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Diffstat (limited to 'gfx/wr/wr_glyph_rasterizer/src/gamma_lut.rs')
| -rw-r--r-- | gfx/wr/wr_glyph_rasterizer/src/gamma_lut.rs | 413 |
1 files changed, 413 insertions, 0 deletions
diff --git a/gfx/wr/wr_glyph_rasterizer/src/gamma_lut.rs b/gfx/wr/wr_glyph_rasterizer/src/gamma_lut.rs new file mode 100644 index 0000000000..15075bacbf --- /dev/null +++ b/gfx/wr/wr_glyph_rasterizer/src/gamma_lut.rs @@ -0,0 +1,413 @@ +/* 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 |