Question

I’ve already asked this question, but that was about FreeImage. Now I’m trying to do the same thing with ImageMagick (to be more correct, with Magick++).

All I need is to get the RGB value of pixels in an image with the ability to print it out onto the screen. I asked this in the ImageMagick forum, but it seems there is nobody there. 🙁 Can anybody help, please?

Answer 1

Version 6 API

Given an “Image” object, you have to request a “pixel cache“, then work with it. Documentation is here and here:

// load an image
Magick::Image image("test.jpg");
int w = image.columns();
int h = image.rows();

// get a "pixel cache" for the entire image
Magick::PixelPacket *pixels = image.getPixels(0, 0, w, h);

// now you can access single pixels like a vector
int row = 0;
int column = 0;
Magick::Color color = pixels[w * row + column];

// if you make changes, don't forget to save them to the underlying image
pixels[0] = Magick::Color(255, 0, 0);
image.syncPixels();

// ...and maybe write the image to file.
image.write("test_modified.jpg");

Version 7 API

Access to pixels has changed in version 7 (see: porting), but low-level access is still present:

MagickCore::Quantum *pixels = image.getPixels(0, 0, w, h);

int row = 0;
int column = 0;
unsigned offset = image.channels() * (w * row + column);
pixels[offset + 0] = 255; // red
pixels[offset + 1] = 0;   // green
pixels[offset + 2] = 0;   // blue