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/* 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.
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.
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.
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.
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);
//println!("{} -- {} {} = {}", alpha, preblend_result, true_result, diff);
max_diff = max(max_diff, diff);
}
//println!("{} {} max {}", src, dst, max_diff);
assert!(max_diff <= 33);
dst += 1;
}
src += 1;
}
}
} // end mod