Re:creation dev log. March-June 2016. Part one. Game stuff and and some awesome Lua scripting

The last few months I was very busy finishing my bachelor’s degree (it’s over, I got an “A”!). But still, I’ve managed to do some cool stuff and improve the engine structure and tools in the last couple of months.

This dev log will be released in two parts. In this part I’m going to talk about game stuff I did and stuff I changed about Lua/C++ integration. In the second part I’ll talk about engine tools I’ve made with ImGui and different refactoring I’ve done.

Game stuff

Here’s one of the latest screenshots of the game. Houses were previously just big sprites, but right now they’re composed out of tiles which lets me reuse them for other houses and build new buildings more quickly.
i2yn8x4

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Using Lua with C++ in Practice. Part3. Controlling entities in Lua by calling C++ functions.

If you haven’t read the first part of the tutorial, I suggest you to read it, so you have a clear idea what’s going on. See the second part for the implementation details.

This article uses LuaBridge and if you’re not familiar with it, I suggest you to check out my tutorials about it. (Pt1, Pt2). If you’re using another Lua/C++ binding: that’s fine, you can totally implement everything I’m talking about with other bindings.

Intro

One of the coolest things in Lua is that you can call C++ functions from it. You can even register your own classes and call their member functions!

Suppose you have an entity class which contains a list of components:

class Entity {
    ...
private:
    std::vector<std::unique_ptr<Component>> components;
};

Suppose that you have a function which lets you get a component by its name.
What if you want to change animations in some Lua script? You can do it like this:

function someLuaFunction(entity)
  ...
    local graphics = entity:getComponent("Graphics")
    if(graphics) then
        graphics:setAnimation("someAnimation")
    end
  ...
end

I believe that this is not a greatest way to do things, because exposing components to Lua is not really necessary. Woudn’t it be nice to hide all the implementation details in C++? Here’s how the code above may look:

function someLuaFunction(entity)
  ...
    entity:setAnimation("someAnimation")
  ...
end

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Using Lua with C++ in practice. Part 2. Implementing the basics

Source code for this article

Hello, this is a second part of “Using Lua with C++ in Practice” series. I recommend to read the first part to know what’s going on and why using Lua with C++ is awesome! (You’ll see it in this arcticle too, of course).

This tutorial will implement a simple ECS model which I’ll improve and use in other parts of tutorials. Note that this is not the best way to implement ECS. I’m making the most basic ECS which can be used to show the main principles of how things work in my game and how you can use them in general. This is not a tutorial about C++, so I won’t spend too much time discussing C++ parts and I’ll mostly focus on Lua/C++ interaction and binding. Let’s start!

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Using Lua with C++. LuaBridge makes everything easier. Variables and functions

Why use Lua?

Using scripts with C++ is great. They provide lots of flexibility and free you from using those .txt files for your configuration files and also let you write complex functions. You can modify your scripts without recompiling. This is really important because you get distracted less often. Even a short compilation can break your concentration.
And you can even design your system that even those people who don’t know how to code can create new scripts or modify existing object behavior without modifying your source code! If you want to know more reasons of what makes Lua great read the beginning of this article.

I wrote about how you can develop your own binding library in previous chapters. But this library is very basic, it doesn’t have the power some other libraries have. It’s really hard to write your own binding because it usually involves lots of templates magic and meta-programming.

I’ve tested lots of libraries and found LuaBridge to be the most awesome. It has MIT license, doesn’t have any dependencies(like Boost which some libraries use) and it doesn’t require C++11. You don’t need to build it. Just drop LuaBridge folder in your project folder, include one header and you’re ready to go!

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Using Lua with C++(Part 3)

Part 1. Why Lua is great and why you should use it. Writing simple wrapper/binding and using Lua for configuration files

Part 2. Getting arrays and calling Lua functions from C++

Part 2.5. Template getArray function and other modifications

Part 3. Calling C++ functions and creating C++ objects

I am very excited to write about this topic because it shows how great Lua is. While Lua is great for configuration files (as I’ve shown in my previous tutorials) it’s even greater when you use it to call C++ functions and you can even create C++ objects classes and modify them with Lua (I’ll cover this in part 4)
Let’s begin.

Note: if you don’t know about how Lua stack works, I recommend you to read this chapter of great “Programming in Lua” book : http://www.lua.org/pil/24.2.html

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Using Lua with C++(Part 2.5)

Part 1. Why Lua is great and why you should use it. Writing simple wrapper/binding and using Lua for configuration files

Part 2. Getting arrays and calling Lua functions from C++

Part 2.5. Template getArray function and other modifications

Part 3. Calling C++ functions and creating C++ objects

This is part 2.5 of my Lua tutorial. Why 2.5 and not 3? Well, because this tutorial doesn’t introduce any new concepts. I’ll point out some bug fixes I made and what improvements can be done. It won’t be too big.

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Using Lua with C++ (Part 1)

You can view full source code of this article here

Russian version of this tutorial here

If you want to see Lua in real world practice, check out Using Lua with C++ in practice series.

 

This article is about building your own  Lua/C++ binding. If you want to use an existing library (which is a lot easier and faster), check out this article : Using Lua with C++: LuaBridge

So, why use LUA?

I’ve seen game devs using different data formats. Some of them use JSON, some use XML, some use plain .txt files etc.. As for me, I use Lua for data, because:

  • It is easy to use without any additional libraries(well, except of lua libraries, of course…)
  • You can use different formulas in your files, for example: some_variable = math.sqrt(2) * 2
  • It’s extremely lightweight and fast
  • It’s under MIT license, so you can use it in any way you want. Just download it and use it
  • It’s ported almost everywhere, because it’s written in C and compiles with almost any C compiler
  • You can use tables to categorize your data which is easy to edit and read

Let’s look at example Lua file:

player = {
    pos = {
         X = 20,
         Y = 30,
    },
    filename = "res/images/player.png",
    HP = 20,
-- you can also have comments
}

With a little class(implementation is below) you can get data in this way:

LuaScript script("player.lua");
std::string filename = script.get("player.filename");
int posX = script.get("player.pos.X");

Pretty neat.

Bindings

You can find lots of bindings here.
But I wanted to write my own, so here it is.

Note: my code is not perfect, but it works. I appreciate your suggestions on how I can improve my code. E-mail me if you find some errors or just to say thanks: eliasdaler@yandex.ru

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