Moonfall

cpp games gpl license lua moonfall sdl tinyxml wxwidgets

Moonfall is the name of a game engine that I spent several months developing back in 2008. It leveraged SDL for graphics, Lua for in-game scripting, and wxWidgets for the built-in developer tools. The cross-platform engine was written using the Code::Blocks IDE, but Visual Studio project files are also available.

The game was partially inspired by Harvest Moon for the Super Nintendo and When They Cry by 07th Expansion. The protagonist is a city dweller, who upon inheriting a small farm from a distant relative, decides to try his hand in agriculture. He finds the residents of the nearby small town a friendly bunch, but something about them does not seem right. Everyone is appears to be hiding something from the player, who must uncover the truth to survive. In short, this is (as I have described it to my friends), “Harvest Moon with Murder.”

Technology

From a technical point of view, the game engine was an exercise in creating a completely data driven game. In retrospect, it was probably over-engineered, but in general it was flexible and I liked what I could do with it. At the core of the engine was the Actor class, the functionality of which could be expanded through add-on properties. The property definitions were specified in XML; for example, below is the definition for the playable character:

<Actor alias = "Player.01" dynamic = "1" layer = "4" thumbnail = "Player.01.Idle.S.01">
    <Properties>
        <Animation />
        <Script resource = "Player.01.Default" />
        <Physics />
    </Properties>
</Actor>

In addition to the self-explainable Animation and Physics properties, you can see a Script node which associates a Lua script with the actor. Scripts would get notifications about events such as OnActorUpdate, during which custom processing could take place:

function OnActorUpdate(elapsed)
    if InputIsKeyTriggered(META_KEY_USE) then
        DoActorUse()
        return
    elseif InputIsKeyPressed(META_KEY_UP) then
        DoActorWalk(DIRECTION_NORTH)
        return
    elseif InputIsKeyPressed(META_KEY_DOWN) then
        DoActorWalk(DIRECTION_SOUTH)
        return
    elseif InputIsKeyPressed(META_KEY_LEFT) then
        DoActorWalk(DIRECTION_WEST)
        return
    elseif InputIsKeyPressed(META_KEY_RIGHT) then
        DoActorWalk(DIRECTION_EAST)
        return
    end

    DoActorIdle(playerDirection)
end

Scripts can directly interact with properties on the parent Actor and can communicate with surrounding world via a messaging system. This made it possible to build any game object imaginable by editing a couple of XML files and some simple scripting.

function DoActorWalk(direction)
    if playerAction == ACTION_WALK and playerDirection == direction then
        return
    end

    ActorPhysicsSetVelocity(playerId, playerWalkVelocities[direction])
    ActorAnimationSet(playerId, playerWalkAnimations[direction])
    ActorAnimationPlay(playerId, true)

    playerDirection = direction
    playerAction = ACTION_WALK
end

function DoActorUse()
    actorsIds = ActorPhysicsQueryShapeCollisions(
        playerId,
        ACTOR_SHAPE_TYPE_COLLISION_INTERACT
    )

    for i, v in ipairs(actorsIds) do
        ActorScriptSendMessage(v, "interact", { user = playerId })
    end
end

I eventually stopped work on Moonfall due to a lack of time, combined with the fact that I am not an artist and could not author the assets required to develop this into a complete game.

Screenshots

Gameplay window Sprite editor Console window Asset editor Map editor

License

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.