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Assetman is a Ninja generator that lets you use wildcard/glob patterns. This is useful when building asset files such as images and audio because there are often many asset additions and deletions.

Assetman is a build configure tool similar to CMake, but only works with Ninja by generating files. Assetman has a focus on building/converting assets such as images and audio, and adds wildcard/glob pattern features to support workflows related to asset production. Assetman is able to produce files that re-generate themselves when files are added or removed from the source directory, something that CMake does not support.

Getting Started

Install assetman with npm install -g assetman and install ninja, there are instructions on the website.

In the source asset folder create a assets.js configure script that specifies which files need to be built and how to build them. Then in the asset build folder run the following command:

assetman ../path/to/asset/source

This will generate a file in the current directory. Running ninja in the command line will then build your files.


For this example we will be using a file structure that looks like this:

│  ├──sprites
│  │  ├──character.psd
│  │  └──bullet.psd
│  └──background.psd

The first step is to create an assets.js in the source directory. In our case that is the raw_assets folder.

To start off we want to convert only to convert the background.psd file in raw_assets to a background.png file in the assets folder. We will be using ImageMagick's convert tool to build out .psd into a .png.

To do that add the following to your assets.js file:

rule('convert').command('convert $in[0] $out');

The first line says to create a rule named "convert" that runs the command convert $in[0] $out. The second line says to take background.psd as an input file and produce background.png as an output file using the rule convert. The words $in and $out are ninja variables that are replaced with the input and output files respectively. The .single() command only ever produces one input file and one output file so the resulting command becomes:

convert ../raw_assets/background.psd[0] background.png

Note: the '[0]' sequence is used by the convert command, telling it to flatten all of the layers in the psd.

Since the .single(pattern) command only ever has one input and one output, we can infer the filename for the output. The filename without an extension is then stored in the variable $filename which can be used in the .to() command:


There is also a shortcut command, .toExt(), that uses the inferred filename and lets you specify an extension/suffix:


Configure and build

Now in the build directory, in our case the assets folder, run the following command:

assetman ../raw_assets

This tells assetman to run the assets.js configure script found in the raw_assets folder and generate a file in the current directory. This file is used by the Ninja build system; running ninja in the assets folder will execute the final build step producing background.png.

You only have to run the assetman command once. Ninja will be able to detect any changes to the assets.js file, and re-generate itself.


Sometimes you may want to use the same rule to convert/build all files that match a specific wildcard pattern, also known as globbing.

rule('convertHalf').command('convert $in[0] -resize 50% $out');

Here we have a rule that converts images to half their original size. The second line says to look for all the .psd files in the sprites folder and output them with the .png extension using the convertHalf rule. The resulting commands would look like this:

convert ../raw_assets/sprites/character.psd[0] -resize 50% sprites/character.png
convert ../raw_assets/sprites/bullet.psd[0] -resize 50% sprites/bullet.png

The file that assetman produces is able to recognize file additions and deletions in the source directory and re-generates itself, so there is no need to run asssetman again.

Note: To match all of the .psd files in the source directory you could use the pattern **/*.psd.


In some situation you may need multiple input files and/or multiple output files. The common scenario is building atlases from sprite images. For us, this also poses the issue of building from files that need to be built. First we need to build our .psd files into .png files. Then we need to combine the .png files into an atlas.

rule('convert').command('image-convert $in[0] $out');
rule('atlas').command('binpack $in -o $name');
  .to(['atlas.png', 'atlas.csv'])
  .assign('name', 'atlas')

This tells assetman to build the .psd files in the sprites folder into .png files. Then it it says to take all of the .png files in the sprites folder in the build directory and produce a atlas.png file and a atlas.csv file.

The .fromBuild(true) command says that the files we are searching for are in the build directory, not the source directory. The bundle(...).to(...) command does not support the $filename variable because it is possible to have more then one input file.

The .assign('name', 'atlas') command tells assetman to create a variable called name with the value atlas. The $name variable is then used by our rule.

The resulting command would look something like this:

binpack sprites/character.png sprites/bullet.png -o atlas


Copyright (c) 2013 Tylor Reynolds Licensed under the MIT license.