activitypub / includes / class-blurhash-encoder.php

class-blurhash-encoder.php in ActivityPub 9.3.0, at includes/class-blurhash-encoder.php

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1	<?php
2	/**
3	* Blurhash encoder.
4	*
5	* @package Activitypub
6	*/
7
8	namespace Activitypub;
9
10	/**
11	* Encodes pixel data into a Blurhash placeholder string.
12	*
13	* A first-party port of the public Blurhash encode algorithm
14	* (https://github.com/woltapp/blurhash, MIT). Adapted from
15	* Automattic/FOSSE (https://github.com/Automattic/fosse), which used the
16	* kornrunner/blurhash library; this replaces that runtime dependency so
17	* the plugin ships Blurhash support out of the box.
18	*
19	* @since 9.0.0
20	*/
21	class Blurhash_Encoder {
22
23	/**
24	* Base83 alphabet defined by the Blurhash spec.
25	*
26	* @var string
27	*/
28	const ALPHABET = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz#$%*+,-.:;=?@[]^_{\|}~';
29
30	/**
31	* Encode a pixel array into a Blurhash string.
32	*
33	* @param array $pixels Row-major `[r,g,b][][]` array (0-255 per channel), indexed `[$y][$x]`.
34	* @param int $components_x Horizontal component count (1-9).
35	* @param int $components_y Vertical component count (1-9).
36	* @return string Blurhash string, or '' on invalid input.
37	*/
38	public static function encode( $pixels, $components_x, $components_y ) {
39	$components_x = (int) $components_x;
40	$components_y = (int) $components_y;
41
42	if ( $components_x < 1 \|\| $components_x > 9 \|\| $components_y < 1 \|\| $components_y > 9 ) {
43	return '';
44	}
45
46	$height = \count( $pixels );
47	if ( $height < 1 \|\| ! isset( $pixels[0] ) \|\| ! \is_array( $pixels[0] ) ) {
48	return '';
49	}
50	$width = \count( $pixels[0] );
51	if ( $width < 1 ) {
52	return '';
53	}
54
55	$factors = array();
56	for ( $y = 0; $y < $components_y; $y++ ) {
57	for ( $x = 0; $x < $components_x; $x++ ) {
58	$normalisation = ( 0 === $x && 0 === $y ) ? 1.0 : 2.0;
59	$r = 0.0;
60	$g = 0.0;
61	$b = 0.0;
62	for ( $i = 0; $i < $width; $i++ ) {
63	for ( $j = 0; $j < $height; $j++ ) {
64	$basis = $normalisation
65	* \cos( \pi() * $x * $i / $width )
66	* \cos( \pi() * $y * $j / $height );
67	$pixel = $pixels[ $j ][ $i ];
68	$r += $basis * self::srgb_to_linear( (int) $pixel[0] );
69	$g += $basis * self::srgb_to_linear( (int) $pixel[1] );
70	$b += $basis * self::srgb_to_linear( (int) $pixel[2] );
71	}
72	}
73	$scale = 1.0 / ( $width * $height );
74	$factors[] = array( $r * $scale, $g * $scale, $b * $scale );
75	}
76	}
77
78	$dc = $factors[0];
79	$ac = \array_slice( $factors, 1 );
80
81	$hash = self::encode83( ( $components_x - 1 ) + ( $components_y - 1 ) * 9, 1 );
82	$max_value = 1.0;
83
84	if ( \count( $ac ) > 0 ) {
85	$actual_max = 0.0;
86	foreach ( $ac as $factor ) {
87	$actual_max = \max( $actual_max, \abs( $factor[0] ), \abs( $factor[1] ), \abs( $factor[2] ) );
88	}
89	$quantised_max = (int) \max( 0, \min( 82, \floor( $actual_max * 166 - 0.5 ) ) );
90	$max_value = ( $quantised_max + 1 ) / 166;
91	$hash .= self::encode83( $quantised_max, 1 );
92	} else {
93	$hash .= self::encode83( 0, 1 );
94	}
95
96	$hash .= self::encode83( self::encode_dc( $dc ), 4 );
97	foreach ( $ac as $factor ) {
98	$hash .= self::encode83( self::encode_ac( $factor, $max_value ), 2 );
99	}
100
101	return $hash;
102	}
103
104	/**
105	* Encode an integer to a fixed-length base83 string.
106	*
107	* @param int $value Value.
108	* @param int $length Output length.
109	* @return string
110	*/
111	private static function encode83( $value, $length ) {
112	$value = (int) $value;
113	$result = '';
114	for ( $i = 1; $i <= $length; $i++ ) {
115	$digit = (int) ( $value / ( 83 ** ( $length - $i ) ) ) % 83;
116	$result .= self::ALPHABET[ $digit ];
117	}
118	return $result;
119	}
120
121	/**
122	* Convert an sRGB 0-255 channel to linear 0-1.
123	*
124	* @param int $value Channel value.
125	* @return float
126	*/
127	private static function srgb_to_linear( $value ) {
128	$v = $value / 255.0;
129	if ( $v <= 0.04045 ) {
130	return $v / 12.92;
131	}
132	return \pow( ( $v + 0.055 ) / 1.055, 2.4 );
133	}
134
135	/**
136	* Convert a linear 0-1 channel to sRGB 0-255.
137	*
138	* @param float $value Linear value.
139	* @return int
140	*/
141	private static function linear_to_srgb( $value ) {
142	$v = \max( 0.0, \min( 1.0, $value ) );
143	if ( $v <= 0.0031308 ) {
144	return (int) ( $v * 12.92 * 255 + 0.5 );
145	}
146	return (int) ( ( 1.055 * \pow( $v, 1 / 2.4 ) - 0.055 ) * 255 + 0.5 );
147	}
148
149	/**
150	* Encode the DC (average color) factor.
151	*
152	* @param array $factor `[r,g,b]` linear floats.
153	* @return int
154	*/
155	private static function encode_dc( $factor ) {
156	$r = self::linear_to_srgb( $factor[0] );
157	$g = self::linear_to_srgb( $factor[1] );
158	$b = self::linear_to_srgb( $factor[2] );
159	return ( $r << 16 ) + ( $g << 8 ) + $b;
160	}
161
162	/**
163	* Encode an AC factor against the maximum value.
164	*
165	* @param array $factor `[r,g,b]` linear floats.
166	* @param float $max_value Quantisation maximum.
167	* @return int
168	*/
169	private static function encode_ac( $factor, $max_value ) {
170	$quant_r = (int) \max( 0, \min( 18, \floor( self::sign_pow( $factor[0] / $max_value, 0.5 ) * 9 + 9.5 ) ) );
171	$quant_g = (int) \max( 0, \min( 18, \floor( self::sign_pow( $factor[1] / $max_value, 0.5 ) * 9 + 9.5 ) ) );
172	$quant_b = (int) \max( 0, \min( 18, \floor( self::sign_pow( $factor[2] / $max_value, 0.5 ) * 9 + 9.5 ) ) );
173	return $quant_r * 19 * 19 + $quant_g * 19 + $quant_b;
174	}
175
176	/**
177	* Sign-preserving power.
178	*
179	* @param float $value Base.
180	* @param float $exp Exponent.
181	* @return float
182	*/
183	private static function sign_pow( $value, $exp ) {
184	$result = \pow( \abs( $value ), $exp );
185	return $value < 0 ? -$result : $result;
186	}
187	}
188