CImg error : Failed to recognize format of file "xxx.png"

Question

I've been looking for a code in C++ which gets me the RGB values of all pixels of a png file.

So I searched a lot through the internet and found different ways of doing such a thing, like using libpng , OpenCV, Magick++, CImg , etc. and I just decided to go with the CImg one (mostly because I don't know how to use OpenCV although I searched a lot about how to install and use it in Code::Blocks and it just has something to do with mingw64 which the installer I downloaded from Sorceforge, keeps giving me errors in the middle of downloading some files, and about the rest of them I just can't find the appropriate header file (like Magick++.h) which's compatible with Code::Blocks... So if you think some of these would be better and simpler than CImg.h, then I'd appreciate if you provided me a download link of the header or any other necessary thing)

So here is the code I found with Cimg :

#include <windows.h>
#include <iostream>
using namespace std;

#include <CImg.h>
using namespace cimg_library;

int main()
{
CImg<unsigned char>src("twittericon.png");
int width = src.width();
int height = src.height();
cout << width << "x" << height << endl;
for (int r = 0; r < height; r++)
for (int c = 0; c < width; c++)
   cout << "(" << r << "," << c << ") ="
        << " R" << (int)src(c,r,0,0)
        << " G" << (int)src(c,r,0,1)
        << " B" << (int)src(c,r,0,2) << endl;
return 0;
}

But the problem is after running the program, it gives me the error : CImg::load(): Failed to recognize format of file 'twittericon.png' 

What should I do with the error?

And by the way, do you know any better way to read these RGB values of a png file? If you do, could you please also provide me a little bit of an instruction for it? Thanks.

Answer 1

There are many ways to get the RGB pixel values in a PNG file, and the best one depends what your actual usage scenario is.

---

Option 1

If you have one or two PNG files and you just want to get the RGB pixels easily one time, then I would suggest you install any of the ready-built ImageMagick binaries you have already given links for. Then we can make a test PNG file like this at the command line in Terminal or Command Prompt:

convert xc:red xc:lime xc:blue +append \( xc:cyan xc:magenta xc:yellow +append \) -append image.png

and it will look like this (just smaller):

(I think you omit the backslashes under Windows in the above command)

Ok, now you want to see the RGB values, so you can either dump the file as text:

convert image.png -depth 8 txt:

Sample Output

# ImageMagick pixel enumeration: 3,2,65535,srgb
0,0: (65535,0,0)  #FF0000  red
1,0: (0,65535,0)  #00FF00  lime
2,0: (0,0,65535)  #0000FF  blue
0,1: (0,65535,65535)  #00FFFF  cyan
1,1: (65535,0,65535)  #FF00FF  magenta
2,1: (65535,65535,0)  #FFFF00  yellow

Or, you can convert the file to the venerable (but superb) NetPBM format Wikipedia description of NetPBMin ASCII mode:

convert image.png -compress none image.pnm

Now you get a file like this that you can easily read:

P3                              # PPM in ASCII mode
3 2                             # width=3 height=2
255                             # 8-bit
255 0 0 0 255 0 0 0 255 
0 255 255 255 0 255 255 255 0 

If you omit the -compress none you will get the same file but the RGB data will be in 8-bit unsigned binary and then the whole file is very easy to read in C/C++. In fact, CImg knows how to read/write these anyway.

So, you could feasibly, convert any images (TIF, BMP, JPEG, GIF) you want to access into NetBM PPM files in advance and just read and write PPM files using the built-in routines in CImg. Job done!

---

Option 2

At the next level, you could install ImageMagick from the pre-built binaries on the ImageMagick website and then use CImg. CImg will then locate ImageMagick's convert/magick program use that to convert anything it can't read itself on its behalf. That way you get instant access to around 200+ formats that ImageMagick knows about - for free.

---

Option 3

Alternatively, you could build and install libpng and then when you write code with CImg you define a macro before including CImg.h and it uses libpng to read and write PNG files.

That is quite a lot of trouble to go to when you could just let ImageMagick do all the work, and at the end of it you only get PNG support not 200+ formats for all that work.

This scenario looks like this:

#define cimg_use_png 1
#include "CImg.h"
...
...

You would compile with:

c++ -I CIMG_DIRECTORY -L LIBPNG_DIRECTORY -lpng -lz

I did another answer, using this option for libjpeg and getting CIMg to read and write JPEGs. I can only say it was a total pain - that answer is here.