Creating a .bmp grayscale image from a vector of uint8_t - c++

Question

I'm trying read a .bmp grayscale image from a file with a given width and height, convert it to std::vector<uint8_t>, run some sort of filter function on that vector, and then create a new image from that std::vector. I'm stuck in the last part.
How do I create a .bmp file from a given std::vector<uint8_t>, height and width?
P.S. I'm trying to do this without using external libraries.
This is the code I have thus far:
class Image {
int weight;
int width;
std::vector<uint8_t> image;
Image(int weight,int width) : weight(weight),width(width);
void read_image(char* pic);
void save_image(const std::vector<uint8_t>& new_image);
std::vector<uint8_t> filter_image(int ww,int wh,double filter);
};
void Image::read_image(char *pic) {
std::ifstream file(pic,std::ios_base::in | std::ios_base::binary);
if(!file.is_open()) return;
while(file.peek()!=EOF){
this->image.push_back(static_cast<uint8_t>(file.get()));
}
}
void Image::save_image(const std::vector<uint8_t> &new_image) {
//what to do here?
}

Answer 1

A .bmp file does not only store raw pixel data. It begins with a header describing the image stored inside the file: width, height, pixel size, color type, etc... The read_image() function you wrote reads the whole file, including the header, and running any image processing algorithm on your vector will ruin your data and produce garbage.
If you are learning image processing, it would be far easier to use raw image files. A raw image file contains only pixel data, without any metadata. When working with a raw image file, it is your responsibility to know the width and height of the image, as well as the pixel encoding.
Converting an image file to a raw image file, and vice versa, involves the use of an external tool. ffmpeg is such a tool. ffmpeg is a linux tool, but it should be easy to find ffmpeg packaged for any operating system.
For converting from a file in almost any format to a raw image file (ffmpeg deduces the size of the image from the input file). The order of each parameter is important:
ffmpeg -i your_file.jpeg -f rawvideo -c rawvideo -pix_fmt gray output.raw
When converting back to your input format, you have to explicitly tell ffmpeg the size of your picture. Again the order of each parameter is important:
ffmpeg -f rawvideo -c rawvideo -pix_fmt gray -s 1280x720 -i input.raw your_processed_file.jpeg
Adapt the width and height to the real size of your image, or ffmpeg will resize the image. you can also play with the pixel type: gray specifies an 8 bits per pixel grayscale format, but you can use rgb24 to keep color information (use ffmpeg -pix_fmts to see a list of all available formats).
If you are lucky enough to have ffplay availabel in your ffmpeg package, you can view the raw file directly on screen:
ffplay -f rawvideo -c rawvideo -pix_fmt gray -s 1280x720 input.raw
Additionally some image processing software are able to open a raw image file: gimp, photoshop, ...