How to read images using C-CodePudding

is there a way to read image pixels or any data considering the image using FILE fopen in C? I'm not interested in libraries. I tried to read the image data with fgetc but it prints out some numbers which don't make any sense.

CodePudding user response：

I don't quite understand what you want to do, but C language is a low level programming language. Of course, there will be nothing but numbers in memory.

The easiest way to store an image in C is creating a matrix structure and keeping the RGB values. The numbers you mention can be the RGB values of each pixel.

CodePudding user response：

If you really want to be able to read and write images, simply, via a FILE pointer, you'll want to look at a file format called a .ppm. (Portable Pixel Map.) This assumes, of course, that you can convert your image files to a .ppm, but that is easily achievable with the imagemagick command-line tool. You can use imagemagick to convert the .ppms you write to any output type you like.

The format is insanely simple:

Starts with the two character (the "magic number") "P6"
The P6 is followed by a whitespace, then the width (in ascii) of the image.
The width is followed by a whitespace, then the height (in ascii) of the image.
The last bit of the header is the "maximum value" of an entry. Just use 255. Follow this with a whitespace.
After that, you just write, in binary, RGB values. One unsigned char per channel.

Example:

P6 128 128 255
[128 * 128 * 3 bytes of data go here, in row major order. The top-left 
 pixel is first, then the pixel to the right of it. When you get to the 
 end of a row, just write the first pixel of the next row. That's all 
 there is. No other header info, no terminators, etc.]

Example:

#include <stdio.h>
void main() {
  FILE * out;
  out = fopen("color_test.ppm", "wb");
  fprintf(out, "P6 256 256 255\n");
  for(int r=0; r<256; r  ) {
    for(int b=0; b<256; b  ) {
      fputc(r, out);
      fputc(0, out);
      fputc(b, out);
    }
  }
  fclose(out);
}

This generates:

Reading files is similarly easy. You can just pull them into a 3d array of unsigned chars, manipulate them in memory however you like, then write that data back out to another file much as my example does here.