在C中裁剪.ppm文件

#include<stdio.h>
#include <stdlib.h>
#include<string.h>

#define MAX_COMMENT_LENGTH 70
#define MAX_LINE_LENGTH 70

typedef struct {
    int width;
    int height;
    int max_color_value;
    char comment[MAX_COMMENT_LENGTH + 1];
    unsigned char *pixels;
} PPMImage;

void read_ppm_header(FILE *file, PPMImage *image) {
    char line[MAX_LINE_LENGTH + 1];
    fgets(line, MAX_LINE_LENGTH + 1, file);
    if (strcmp(line, "P6\n") != 0) {
        fprintf(stderr, "Error: Invalid PPM file format.\n");
        exit(1);
    }

    fgets(line, MAX_LINE_LENGTH + 1, file);
    sscanf(line, "%d %d\n", &image->width, &image->height);

    fgets(line, MAX_LINE_LENGTH + 1, file);
    sscanf(line, "%d\n", &image->max_color_value);

    fgets(line, MAX_LINE_LENGTH + 1, file);
    strncpy(image->comment, line, MAX_COMMENT_LENGTH);
    image->comment[MAX_COMMENT_LENGTH] = '\0';
}

PPMImage *read_ppm_image(const char *filename) {
    FILE *file = fopen(filename, "rb");
    if (file == NULL) {
        fprintf(stderr, "Error: Cannot open file %s.\n", filename);
        exit(1);
    }

    PPMImage *image = (PPMImage *)malloc(sizeof(PPMImage));
    read_ppm_header(file, image);

    int pixel_count = image->width * image->height;
    image->pixels = (unsigned char *)malloc(pixel_count * 3);
    fread(image->pixels, 1, pixel_count * 3, file);

    fclose(file);
    return image;
}

void write_ppm_image(const char *filename, PPMImage *image) {
    FILE *file = fopen(filename, "wb");
    if (file == NULL) {
        fprintf(stderr, "Error: Cannot open file %s.\n", filename);
        exit(1);
    }

    fprintf(file, "P6\n");
    fprintf(file, "%d %d\n", image->width, image->height);
    fprintf(file, "%d\n", image->max_color_value);
    fprintf(file, "%s", image->comment);

    int pixel_count = image->width * image->height;
    fwrite(image->pixels, 1, pixel_count * 3, file);

    fclose(file);
}

void crop_ppm_image(PPMImage *image, int x, int y, int width, int height) {
    unsigned char *cropped_pixels = (unsigned char *)malloc(width * height * 3);
    int src_row_stride = image->width * 3;
    int dst_row_stride = width * 3;

    for (int i = 0; i< height; i++) {
        memcpy(cropped_pixels + i * dst_row_stride,
               image->pixels + (y + i) * src_row_stride + x * 3,
               width * 3);
    }

    free(image->pixels);
    image->pixels = cropped_pixels;
    image->width = width;
    image->height = height;
}

int main() {
    PPMImage *image = read_ppm_image("input.ppm");
    crop_ppm_image(image, 10, 10, 100, 100);
    write_ppm_image("output.ppm", image);
    free(image->pixels);
    free(image);
    return 0;
}