Answering my own question

I spent two days digging through Node.js graphics libraries.

node-canvas

• I tried it first since I'm quite familiar with <canvas> API. It's a huge plus for a library.


• it requires Cairo which doesn't have an easy Windows download. I found it in GTK+ distribution though.


• moreover it needs native library binding code to be compiled on module installation. It uses Node-Waf which hasn't being ported to Windows yet.

gm

• mature


• runs on Windows smoothly


• docs are ok but not thorough: I had to look up into source code to figure out what API is available


• unfortunately there's no easy way to combine images with gm. Maybe there's some way to achieve that but I haven't found one after two hours spent with it.

node-imagemagick

• The official repo has very few basic ImageMagick commands covered but I used this fork (good thing that NPM can pull libraries directly from git repositories). It has bindings for montage which does exactly what I need.


• ImageMagick is quite slow, though it works on Windows.

Node-Vips

• Huge plus: it uses an incredible VIPS library which I'm familiar with. VIPS is very fast and optimized for large images. It's very smart about utilizing hardware resources: if your machine has a lot of RAM it'll do all processing in memory but will switch to hard-drive caches if memory is scarce or required for other applications.


• same as node-canvas it requires Node-Waf so it's not available for Windows yet.

I also looked at other libraries from the list but most of them are either very immature or do not suit my use case. I would really like to try migrating to either Node-Canvas or Node-Vips when Node-Waf gets ported to Windows but until then I'll stick to node-imagemagick.