from-context
) {
$from-unit: unit($length);
// Optimize for cases where `from` and `to` units are accidentally the same.
@if $from-unit == $to-unit { @return $length; }
// Context values must be in px so we can determine a conversion ratio for
// relative units.
@if unit($from-context) != 'px' { @warn "Paremeter $from-context must resolve to a value in pixel units."; }
@if unit($to-context) != 'px' { @warn "Parameter $to-context must resolve to a value in pixel units."; }
// Convert input length to pixels
$px-length: $length;
@if $from-unit != 'px' {
// Convert relative units using the from-context parameter.
@if $from-unit == 'em' { $px-length: $length * $from-context / 1em }
@else if $from-unit == 'rem' { $px-length: $length * $base-font-size / 1rem }
@else if $from-unit == '%' { $px-length: $length * $from-context / 100% }
@else if $from-unit == 'ex' { $px-length: $length * $from-context / 2ex }
// Convert absolute units using Sass' conversion table.
@else if $from-unit == 'in' or
$from-unit == 'mm' or
$from-unit == 'cm' or
$from-unit == 'pt' or
$from-unit == 'pc' { $px-length: 0px + $length }
// Certain units can't be converted.
@else if $from-unit == 'ch' or
$from-unit == 'vw' or
$from-unit == 'vh' or
$from-unit == 'vmin' {
@warn "#{$from-unit} units can't be reliably converted; Returning original value.";
@return $length;
}
@else {
@warn "#{$from-unit} is an unknown length unit. Returning original value.";
@return $length;
}
}
// Convert length in pixels to the output unit
$output-length: $px-length;
@if $to-unit != 'px' {
// Relative units
@if $to-unit == 'em' { $output-length: $px-length * 1em / $to-context }
@else if $to-unit == 'rem' { $output-length: $px-length * 1rem / $base-font-size }
@else if $to-unit == '%' { $output-length: $px-length * 100% / $to-context }
@else if $to-unit == 'ex' { $output-length: $px-length * 2ex / $to-context }
// Absolute units
@else if $to-unit == 'in' { $output-length: 0in + $px-length }
@else if $to-unit == 'mm' { $output-length: 0mm + $px-length }
@else if $to-unit == 'cm' { $output-length: 0cm + $px-length }
@else if $to-unit == 'pt' { $output-length: 0pt + $px-length }
@else if $to-unit == 'pc' { $output-length: 0pc + $px-length }
// Non-convertible units
@else if $to-unit == 'ch' or
$to-unit == 'vw' or
$to-unit == 'vh' or
$to-unit == 'vmin' {
@warn "#{$to-unit} units can't be reliably converted; Returning original value.";
@return $length;
}
@else {
@warn "#{$to-unit} is an unknown length unit. Returning original value.";
@return $length;
}
}
@return $output-length;
}
// @private Get the px/rem versions of a value.
@function rem-fallback-values($value) {
$_return: (
px: $value,
rem: $value,
);
@if type-of($value) == number and not unitless($value) {
@if unit($value) == rem {
$_return: map-merge($_return, (
px: round(convert-length($value, px)),
));
} @else if unit($value) == px {
$_return: map-merge($_return, (
px: round($value),
rem: convert-length($value, rem),
));
}
}
@return $_return;
}
// @private Get the px/rem versions of a list (or nested lists).
@function list-convert-rems($value) {
$_empty: (
px: (),
rem: (),
);
$_return: $_empty;
$_sep: null;
@if type-of($value) == list {
$_sep: list-separator($value);
@each $sub in $value {
$_this: list-convert-rems($sub);
$_return: (
px: append(map-get($_return, px), map-get($_this, px), $_sep),
rem: append(map-get($_return, rem), map-get($_this, rem), $_sep),
);
}
} @else {
$_this: rem-fallback-values($value);
$_return: map-merge($_return, $_this);
}
@return $_return;
}
// Output a given style rule containing rem values along with an (optional)
// fallback rule for older browsers (with rem values converted to px).
//
// @param $property
// The css property name.
//
// @param $values
// The value or list of values for the property.
//
// @param $use-px-fallback
// [ true | false ]
//
@mixin rem($property, $values, $use-px-fallback: $rem-with-px-fallback) {
// get converted values.
$values: list-convert-rems($values);
$px-values: map-get($values, px);
$values: map-get($values, rem);
// Use pixel fallback for browsers that don't understand rem units.
@if $use-px-fallback and $px-values != $values {
#{$property}: $px-values;
}
// Use rem values for everyone else (overrides pixel values).
#{$property}: $values;
}