11315：灰度字符画降噪

题目

依据四邻域规则修正孤立灰度值，并把取值 0～9 的二维图像写成字符画。

解析

函数不会修改像素数组，因此每个位置读取到的始终是原图数据，降噪不会沿遍历方向传播。尺寸和所有像素在首次写入前统一检查，使参数错误不会留下部分输出。

一个位置最多有四个相邻值。先累计实际存在的相邻值，同时判断当前值是否与每一个相邻值都相差 1 以上；条件成立时，用 (sum + count / 2) / count 完成非负整数的四舍五入。

解析

#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>

typedef enum {
    DIGIT_IMAGE_OK,
    DIGIT_IMAGE_INVALID,
    DIGIT_IMAGE_WRITE_ERROR
} DigitImageResult;

static unsigned int distance_between(
    unsigned int left,
    unsigned int right
) {
    return left > right ? left - right : right - left;
}

static void include_neighbor(
    unsigned int current,
    unsigned int neighbor,
    unsigned int *sum,
    unsigned int *count,
    bool *replace
) {
    *sum += neighbor;
    ++*count;

    if (distance_between(current, neighbor) <= 1) {
        *replace = false;
    }
}

static unsigned int denoised_at(
    const unsigned char *pixels,
    size_t width,
    size_t height,
    size_t row,
    size_t column
) {
    size_t index = row * width + column;
    unsigned int current = pixels[index];
    unsigned int sum = 0;
    unsigned int count = 0;
    bool replace = true;

    if (row > 0) {
        include_neighbor(
            current,
            pixels[index - width],
            &sum,
            &count,
            &replace
        );
    }
    if (row + 1 < height) {
        include_neighbor(
            current,
            pixels[index + width],
            &sum,
            &count,
            &replace
        );
    }
    if (column > 0) {
        include_neighbor(
            current,
            pixels[index - 1],
            &sum,
            &count,
            &replace
        );
    }
    if (column + 1 < width) {
        include_neighbor(
            current,
            pixels[index + 1],
            &sum,
            &count,
            &replace
        );
    }

    if (count == 0 || !replace) {
        return current;
    }
    return (sum + count / 2) / count;
}

DigitImageResult write_denoised_digit_image(
    FILE *output,
    const unsigned char *pixels,
    size_t width,
    size_t height
) {
    static const char palette[] = " .:-=+*#%@";

    if (output == NULL
        || pixels == NULL
        || width == 0
        || height == 0
        || width > SIZE_MAX / height) {
        return DIGIT_IMAGE_INVALID;
    }

    size_t pixel_count = width * height;
    for (size_t i = 0; i < pixel_count; ++i) {
        if (pixels[i] > 9) {
            return DIGIT_IMAGE_INVALID;
        }
    }

    for (size_t row = 0; row < height; ++row) {
        for (size_t column = 0; column < width; ++column) {
            unsigned int value = denoised_at(
                pixels,
                width,
                height,
                row,
                column
            );

            if (fputc(
                    (unsigned char)palette[value],
                    output
                ) == EOF) {
                return DIGIT_IMAGE_WRITE_ERROR;
            }
        }

        if (fputc('\n', output) == EOF) {
            return DIGIT_IMAGE_WRITE_ERROR;
        }
    }

    if (fflush(output) == EOF) {
        return DIGIT_IMAGE_WRITE_ERROR;
    }
    return DIGIT_IMAGE_OK;
}