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<?php
/**
* Blurhash encoder.
*
* @package Activitypub
*/
namespace Activitypub;
/**
* Encodes pixel data into a Blurhash placeholder string.
*
* A first-party port of the public Blurhash encode algorithm
* (https://github.com/woltapp/blurhash, MIT). Adapted from
* Automattic/FOSSE (https://github.com/Automattic/fosse), which used the
* kornrunner/blurhash library; this replaces that runtime dependency so
* the plugin ships Blurhash support out of the box.
*
* @since 9.0.0
*/
class Blurhash_Encoder {
/**
* Base83 alphabet defined by the Blurhash spec.
*
* @var string
*/
const ALPHABET = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz#$%*+,-.:;=?@[]^_{|}~';
/**
* Encode a pixel array into a Blurhash string.
*
* @param array $pixels Row-major `[r,g,b][][]` array (0-255 per channel), indexed `[$y][$x]`.
* @param int $components_x Horizontal component count (1-9).
* @param int $components_y Vertical component count (1-9).
* @return string Blurhash string, or '' on invalid input.
*/
public static function encode( $pixels, $components_x, $components_y ) {
$components_x = (int) $components_x;
$components_y = (int) $components_y;
if ( $components_x < 1 || $components_x > 9 || $components_y < 1 || $components_y > 9 ) {
return '';
}
$height = \count( $pixels );
if ( $height < 1 || ! isset( $pixels[0] ) || ! \is_array( $pixels[0] ) ) {
return '';
}
$width = \count( $pixels[0] );
if ( $width < 1 ) {
return '';
}
$factors = array();
for ( $y = 0; $y < $components_y; $y++ ) {
for ( $x = 0; $x < $components_x; $x++ ) {
$normalisation = ( 0 === $x && 0 === $y ) ? 1.0 : 2.0;
$r = 0.0;
$g = 0.0;
$b = 0.0;
for ( $i = 0; $i < $width; $i++ ) {
for ( $j = 0; $j < $height; $j++ ) {
$basis = $normalisation
* \cos( \pi() * $x * $i / $width )
* \cos( \pi() * $y * $j / $height );
$pixel = $pixels[ $j ][ $i ];
$r += $basis * self::srgb_to_linear( (int) $pixel[0] );
$g += $basis * self::srgb_to_linear( (int) $pixel[1] );
$b += $basis * self::srgb_to_linear( (int) $pixel[2] );
}
}
$scale = 1.0 / ( $width * $height );
$factors[] = array( $r * $scale, $g * $scale, $b * $scale );
}
}
$dc = $factors[0];
$ac = \array_slice( $factors, 1 );
$hash = self::encode83( ( $components_x - 1 ) + ( $components_y - 1 ) * 9, 1 );
$max_value = 1.0;
if ( \count( $ac ) > 0 ) {
$actual_max = 0.0;
foreach ( $ac as $factor ) {
$actual_max = \max( $actual_max, \abs( $factor[0] ), \abs( $factor[1] ), \abs( $factor[2] ) );
}
$quantised_max = (int) \max( 0, \min( 82, \floor( $actual_max * 166 - 0.5 ) ) );
$max_value = ( $quantised_max + 1 ) / 166;
$hash .= self::encode83( $quantised_max, 1 );
} else {
$hash .= self::encode83( 0, 1 );
}
$hash .= self::encode83( self::encode_dc( $dc ), 4 );
foreach ( $ac as $factor ) {
$hash .= self::encode83( self::encode_ac( $factor, $max_value ), 2 );
}
return $hash;
}
/**
* Encode an integer to a fixed-length base83 string.
*
* @param int $value Value.
* @param int $length Output length.
* @return string
*/
private static function encode83( $value, $length ) {
$value = (int) $value;
$result = '';
for ( $i = 1; $i <= $length; $i++ ) {
$digit = (int) ( $value / ( 83 ** ( $length - $i ) ) ) % 83;
$result .= self::ALPHABET[ $digit ];
}
return $result;
}
/**
* Convert an sRGB 0-255 channel to linear 0-1.
*
* @param int $value Channel value.
* @return float
*/
private static function srgb_to_linear( $value ) {
$v = $value / 255.0;
if ( $v <= 0.04045 ) {
return $v / 12.92;
}
return \pow( ( $v + 0.055 ) / 1.055, 2.4 );
}
/**
* Convert a linear 0-1 channel to sRGB 0-255.
*
* @param float $value Linear value.
* @return int
*/
private static function linear_to_srgb( $value ) {
$v = \max( 0.0, \min( 1.0, $value ) );
if ( $v <= 0.0031308 ) {
return (int) ( $v * 12.92 * 255 + 0.5 );
}
return (int) ( ( 1.055 * \pow( $v, 1 / 2.4 ) - 0.055 ) * 255 + 0.5 );
}
/**
* Encode the DC (average color) factor.
*
* @param array $factor `[r,g,b]` linear floats.
* @return int
*/
private static function encode_dc( $factor ) {
$r = self::linear_to_srgb( $factor[0] );
$g = self::linear_to_srgb( $factor[1] );
$b = self::linear_to_srgb( $factor[2] );
return ( $r << 16 ) + ( $g << 8 ) + $b;
}
/**
* Encode an AC factor against the maximum value.
*
* @param array $factor `[r,g,b]` linear floats.
* @param float $max_value Quantisation maximum.
* @return int
*/
private static function encode_ac( $factor, $max_value ) {
$quant_r = (int) \max( 0, \min( 18, \floor( self::sign_pow( $factor[0] / $max_value, 0.5 ) * 9 + 9.5 ) ) );
$quant_g = (int) \max( 0, \min( 18, \floor( self::sign_pow( $factor[1] / $max_value, 0.5 ) * 9 + 9.5 ) ) );
$quant_b = (int) \max( 0, \min( 18, \floor( self::sign_pow( $factor[2] / $max_value, 0.5 ) * 9 + 9.5 ) ) );
return $quant_r * 19 * 19 + $quant_g * 19 + $quant_b;
}
/**
* Sign-preserving power.
*
* @param float $value Base.
* @param float $exp Exponent.
* @return float
*/
private static function sign_pow( $value, $exp ) {
$result = \pow( \abs( $value ), $exp );
return $value < 0 ? -$result : $result;
}
}