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use crate::{CHANNELS_IN_RGB, Histogram, Image, Pixel};
use std::f64::consts::E;
impl Image<u16> {
pub fn gamma_correction_fn(&self, histogram: &Histogram) -> impl Fn(Pixel) -> [u16; CHANNELS_IN_RGB] + use<> {
let percentage = self.width * self.height;
let mut white = 0;
for channel_histogram in histogram {
let mut total = 0;
for i in (0x20..0x2000).rev() {
total += channel_histogram[i] as u64;
if total * 100 > percentage as u64 {
white = white.max(i);
break;
}
}
}
let curve = generate_gamma_curve(0.45, 4.5, (white << 3) as f64);
move |pixel: Pixel| pixel.values.map(|value| curve[value as usize])
}
}
/// `max_intensity` must be non-zero.
fn generate_gamma_curve(power: f64, threshold: f64, max_intensity: f64) -> Vec<u16> {
debug_assert!(max_intensity != 0.);
let (mut bound_start, mut bound_end) = if threshold >= 1. { (0., 1.) } else { (1., 0.) };
let mut transition_point = 0.;
let mut transition_ratio = 0.;
let mut curve_adjustment = 0.;
if threshold != 0. && (threshold - 1.) * (power - 1.) <= 0. {
for _ in 0..48 {
transition_point = (bound_start + bound_end) / 2.;
if power != 0. {
let temp_transition_ratio = transition_point / threshold;
let exponential_power = temp_transition_ratio.powf(-power);
let normalized_exponential_power = (exponential_power - 1.) / power;
let comparison_result = normalized_exponential_power - (1. / transition_point);
let bound_to_update = if comparison_result > -1. { &mut bound_end } else { &mut bound_start };
*bound_to_update = transition_point;
} else {
let adjusted_transition_point = E.powf(1. - 1. / transition_point);
let transition_point_ratio = transition_point / adjusted_transition_point;
let bound_to_update = if transition_point_ratio < threshold { &mut bound_end } else { &mut bound_start };
*bound_to_update = transition_point;
}
}
transition_ratio = transition_point / threshold;
if power != 0. {
curve_adjustment = transition_point * ((1. / power) - 1.);
}
}
let mut curve = vec![0xffff; 0x1_0000];
let length = curve.len() as f64;
for (i, entry) in curve.iter_mut().enumerate() {
let ratio = (i as f64) / max_intensity;
if ratio < 1. {
let altered_ratio = if ratio < transition_ratio {
ratio * threshold
} else if power != 0. {
ratio.powf(power) * (1. + curve_adjustment) - curve_adjustment
} else {
ratio.ln() * transition_point + 1.
};
*entry = (length * altered_ratio) as u16;
}
}
curve
}
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