给定一个图像文件和pixel_array_offset，如何初始化实际的结构像素值？

Question

Following this requirement

I also need to return the address of the first "struct pixel *". Not sure whether I'm implementing it correctly.

struct pixel **read_pixel_array(FILE *image, int pixel_array_offset, int width, int height) {

    struct pixel **ptrs = malloc(height * sizeof(struct pixel));

    struct pixel *first_pixel;
    fseek(image, pixel_array_offset, SEEK_SET);

    for (int i=0, i<height, i++) {
        ptrs[i] = malloc(width * sizeof(struct pixel));
        for (int j=0, j<width, j++) {
            fread(ptrs[i][j]->blue, 1, 1, image);
            fread(ptrs[i][j]->green, 1, 1, image);
            fread(ptrs[i][j]->red, 1, 1, image);
        }

    }

    fseek(image, pixel_array_offset, SEEK_SET);
    fread(first_pixel, 3, 1, image);

    return &first_pixel;

}

Answer 1

You're going to want to figure out how you want to store the pixel data in RAM. Specifically, do you want:

• a 1 dimensional array of "pointers to 1 dimensional array of pixels" for each row
• a 2 dimensional array of pixels
• something else

Your code looks like it's trying to do the first option (and looks like it's doing it wrong - eg malloc(height * sizeof(struct pixel)); when it should be malloc(height * sizeof(struct pixel *)); . However your code also looks like it's trying to do the second option (eg ptrs[i][j]->blue and not row = ptrs[i]; and row[j]->blue ).

For the first option (1 dimensional array of pointers) it'd be like:

struct pixel **read_pixel_array(FILE *image, int pixel_array_offset, int width, int height) {

    struct pixel **ptrs;
    struct pixel *row;

    ptrs = malloc(height * sizeof(struct pixel *));
    if(ptrs == NULL) {
        // Failed to allocate memory for array of pointers
        return NULL;
    }

    fseek(image, pixel_array_offset, SEEK_SET);

    for (int i=0, i<height, i++) {
        row = malloc(width * sizeof(struct pixel));
        if(row == NULL) {
            // Failed to allocate memory for row. Need to free everything
            //   that was previously allocated to avoid memory leaks
            while(i > 0) {
                i--;
                free(ptrs[i]);
            }
            free(ptrs);
            return NULL;
        }
        ptrs[i] = row;
        for (int j=0, j<width, j++) {
            fread(row[j]->blue, 1, 1, image);
            fread(row[j]->green, 1, 1, image);
            fread(row[j]->red, 1, 1, image);
        }
    }
    return ptrs;
}

Myself, I'd use the second option (one 2D array) because it's easier and faster. The problem is that C doesn't know the dimensions at compile time, so you need to cheat and use "2D array implemented as 1D array". In that case it'd be like:

struct pixel *read_pixel_array(FILE *image, int pixel_array_offset, int width, int height) {
    struct pixel *myArray;

    myArray = malloc(height * width * sizeof(struct pixel));
    if(myArray == NULL) {
        // Failed to allocate memory
        return NULL;
    }

    fseek(image, pixel_array_offset, SEEK_SET);

    for (int i=0, i<height, i++) {
        for (int j=0, j<width, j++) {
            fread(myArray [i*width + j]->blue, 1, 1, image);
            fread(myArray [i*width + j]->green, 1, 1, image);
            fread(myArray [i*width + j]->red, 1, 1, image);
        }
    }
    return myArray;
}

NOTE: Neither of these examples were tested and neither can be assumed "correct" without testing.