Read metadata from a PNG file with imagemagick++ - imagemagick

Question

I want to do this (as subject indicates) using the imagemagick++ API. I specifically want to read the focal length used by the (DSLR camera) to shoot. [EDIT] This is part of a program that manipulates the images [END EDIT]
I know I can do it through a system call, invoking the command identify -verbose <image_filename> (e.g., with popen so that I can read the output and look for the line containing FocalLengthIn35mmFilm). But this is gross for two reasons:
It is just gross and inefficient.
identify is an ImageMagick utility, so if that command can do it, then I should be able to do it using ImageMagick's API.
Notice that, as the subject says, I am working with PNG files; however, a solution that could work with any lossless image format would be preferred.

Answer 1

ImageMagick is not a EXIF editor, so the best option is to dump the profile EXIF payload into libexif API. But if you are just peaking at a value, you can use ImageMagick's percent escape mechanics to retrieve a value.
For example:
#include <iostream>
#include <string>
#include <Magick++.h>
using namespace Magick;
int main()
{
InitializeMagick((const char *)nullptr);
Image img("DSCF2763.JPG");
Blob data;
MagickCore::SetImageOption(img.imageInfo(), "format", "%[EXIF:FocalLengthIn35mmFilm]");
img.write(&data, "INFO");
std::string value((const char *)data.data(), data.length());
std::cout << value << std::endl;
}
The above is the same as the CLI command:
convert DSCF2763.JPG -define "format=%[EXIF:FocalLengthIn35mmFilm]" info:
Update
An even easier way would be to use Magick::Image.attribute()
#include <iostream>
#include <string>
#include <Magick++.h>
using namespace Magick;
int main()
{
InitializeMagick((const char *)nullptr);
Image img("DSCF2763.JPG");
std::string value = img.attribute("EXIF:FocalLengthIn35mmFilm");
std::cout << value << std::endl;
}