Crop BMP file image in c program - c

Question

My function is getting an Image and I am trying to crop the BMP image
by size: 100 x 100, 200 x 200, 300 x 300, 400 x 400 by Pixels ,
i don't know what to do make it work. please help me
The size of the image is int height and int width and the function knows the values in pixels.
Here is my code:
void re_allocate_pixels(struct RGB_Image *image, int new_height, int new_width)
{
int org_height = image->height;
int org_width = image->width;
int org_size = image->size;
int i;
struct Pixel **pxls;
pxls = (struct Pixel **)malloc((org_height) * sizeof(struct Pixel *));
if (pxls == NULL)
{
printf("Memory allocation failed\n");
exit(1);
}
for (i = 0; i < org_height; i++)
{
pxls[i] = (struct Pixel *)malloc(org_width * sizeof(struct Pixel));
if (pxls[i] == NULL)
{
printf("Memory allocation failed\n");
exit(1);
}
}
//i have no idea what to do next to crop the image and pixecl
/*for (int j = 0; j < org_height; j++)
{
for (int k = 0; k < org_width; k++)
{
pxls[i][j] = pxls[k][j];
}
}*/
}
here are struct data:
struct Pixel
{
unsigned char red;
unsigned char green;
unsigned char blue;
};
struct RGB_Image
{
char file_name[MAX_FILE_NAME_SIZE];
long height;
long width;
long size;
struct Pixel **pixels;
};
here is how am calling this function
struct RGB_Image *rgb_img_ptr;
struct RGB_Image image;
rgb_img_ptr = ℑ
int image_load_ret_value = load_image(rgb_img_ptr);
re_allocate_pixels(rgb_img_ptr, 100,100); // here is calling

Answer 1

You have to delete the old allocation and also assign the new size value. size usually refers to "width in bytes" multiplied by height.
In this case the value for "width in bytes" should be "width * 3", and it should always be a multiple of 4.
Read the image one row at a time. Use mempcy to copy each row.
Note that bitmaps are usually upside down. You may have to change the loop to for (int y = dst.height - 1; y >=0; y--){memcpy...}
void re_allocate_pixels(struct RGB_Image *image, int new_height, int new_width)
{
if(new_width > image->width) return;
if(new_height > image->height) return;
struct RGB_Image *src = image;
struct RGB_Image dst;
dst.width = new_width;
dst.height = new_height;
//allocate memory
dst.pixels = malloc(dst.height * sizeof(struct Pixel*));
for(int y = 0; y < dst.height; y++)
dst.pixels[y] = malloc(dst.width * sizeof(struct Pixel));
//copy from source to destination
for(int y = 0; y < dst.height; y++)
memcpy(dst.pixels[y], src->pixels[y], dst.width * sizeof(struct Pixel));
//free the old allocation
for(int y = 0; y < src->height; y++)
free(image->pixels[y]);
free(image->pixels);
//assing new allocation
image->pixels = dst.pixels;
//set the new width, height, size
image->width = dst.width;
image->height = dst.height;
int bitcount = 24;
int bytecount = 3; //<- same as sizeof(struct Pixel)
//calculate width in bytes
int width_in_bytes = ((dst.width * bitcount + 31) / 32) * 4;
image->size = width_in_bytes * dst.height;
}