alpha-lut.c

#include <stdio.h>
#include <stdint.h>
#include <assert.h>
#include <math.h>

static uint8_t reference[256][256] = {
#include "reference.h"
};


uint8_t to_premultiplied_alpha(uint8_t color, uint8_t alpha) {
    return (((unsigned) color) * alpha + 127) / 255;
}

uint8_t to_unmultiplied_alpha(uint8_t color, int inv, int alpha) {
    int r = (((int)color * inv + 0x8000) >> 16);
    return r;
}

int main() {
    int max_inv = 256*256*256;
    for (int alpha = 1; alpha < 256; ++alpha) {
        int best_inv = 0;
        int best_errors = 256*256*2;
        for (int inv = 0; inv < max_inv; ++inv) {
            int errors = 0;
            for (int color = 0; color < 256; ++color) {
                int result = to_unmultiplied_alpha(to_premultiplied_alpha(color, alpha), inv, alpha);
                int expected = reference[alpha][color];
                if (result != expected) errors++;
            }
            if (errors < best_errors) {
                best_inv=inv;
                best_errors=errors;
            }
        }
        fprintf(stderr, "alpha %d: %d [%d]\n", alpha, best_inv, best_errors);
        max_inv=best_inv;
    }
}