How to convert the imagemagick sparse-color Barycentric command line to magick++ code?

Question

I am trying to convert a simple ImageMagick command line to Magick++/c++ code. However it didn't work as expected. The offical website lack of API example. I can not figure out the problem even I dive into the related implement code. And Google doesn't give any hint.

ImageMagick command:

convert -size 543x114 xc: -sparse-color Barycentric '0,0 red 0,%h blue' gradient.png

I want to copy its effect by following code, but the output is inconsistent.

Magick++ code:

#include <Magick++.h>

int main()
{
    int width = 543;
    int height = 114;
    Magick::Image gradImage(Magick::Geometry(width, height), Magick::Color("white"));
    // installed magick++ is built --with-quantum-depth=8
    double maxRGB = 255;

    double args[] = {
        0, 0,
        maxRGB, 0, 0,
        0, height,
        0, 0, maxRGB
    };

    gradImage.sparseColor(
        (Magick::ChannelType)((Magick::DefaultChannels & ~Magick::OpacityChannel) & ~Magick::IndexChannel),
        Magick::BarycentricColorInterpolate,
        sizeof(args) / sizeof(double),
        args
    );

    gradImage.write("gradient.png");
}

CMakelist.txt:

cmake_minimum_required(VERSION 3.16)

project(demo)
find_package(ImageMagick COMPONENTS Magick++)
include_directories(
    ${ImageMagick_INCLUDE_DIRS}
)

add_executable(
    demo
    main.cpp
)

target_link_libraries(
    demo
    ${ImageMagick_Magick++_LIBRARY}
)

generated by command:

generated by magick++:

Other Info:

• OS: Ubuntu 20.04
• ImageMagick: 7.1.1 (built from source: ./configure --with-quantum-depth=8 && make)

Any help is appreciated!

I want the magick++ code have same effect with the command line

Answer 1

You've almost got it! It might be worth peaking at the source code for GenerateCoefficients to read the expected arguments, and SparseColorOption to see how the CLI options get parsed.

• The R, G, B values should be in a range between 0.0 & 1.0.
• You'll need at least 3 points, as the coefficients for Barycentric are the same as Affine.
• If you're not targeting color traits / masks, just use DefaultChannels.

Try the following...

#include <Magick++.h>

int main(int argc, char **argv)
{
  double
    width = 543.0,
    height = 114.0;
  Magick::InitializeMagick(*argv);
  Magick::Geometry rect((int)width, (int)height);
  Magick::Image gradImage(rect, Magick::Color("white"));
  Magick::ColorRGB color1("red");
  Magick::ColorRGB color2("blue");
  double args[15] = {
        0,      0, color1.red(), color1.green(), color1.blue(),
    width,      0, color1.red(), color1.green(), color1.blue(),
        0, height, color2.red(), color2.green(), color2.blue()
  };
  gradImage.sparseColor(
    Magick::DefaultChannels,
    Magick::BarycentricColorInterpolate,
    15,
    args
  );
  gradImage.write("gradient.png");
}

Don't forget to call Magick::InitializeMagick() first thing!