create dimmed icon with magick++ (ImageMagick)

I’m trying to use ImageMagick for what I believed would be a simple task. I want to generate a dimmed version of an image/icon at runtime in the application. I use the magick++ c plus api.

I have found some commands that give me an ok result when run from the command line. Converting the commands to the c++ api was a bit challenging, and the result is then not as hoped.

// Command example
convert -size 32x32 xc:"#999999" gray32.png
composite -dissolve 50% logo32.png gray32.png dim_logo32.png

How would this look in c++? I came up with this.

Magick::Image gray;
gray.size( Magick::Geometry(image.columns(), image.rows()));
gray.read( "xc:#999999");
gray.label( "gray" );

if(gray.isValid()) {
gray.opacity(QuantumRange/2);
image.composite(gray, Magick::Geometry(image.columns(),image.rows()), Magick::DissolveCompositeOp );

But the transparency in the picture is lost.

A other suggestion as to make a dimmed image, is to make the full image semi transparent.

convert input.png -channel Alpha -evaluate Set 50% output.png

This could work. The transparency is kept when I tried this from command line. Changing this to c++ api confused me a lot.

I ended up with this single line.

image.opacity(QuantumRange/ 2);

Now the r开发者_StackOverflow社区esult from this confuses me. The image is semi transparent, but the background that was originally transparent is now magenta.

I was not allowed to post the images, to show the result. :(