summaryrefslogtreecommitdiffstats
path: root/gfx/wr/wr_glyph_rasterizer/src/gamma_lut.rs
blob: 15075bacbf524dc99894d97a1e796e229465605d (plain)
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
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/*!
Gamma correction lookup tables.

This is a port of Skia gamma LUT logic into Rust, used by WebRender.
*/
//#![warn(missing_docs)] //TODO
#![allow(dead_code)]

use api::ColorU;
use std::cmp::max;

/// Color space responsible for converting between lumas and luminances.
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum LuminanceColorSpace {
    /// Linear space - no conversion involved.
    Linear,
    /// Simple gamma space - uses the `luminance ^ gamma` function.
    Gamma(f32),
    /// Srgb space.
    Srgb,
}

impl LuminanceColorSpace {
    pub fn new(gamma: f32) -> LuminanceColorSpace {
        if gamma == 1.0 {
            LuminanceColorSpace::Linear
        } else if gamma == 0.0 {
            LuminanceColorSpace::Srgb
        } else {
            LuminanceColorSpace::Gamma(gamma)
        }
    }

    pub fn to_luma(&self, luminance: f32) -> f32 {
        match *self {
            LuminanceColorSpace::Linear => luminance,
            LuminanceColorSpace::Gamma(gamma) => luminance.powf(gamma),
            LuminanceColorSpace::Srgb => {
                //The magic numbers are derived from the sRGB specification.
                //See http://www.color.org/chardata/rgb/srgb.xalter .
                if luminance <= 0.04045 {
                    luminance / 12.92
                } else {
                    ((luminance + 0.055) / 1.055).powf(2.4)
                }
            }
        }
    }

    pub fn from_luma(&self, luma: f32) -> f32 {
        match *self {
            LuminanceColorSpace::Linear => luma,
            LuminanceColorSpace::Gamma(gamma) => luma.powf(1. / gamma),
            LuminanceColorSpace::Srgb => {
                //The magic numbers are derived from the sRGB specification.
                //See http://www.color.org/chardata/rgb/srgb.xalter .
                if luma <= 0.0031308 {
                    luma * 12.92
                } else {
                    1.055 * luma.powf(1./2.4) - 0.055
                }
            }
        }
    }
}

//TODO: tests
fn round_to_u8(x : f32) -> u8 {
    let v = (x + 0.5).floor() as i32;
    assert!(0 <= v && v < 0x100);
    v as u8
}

//TODO: tests
/*
 * Scales base <= 2^N-1 to 2^8-1
 * @param N [1, 8] the number of bits used by base.
 * @param base the number to be scaled to [0, 255].
 */
fn scale255(n: u8, mut base: u8) -> u8 {
    base <<= 8 - n;
    let mut lum = base;
    let mut i = n;

    while i < 8 {
        lum |= base >> i;
        i += n;
    }

    lum
}

// Computes the luminance from the given r, g, and b in accordance with
// SK_LUM_COEFF_X. For correct results, r, g, and b should be in linear space.
fn compute_luminance(r: u8, g: u8, b: u8) -> u8 {
    // The following is
    // r * SK_LUM_COEFF_R + g * SK_LUM_COEFF_G + b * SK_LUM_COEFF_B
    // with SK_LUM_COEFF_X in 1.8 fixed point (rounding adjusted to sum to 256).
    let val: u32 = r as u32 * 54 + g as u32 * 183 + b as u32 * 19;
    assert!(val < 0x10000);
    (val >> 8) as u8
}

// Skia uses 3 bits per channel for luminance.
const LUM_BITS: u8 = 3;
// Mask of the highest used bits.
const LUM_MASK: u8 = ((1 << LUM_BITS) - 1) << (8 - LUM_BITS);

pub trait ColorLut {
    fn quantize(&self) -> ColorU;
    fn quantized_floor(&self) -> ColorU;
    fn quantized_ceil(&self) -> ColorU;
    fn luminance(&self) -> u8;
    fn luminance_color(&self) -> ColorU;
}

impl ColorLut for ColorU {
    // Compute a canonical color that is equivalent to the input color
    // for preblend table lookups. The alpha channel is never used for
    // preblending, so overwrite it with opaque.
    fn quantize(&self) -> ColorU {
        ColorU::new(
            scale255(LUM_BITS, self.r >> (8 - LUM_BITS)),
            scale255(LUM_BITS, self.g >> (8 - LUM_BITS)),
            scale255(LUM_BITS, self.b >> (8 - LUM_BITS)),
            255,
        )
    }

    // Quantize to the smallest value that yields the same table index.
    fn quantized_floor(&self) -> ColorU {
        ColorU::new(
            self.r & LUM_MASK,
            self.g & LUM_MASK,
            self.b & LUM_MASK,
            255,
        )
    }

    // Quantize to the largest value that yields the same table index.
    fn quantized_ceil(&self) -> ColorU {
        ColorU::new(
            self.r | !LUM_MASK,
            self.g | !LUM_MASK,
            self.b | !LUM_MASK,
            255,
        )
    }

    // Compute a luminance value suitable for grayscale preblend table
    // lookups.
    fn luminance(&self) -> u8 {
        compute_luminance(self.r, self.g, self.b)
    }

    // Make a grayscale color from the computed luminance.
    fn luminance_color(&self) -> ColorU {
        let lum = self.luminance();
        ColorU::new(lum, lum, lum, self.a)
    }
}

// This will invert the gamma applied by CoreGraphics,
// so we can get linear values.
// CoreGraphics obscurely defaults to 2.0 as the smoothing gamma value.
// The color space used does not appear to affect this choice.
#[cfg(target_os="macos")]
fn get_inverse_gamma_table_coregraphics_smoothing() -> [u8; 256] {
    let mut table = [0u8; 256];

    for (i, v) in table.iter_mut().enumerate() {
        let x = i as f32 / 255.0;
        *v = round_to_u8(x * x * 255.0);
    }

    table
}

// A value of 0.5 for SK_GAMMA_CONTRAST appears to be a good compromise.
// With lower values small text appears washed out (though correctly so).
// With higher values lcd fringing is worse and the smoothing effect of
// partial coverage is diminished.
fn apply_contrast(srca: f32, contrast: f32) -> f32 {
    srca + ((1.0 - srca) * contrast * srca)
}

// The approach here is not necessarily the one with the lowest error
// See https://bel.fi/alankila/lcd/alpcor.html for a similar kind of thing
// that just search for the adjusted alpha value
pub fn build_gamma_correcting_lut(table: &mut [u8; 256], src: u8, contrast: f32,
                                  src_space: LuminanceColorSpace,
                                  dst_convert: LuminanceColorSpace) {
    let src = src as f32 / 255.0;
    let lin_src = src_space.to_luma(src);
    // Guess at the dst. The perceptual inverse provides smaller visual
    // discontinuities when slight changes to desaturated colors cause a channel
    // to map to a different correcting lut with neighboring srcI.
    // See https://code.google.com/p/chromium/issues/detail?id=141425#c59 .
    let dst = 1.0 - src;
    let lin_dst = dst_convert.to_luma(dst);

    // Contrast value tapers off to 0 as the src luminance becomes white
    let adjusted_contrast = contrast * lin_dst;

    // Remove discontinuity and instability when src is close to dst.
    // The value 1/256 is arbitrary and appears to contain the instability.
    if (src - dst).abs() < (1.0 / 256.0) {
        let mut ii : f32 = 0.0;
        for v in table.iter_mut() {
            let raw_srca = ii / 255.0;
            let srca = apply_contrast(raw_srca, adjusted_contrast);

            *v = round_to_u8(255.0 * srca);
            ii += 1.0;
        }
    } else {
        // Avoid slow int to float conversion.
        let mut ii : f32 = 0.0;
        for v in table.iter_mut() {
            // 'raw_srca += 1.0f / 255.0f' and even
            // 'raw_srca = i * (1.0f / 255.0f)' can add up to more than 1.0f.
            // When this happens the table[255] == 0x0 instead of 0xff.
            // See http://code.google.com/p/chromium/issues/detail?id=146466
            let raw_srca = ii / 255.0;
            let srca = apply_contrast(raw_srca, adjusted_contrast);
            assert!(srca <= 1.0);
            let dsta = 1.0 - srca;

            // Calculate the output we want.
            let lin_out = lin_src * srca + dsta * lin_dst;
            assert!(lin_out <= 1.0);
            let out = dst_convert.from_luma(lin_out);

            // Undo what the blit blend will do.
            // i.e. given the formula for OVER: out = src * result + (1 - result) * dst
            // solving for result gives:
            let result = (out - dst) / (src - dst);

            *v = round_to_u8(255.0 * result);
            debug!("Setting {:?} to {:?}", ii as u8, *v);

            ii += 1.0;
        }
    }
}

pub struct GammaLut {
    tables: [[u8; 256]; 1 << LUM_BITS],
    #[cfg(target_os="macos")]
    cg_inverse_gamma: [u8; 256],
}

impl GammaLut {
    // Skia actually makes 9 gamma tables, then based on the luminance color,
    // fetches the RGB gamma table for that color.
    fn generate_tables(&mut self, contrast: f32, paint_gamma: f32, device_gamma: f32) {
        let paint_color_space = LuminanceColorSpace::new(paint_gamma);
        let device_color_space = LuminanceColorSpace::new(device_gamma);

        for (i, entry) in self.tables.iter_mut().enumerate() {
            let luminance = scale255(LUM_BITS, i as u8);
            build_gamma_correcting_lut(entry,
                                       luminance,
                                       contrast,
                                       paint_color_space,
                                       device_color_space);
        }
    }

    pub fn table_count(&self) -> usize {
        self.tables.len()
    }

    pub fn get_table(&self, color: u8) -> &[u8; 256] {
        &self.tables[(color >> (8 - LUM_BITS)) as usize]
    }

    pub fn new(contrast: f32, paint_gamma: f32, device_gamma: f32) -> GammaLut {
        #[cfg(target_os="macos")]
        let mut table = GammaLut {
            tables: [[0; 256]; 1 << LUM_BITS],
            cg_inverse_gamma: get_inverse_gamma_table_coregraphics_smoothing(),
        };
        #[cfg(not(target_os="macos"))]
        let mut table = GammaLut {
            tables: [[0; 256]; 1 << LUM_BITS],
        };

        table.generate_tables(contrast, paint_gamma, device_gamma);

        table
    }

    // Assumes pixels are in BGRA format. Assumes pixel values are in linear space already.
    pub fn preblend(&self, pixels: &mut [u8], color: ColorU) {
        let table_r = self.get_table(color.r);
        let table_g = self.get_table(color.g);
        let table_b = self.get_table(color.b);

        for pixel in pixels.chunks_mut(4) {
            let (b, g, r) = (table_b[pixel[0] as usize], table_g[pixel[1] as usize], table_r[pixel[2] as usize]);
            pixel[0] = b;
            pixel[1] = g;
            pixel[2] = r;
            pixel[3] = max(max(b, g), r);
        }
    }

    // Assumes pixels are in BGRA format. Assumes pixel values are in linear space already.
    pub fn preblend_scaled(&self, pixels: &mut [u8], color: ColorU, percent: u8) {
        if percent >= 100 {
            self.preblend(pixels, color);
            return;
        }

        let table_r = self.get_table(color.r);
        let table_g = self.get_table(color.g);
        let table_b = self.get_table(color.b);
        let scale = (percent as i32 * 256) / 100;

        for pixel in pixels.chunks_mut(4) {
            let (mut b, g, mut r) = (
                table_b[pixel[0] as usize] as i32,
                table_g[pixel[1] as usize] as i32,
                table_r[pixel[2] as usize] as i32,
            );
            b = g + (((b - g) * scale) >> 8);
            r = g + (((r - g) * scale) >> 8);
            pixel[0] = b as u8;
            pixel[1] = g as u8;
            pixel[2] = r as u8;
            pixel[3] = max(max(b, g), r) as u8;
        }
    }

    #[cfg(target_os="macos")]
    pub fn coregraphics_convert_to_linear(&self, pixels: &mut [u8]) {
        for pixel in pixels.chunks_mut(4) {
            pixel[0] = self.cg_inverse_gamma[pixel[0] as usize];
            pixel[1] = self.cg_inverse_gamma[pixel[1] as usize];
            pixel[2] = self.cg_inverse_gamma[pixel[2] as usize];
        }
    }

    // Assumes pixels are in BGRA format. Assumes pixel values are in linear space already.
    pub fn preblend_grayscale(&self, pixels: &mut [u8], color: ColorU) {
        let table_g = self.get_table(color.g);

        for pixel in pixels.chunks_mut(4) {
            let luminance = compute_luminance(pixel[2], pixel[1], pixel[0]);
            let alpha = table_g[luminance as usize];
            pixel[0] = alpha;
            pixel[1] = alpha;
            pixel[2] = alpha;
            pixel[3] = alpha;
        }
    }

} // end impl GammaLut

#[cfg(test)]
mod tests {
    use super::*;

    fn over(dst: u32, src: u32, alpha: u32) -> u32 {
        (src * alpha + dst * (255 - alpha))/255
    }

    fn overf(dst: f32, src: f32, alpha: f32) -> f32 {
        ((src * alpha + dst * (255. - alpha))/255.) as f32
    }


    fn absdiff(a: u32, b: u32) -> u32 {
        if a < b  { b - a } else { a - b }
    }

    #[test]
    fn gamma() {
        let mut table = [0u8; 256];
        let g = 2.0;
        let space = LuminanceColorSpace::Gamma(g);
        let mut src : u32 = 131;
        while src < 256 {
            build_gamma_correcting_lut(&mut table, src as u8, 0., space, space);
            let mut max_diff = 0;
            let mut dst = 0;
            while dst < 256 {
                for alpha in 0u32..256 {
                    let preblend = table[alpha as usize];
                    let lin_dst = (dst as f32 / 255.).powf(g) * 255.;
                    let lin_src = (src as f32 / 255.).powf(g) * 255.;

                    let preblend_result = over(dst, src, preblend as u32);
                    let true_result = ((overf(lin_dst, lin_src, alpha as f32) / 255.).powf(1. / g) * 255.) as u32;
                    let diff = absdiff(preblend_result, true_result);
                    //debug!("{} -- {} {} = {}", alpha, preblend_result, true_result, diff);
                    max_diff = max(max_diff, diff);
                }

                //debug!("{} {} max {}", src, dst, max_diff);
                assert!(max_diff <= 33);
                dst += 1;

            }
            src += 1;
        }
    }
} // end mod