blockhash 1.0.0

A perceptual hashing algorithm for detecting similar images.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261

use crate::Image;

pub(crate) fn blockhash<
    I: Image,
    const BITS: u32,
    const NUM_BLOCKS: usize,
    const DIGEST_SIZE: usize,
>(
    img: &I,
) -> [u8; DIGEST_SIZE] {
    debug_assert_eq!(BITS % 4, 0);
    debug_assert_ne!(BITS, 0);

    let (width, height) = img.dimensions();

    let values = if width % BITS == 0 && height % BITS == 0 {
        get_values_aligned::<I, BITS, NUM_BLOCKS>(img)
    } else if width >= BITS && height >= BITS {
        get_values_larger::<I, BITS, NUM_BLOCKS>(img)
    } else {
        get_values_generic::<I, BITS, NUM_BLOCKS>(img)
    };

    convert_to_bits(width, height, &values, I::MAX_BRIGHTNESS)
}

fn get_values_aligned<I: Image, const BITS: u32, const NUM_BLOCKS: usize>(
    img: &I,
) -> [u64; NUM_BLOCKS] {
    // These values are related, but need to be passed in separately due to
    // limitations with const generics.
    debug_assert_eq!(NUM_BLOCKS, (BITS * BITS) as usize);

    let (width, height) = img.dimensions();
    let block_width = width / BITS;
    let block_height = height / BITS;

    let mut values = [0_u64; NUM_BLOCKS];

    for y in 0..height {
        let block_y = y / block_height;
        let idx_row = (block_y * BITS) as usize;

        for x in 0..width {
            let block_x = x / block_width;
            let idx_x = block_x as usize;

            let brightness = u64::from(img.brightness(x, y));

            values[idx_row + idx_x] += brightness * NUM_BLOCKS as u64;
        }
    }

    values
}

fn get_values_larger<I: Image, const BITS: u32, const NUM_BLOCKS: usize>(
    img: &I,
) -> [u64; NUM_BLOCKS] {
    // These values are related, but need to be passed in separately due to
    // limitations with const generics.
    debug_assert_eq!(NUM_BLOCKS, (BITS * BITS) as usize);

    let (width, height) = img.dimensions();
    let (width, height) = (u64::from(width), u64::from(height));

    let mut values = [0_u64; NUM_BLOCKS];

    let mut block_top;
    let mut block_bottom = 0;

    let mut weight_top = u64::from(BITS);
    let mut weight_bottom = 0;

    for y in 0..height {
        block_top = block_bottom;

        let end_y = (y + 1) * u64::from(BITS) % height;
        if end_y < u64::from(BITS) {
            block_bottom += 1;
            weight_top = u64::from(BITS) - end_y;
            weight_bottom = end_y;
        }

        let idx_top = (block_top * BITS) as usize;
        let idx_bottom = if block_bottom < BITS {
            (block_bottom * BITS) as usize
        } else {
            0 // to avoid out-of-bounds access (the weight will be zero)
        };

        let mut block_left;
        let mut block_right = 0;

        let mut weight_left = u64::from(BITS);
        let mut weight_right = 0;

        for x in 0..width {
            block_left = block_right;

            let end_x = (x + 1) * u64::from(BITS) % width;
            if end_x < u64::from(BITS) {
                block_right += 1;
                weight_left = u64::from(BITS) - end_x;
                weight_right = end_x;
            }

            let idx_left = block_left as usize;
            let idx_right = if block_right < BITS {
                block_right as usize
            } else {
                0 // to avoid out-of-bounds access (the weight will be zero)
            };

            let brightness = u64::from(img.brightness(x as u32, y as u32));

            values[idx_top + idx_left] += brightness * weight_top * weight_left;
            values[idx_top + idx_right] += brightness * weight_top * weight_right;
            values[idx_bottom + idx_left] += brightness * weight_bottom * weight_left;
            values[idx_bottom + idx_right] += brightness * weight_bottom * weight_right;
        }
    }

    values
}

fn get_values_generic<I: Image, const BITS: u32, const NUM_BLOCKS: usize>(
    img: &I,
) -> [u64; NUM_BLOCKS] {
    // These values are related, but need to be passed in separately due to
    // limitations with const generics.
    debug_assert_eq!(NUM_BLOCKS, (BITS * BITS) as usize);

    let (width, height) = img.dimensions();
    let (width, height) = (u64::from(width), u64::from(height));

    let mut values = [0_u64; NUM_BLOCKS];

    let mut block_top;
    let mut block_bottom = 0;

    let mut weight_top = u64::from(BITS);
    let mut weight_bottom = 0;

    for y in 0..height {
        block_top = block_bottom;

        let end_y = (y + 1) * u64::from(BITS) % height;
        if end_y < u64::from(BITS) {
            block_bottom = (y + 1) * u64::from(BITS) / height;
            weight_top = (u64::from(BITS) - 1 - end_y) % height + 1;
            weight_bottom = end_y;
        }

        let idx_top = (block_top * u64::from(BITS)) as usize;
        let idx_bottom = if block_bottom < u64::from(BITS) {
            (block_bottom * u64::from(BITS)) as usize
        } else {
            0 // to avoid out-of-bounds access (the weight will be zero)
        };

        let mut block_left;
        let mut block_right = 0;

        let mut weight_left = u64::from(BITS);
        let mut weight_right = 0;

        for x in 0..width {
            block_left = block_right;

            let end_x = (x + 1) * u64::from(BITS) % width;
            if end_x < u64::from(BITS) {
                block_right = (x + 1) * u64::from(BITS) / width;
                weight_left = (u64::from(BITS) - 1 - end_x) % width + 1;
                weight_right = end_x;
            }

            let idx_left = block_left as usize;
            let idx_right = if block_right < u64::from(BITS) {
                block_right as usize
            } else {
                0 // to avoid out-of-bounds access (the weight will be zero)
            };

            let brightness = u64::from(img.brightness(x as u32, y as u32));

            values[idx_top + idx_left] += brightness * weight_top * weight_left;
            values[idx_top + idx_right] += brightness * weight_top * weight_right;
            values[idx_bottom + idx_left] += brightness * weight_bottom * weight_left;
            values[idx_bottom + idx_right] += brightness * weight_bottom * weight_right;

            for bx in (block_left + 1)..block_right {
                let idx_x = bx as usize;
                values[idx_top + idx_x] += brightness * weight_top * width;
                values[idx_bottom + idx_x] += brightness * weight_bottom * width;
            }

            for by in (block_top + 1)..block_bottom {
                let idx_y = (by * u64::from(BITS)) as usize;
                values[idx_y + idx_left] += brightness * height * weight_left;
                values[idx_y + idx_right] += brightness * height * weight_right;
            }

            let full_value = brightness * width * height;
            for by in (block_top + 1)..block_bottom {
                let idx_y = (by * u64::from(BITS)) as usize;
                for bx in (block_left + 1)..block_right {
                    let idx_x = bx as usize;
                    values[idx_y + idx_x] += full_value;
                }
            }
        }
    }

    values
}

fn convert_to_bits<const NUM_BLOCKS: usize, const DIGEST_SIZE: usize>(
    width: u32,
    height: u32,
    values: &[u64; NUM_BLOCKS],
    max_value: u32,
) -> [u8; DIGEST_SIZE] {
    // These values are related, but need to be passed in separately due to
    // limitations with const generics.
    debug_assert_eq!(NUM_BLOCKS, DIGEST_SIZE * 8);

    let band_size: usize = NUM_BLOCKS / 4;
    let half_value = u64::from(max_value) * u64::from(width) * u64::from(height) / 2;

    let mut bands = *values;
    let mut bits = [0_u8; NUM_BLOCKS];

    for i in 0..4 {
        let offset = i * band_size;

        let band = &mut bands[offset..(offset + band_size)];
        band.sort_unstable();

        let median = (band[band_size / 2 - 1] + band[band_size / 2]) / 2;

        for (n, &val) in values.iter().enumerate().skip(offset).take(band_size) {
            if val > median || (val == median && val > half_value) {
                bits[n] = 1;
            } else {
                bits[n] = 0;
            }
        }
    }

    let mut res = [0_u8; DIGEST_SIZE];

    for (i, octet) in bits.chunks(8).enumerate() {
        for &bit in octet {
            res[i] <<= 1;
            res[i] |= bit;
        }
    }

    res
}