In this page, you’ll be given some pointers on how to start contributing to the Minecraft-Overviewer project. This is useful for people who want to help develop the Overviewer, but don’t quite know where to start.

This page is mostly focused on where to look for things and how to get your changes back into the project, for help on how to compile the Overviewer, check Building.


Ideally you’re familiar with Python (Overviewer uses Python 2), and know the basics of Git. Both have various very good resources online that help you in learning them, but the best way of learning is always to use them in the real world, so don’t hesitate to jump right in after having a basic grasp and ask questions along the way.

Additionally, some parts of Overviewer are written in C, though unless you’re interested in the drawing and compositing routines or the rendermodes, you don’t need to know C.

Last but not least, some of the Overviewer’s code is written in JavaScript, namely the part that runs in your browser when you view the map.

Acquiring the Source Code

First, you’ll need to get the Overviewer source code. We do version management of code through Git, which allows multiple people to work on the code at the same time. Naturally, this means you’ll also be getting the source code through Git. For this to work, you’ll have to install Git on your computer.

Our source code is hosted on GitHub, so it’s a good idea to make an account there if you don’t already have one.

This page won’t go into the details of how to use Git, but it’ll give you some advice on how your workflow should be to avoid some trouble.

Finding Your Way around the Code Base

At first glance, all the code can be a bit overwhelming. So here’s a quick overview of the important parts.

  • is the build script. If you need to make any changes to how the Overviewer is built, you’ll want to look there.
  • is the entry-point of the application. It imports all the other functionality, and does the command line parsing.
  • overviewer_core/ is the directory where the vast majority of the Overviewer’s functionality is. More on that below.
  • overviewer_core/aux_files/ is where the genPOI functionality is implemented. If you’re looking into changing the way markers are generated, look there.
  • overviewer_core/src/ is the directory for all the files that are part of Overviewer’s C extension. This includes things such as rendermodes, which are stored in the primitives sub-directory.
  • overviewer_core/data/ mostly contains the parts that make up Overviewer’s web front-end, with js_src containing the JS files and web_assets containing the index.html, CSS files and image files such as icons or the compass.
  • docs/ contains the documentation, which can be built with the included Makefile if you have sphinx installed.


Let’s take a closer look at the overviewer_core/ directory:

  • controls how the HTML and JS output are written out, as well as the overviewerConfig.js format.
  • implements a Least-Recently-Used (LRU) cache, which is used for caching chunks in memory as the rendering happens.
  • contains some code that sets up how the config is parsed, but is not really involved in the definitions of individual settings therein.
  • is the code that sets up multiprocessing, so Overviewer can use all available CPU threads on a machine.
  • implements helpful routines which allow you to determine whether some file operations such as replacing a file work in a given directory, and also implements the FileReplacer class which can then safely replace a file given the capabilities of the filesystem.
  • is a remnant of the past and entirely unused.
  • sets up and implements Overviewer’s logging facilities.
  • contains the code that is used to parse the Minecraft NBT file structure.
  • defines all the observers that are available. If you want to add a new observer, this is the place where you’ll want to look.
  • defines all the optimizeimg tools and how they’re called.
  • implements the fancy progress bar that the Overviewer has.
  • implements an rcon client for the Minecraft server, used by the RConObserver.
  • contains definitions and glue code for the rendermodes in the C extension.
  • includes all definitions for the Overviewer configuration file. If you want to add a new configuration option, this is where you’ll want to start.
  • contains validation code for the settings definitions, which ensures that the values are all good.
  • is multiprocessing communication code. Scary stuff.
  • contains all the block definitions and how Overviewer should render them. If you want to add a new block to the Overviewer, this is where you’ll want to do it. Additionally, this code also controls how the textures are loaded.
  • contains code that maps a render dict entry to the output tiled image structure.
  • contains random utility code that has no home anywhere else.
  • is a whole lot of code that does things like choosing which chunks to load and to cache, and general functionality revolving around the concept of Minecraft worlds.


The documentation is written in reStructuredText, a markup format. It can be compiled into an HTML output using the Makefile in the docs/ subtree by typing make. You’ll need to have sphinx installed for this to work.

The theme that will be used in the locally generated HTML is different than what is used on However, it should still be sufficient to get a good idea of how your changes will end up looking like when they’re on the main docs page.

Code Style

To be honest, currently the Overviewer’s codebase is a bit of a mess. There is no consistent code style in use right now. However, it’s probably a good idea to stick to PEP8 when writing new code. If you’re refactoring old code, it would be great if you were to fix it to make it PEP8 compliant as well.

To check whether the code is PEP8 compliant, you can use pycodestyle. You can easily install it with pip by using pip2 install pycodestyle.

Example Scenarios

This section will demonstrate by example how a few possible contributions might be made. These serve as guidelines on how to quickly get started if you’re interested in doing a specific task that many others before you have done too in some other form.

Adding a Block

Let’s assume you want to add support for a new block to the Overviewer. This is probably one of the most common ways people start contributing to the project, as all blocks in the Overviewer are currently hardcoded and code to handle them needs to be added by hand.

The place to look here is It contains the block definitions, which are assisted by Python decorators, which make it quite a bit simpler to add new blocks.

The big decorator in question is @material, which takes arguments such as the blockid (a list of block IDs this block definition should handle), and data (a list of possible data values for this block). Additionally, it can also take various additional arguments for the different block properties, such as solid=True to indicate that the block is a solid block.

Simple Solid 6-Sided Block

A lot of times, new blocks are basically just your standard full-height block with a new texture. For a block this simple, we don’t even really need to use the material decorator. As an example, check out the definition of the coal block:

block(blockid=173, top_image="assets/minecraft/textures/blocks/coal_block.png")

Block with a Different Top

Another common theme is a block where the top is a different texture than the sides. Here we use the @material decorator to create the jukebox block:

@material(blockid=84, data=range(16), solid=True)
def jukebox(self, blockid, data):
    return self.build_block(self.load_image_texture("assets/minecraft/textures/blocks/jukebox_top.png"), self.load_image_texture("assets/minecraft/textures/blocks/noteblock.png"))

As you can see, we define a method called jukebox, taking the parameters blockid and data, decorated by a decorator stating that the following definition is a material with a blockid of 84 and a data value range from 0 to 15 (or range(16)), which we won’t use as it doesn’t affect the rendering of the block. We also specify that the block is solid.

Inside the method, we then return the return value of self.build_block(), which is a helper method that takes a texture for the top and a texture for the side as its arguments.

Block with Variable Colors

Occasionally, blocks can have colors stored in their data values. includes an easy mapping list, called color_map, to map between data values and Minecraft color names. Let’s take stained hardened clay as an example of how this is used:

@material(blockid=159, data=range(16), solid=True)
def stained_clay(self, blockid, data):
    texture = self.load_image_texture("assets/minecraft/textures/blocks/hardened_clay_stained_%s.png" % color_map[data])

    return self.build_block(texture,texture)

As you can see, we specify that the block has 16 data values, then depending on the data value we load the right block texture by looking up the color name in the color_map list, formatting a string for the filename with it.

Good Git Practices

How you structure your Git workflow is ultimately up to you, but here are a few recommendations to make your life and the life of the people who want to merge your pull requests easier.

  • Commit your changes in a separate branch, and then submit a pull request from that branch. This makes it easier for you to rebase your changes, and allows you to keep your repository’s master branch in-sync with our master branch, so you can easily split off a new branch from master if you want to develop a new change while your old change still isn’t merged into the master.
  • Format your commit messages properly. The first line should be a 50 character long summary of the change the commit makes, in present tense, e.g. “Add a spinner to the progress bar”. This should be followed by a blank line, and a longer explanation of the change the commit actually does, wrapped at 72 characters.
  • Don’t merge master into your branch. If you plan on submitting a change as a pull request and the master branch has moved in the meantime, then don’t merge the master branch into the branch of your pull request. Instead, rebase your branch on top of the updated master.
  • Keep commits logically separated. Don’t try to cram unrelated changes into just one commit unless it’s a commit full of small fixes. If you find yourself struggling to keep the commit summary below 50 characters, and find yourself using the word “and” in it, rethink whether the changes you’re making should be just one commit.

It’s also a good idea to look at the output of git diff before committing a change, to make sure nothing was unintentionally changed in the file where you weren’t expecting it. git diff will also highlight blank lines with spaces in them with a solid red background.

Talking with other Developers

Occasionally, the issue tracker simply doesn’t cut it. You need to talk with another developer, maybe to brainstorm a new feature or ask a question about the code. For this, we have an IRC channel on freenode, which allows you to talk with other developers that are on the IRC channel in real-time.

Since most developers have jobs or are in college or university, it may sometimes take a few moments to get a reply. So it’s useful to stick around and wait for someone who can help you to be around.