c - 如何在c中读取图像的像素？

Question

Suppose our bitmap image has height M and width N. We'll always assume in this lab that the width N is a multiple of 4, which simplifies the byte layout in the file. For this image, the pixel array stores exactly 3 x N x M bytes, in the following way:

Each group of 3 bytes represents a single pixel, where the bytes store the blue, green, and red colour values of the pixel, in that order.

Pixels are grouped by row. For example, the first 3 x N bytes in the pixel array represent the pixels in the top-most row of the image.

pixel_array_offset 是像素数组的起始位置。

结构体像素如下:

struct pixel {
    unsigned char blue;
    unsigned char green;
    unsigned char red;
};

这里是实现该功能的要求:

/*
 * Read in pixel array by following these instructions:
 *
 * 1. First, allocate space for m "struct pixel *" values, where m is the
 *    height of the image.  Each pointer will eventually point to one row of
 *    pixel data.
 * 2. For each pointer you just allocated, initialize it to point to
 *    heap-allocated space for an entire row of pixel data.
 * 3. ...
 * 4. ...
 */
struct pixel **read_pixel_array(FILE *image, int pixel_array_offset, int width, int height) {

}

第一步，为 m 个“struct Pixel *”值分配空间。我认为它实际上是为像素值数组分配空间。所以我把

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

对于第二步，我不太明白我应该做什么。我想我需要一个 for 循环来为所有像素数据行分配空间。但我不知道该放什么进去。

for (int i=0, i<height, i++) {

        }

Answer 1

既然要分配一个2D数组，那么首先需要分配一个struct Pixel *数组:

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

这里有几处变化需要注意:

1. 您正在分配指向struct Pixel的指针，而不是unsigned char。
2. malloc() 返回一个指针。指针乘以整数是无效的。
3. 乘法放在括号中，因为它有助于计算要分配的正确字节数。

接下来，您需要为 2D 数组中的每一行分配一个 struct Pixel 数组:

for (int i=0, i<height, i++) {
    ptrs[i] = malloc(width * sizeof(struct pixel));
}

现在数组已完全分配，您可以用数据填充它:

ptrs[5][6] = { 255, 0, 0}; // a blue pixel

最后记住在退出程序之前free()所有指针:

for (int i=0, i<height, i++) {
    free(ptrs[i]);
}

free(ptrs);