How to control output bit depth in graphicsmagick (for Node)? - node.js

Question

So I have two PNG images, both non-transparent 24bpp.
One image contains a rainbow, other one contains a single line of text:
I do the same thing with both of them:
var gm = require('gm').subClass({imageMagick: true})
gm("./sources/source.png").bitdepth(24).write("test.png", function(){
console.log("test.png")
});
gm("./sources/source2.png").bitdepth(24).write("test2.png", function(){
console.log("test2.png")
});
where gm is this
And I set both to 24bpp explicitly
In result I have two images with different bit depth:
In some cases I also had 32bpp image.
How can I make it create only 24bpp image (discard alpha channel if needed).
Also, I don't want to create jpgs.
Thanks to #mark-setchell, I could force bit depth. I did it this way in Node:
gm("./sources/source.png")
.out("-define")
.out("png:color-type=2")
.write("test.png", function(){
console.log("test.png")
});
out() is an undocumented method but it basically helps you add custom parameters to commandline. Notice that
.out("-define png:color-type=2")
won't work, it only works if you pass each parameter in individual .out() call
.bitdepth(24) doesn't seem to affect output at all, probably because I did .subClass({imageMagick: true}) above.

Answer 1

My suggestion is to try using -define to set the variable png:color-type=2. As you worked out, and kindly shared with the community, it is done as follows:
gm("./sources/source.png")
.out("-define")
.out("png:color-type=2")
.write("test.png", function(){
console.log("test.png")
});