Gamedev from scratch 1: Scaffolding / fuzzy notepad
Welcome to half 1 of this narrative sequence about writing a whole online game from scratch, utilizing the PICO-8. That is truly the second half, as a result of on this home (in contrast to Lua) we index from 0, so in case you’re new right here chances are you’ll wish to seek the advice of the introductory stuff and desk of contents in part zero.
Should you’ve been following alongside, welcome again, and let’s dive proper in!
Up to now, I’ve… this. Which is one thing, and positively rather more than nothing, however all advised not a lot.
Most conspicuously, that is going to be a platformer, so I want gravity. The issue with gravity is that it means issues are at all times shifting downwards, and if there’s nothing to cease them, they’ll proceed off indefinitely into the void.
What I’m attempting to say right here is that I really feel the looming spectre of collision detection hanging over me. I’m going to wish it, and I’m going to wish it actual quickly.
And, hey, that sucks. Collision detection is an actual huge ache within the ass to write down, so needing it this early is a hell of a giant spike within the studying curve. Fortunately for you, another person has already written it: me!
Earlier than I can get to that, although, I want so as to add some construction to the code I’ve to this point. Every thing I’ve written is designed to work for Star Anise and solely Star Anise. That’s completely high quality when he’s the one factor within the recreation, however I don’t anticipate he’ll keep alone for lengthy! Collision detection specifically is a reasonably main part of a platformer, so I undoubtedly need to have the ability to reuse it for different issues within the recreation. Additionally, collision detection is a giant fucking furry mess, so I undoubtedly need to have the ability to shove it in a nook someplace I’ll by no means have to have a look at it once more.
An excellent begin can be to construct in direction of having a nook to shove it into.
As of the place I left off final time, my particular _update() and _draw() capabilities are largely stuffed with code for updating and drawing Star Anise. That doesn’t actually sit proper with me; as the primary entry factors, they need to be about updating and drawing the sport itself. Star Anise is a part of the sport, however he isn’t the entire recreation. All that code that’s particular to him must be delay in just a little field someplace. Cats like to be in little packing containers, you see.
This raises the query of how I wish to construction this mission on the whole. And, I notice: structuring a software program mission is onerous, and also you solely actually get a superb sense of how one can do it from expertise. I’m nonetheless undecided I have a superb sense of how one can do it. Hell, I’m not satisfied anybody has a superb sense of how one can do it.
Fortunately, it is a recreation, so it’s fairly apparent how one can break it into items. (The tradeoff is that every little thing in a recreation finally ends up entangled with every little thing else regardless of the way you construction it, alas.) Star Anise is a separate factor within the recreation, so he would possibly as properly be a separate factor within the code. Afterward I’ll want some extra summary structuring, however as an especially tough guideline: if I can provide it a reputation, it’s a superb candidate to be made right into a factor.
However what, precisely, is a factor in code? Mostly (however not at all times), a factor is carried out with what’s referred to as an object — just a little bundle of knowledge (what it is) with code (what it may well do). I have already got each of those elements for Star Anise: he has information like his place and which method he’s dealing with, and he has code for doing issues like updating or drawing himself. An incredible first step can be to extract that stuff into an object, after which another construction would possibly reveal itself.
I do must do one factor earlier than I can flip get to that, although. You see, Lua is without doubt one of the few languages in frequent use at this time that doesn’t fairly have built-in help for objects. As a substitute, it has all of the constructing blocks you must craft your personal system for making objects. On the one hand, the best way it does that could be very slick and intelligent. Alternatively, it means you possibly can’t write a lot Lua with out cobbling collectively some arcane nonsense first, and likewise nobody’s code fairly works the identical method.
Which brings me to the next magnificent monstrosity:
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How does this work? What does this imply? What is a prototype chain, anyway? Dearest reader: it extraordinarily doesn’t matter. Nobody cares. I must stare at this for ten minutes to even start to elucidate it. Each line is oozing with subtlety. To be trustworthy, regardless that I describe this sequence as “from scratch”, this is without doubt one of the only a few issues that I copy/pasted wholesale from an earlier recreation. I do know this does the naked minimal I want and I completely don’t wish to waste time reinventing it incorrectly. To drive that time dwelling: I wrote collision detection from scratch, however I copy/pasted this. (However in case you actually wish to know, I’ll clarify it in an appendix.)
Be at liberty to repeat/paste mine, in case you like. You may as well discover a variety of tiny Lua object methods floating round on-line, however with tokens at a premium, I wished one thing microscopic. This principally does constructors, inheritance, and nothing else.
(Oh, I don’t suppose I discussed, however the -- prefix signifies a Lua remark. Feedback are ignored by the pc and have a tendency to include notes which can be useful for people to comply with. They don’t rely in opposition to the PICO-8 token restrict, however they do rely in opposition to the full dimension restrict, alas.)
The upshot is that I can now write stuff like this:
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This creates a… properly, terminology is hard, however I’ll name it a sort whereas doing air-quotes and glancing behind me to see if any Haskell programmers are listening. (It’s not very like the notion of a kind in lots of different languages, however it’s the closest I’m going to get.) Now I can mix an x- and y-coordinate collectively as a single object, a single factor, with out having to juggle them individually. I’m calling that type of factor a vec, quick for vector, the identify mathematicians give to a set of coordinates. (Kind of. That’s not fairly proper, however don’t fear about it but.)
After the above incantation, I can create a vec by calling it like a operate. Notice that the arguments finally arrive in vec:init, loosely referred to as a constructor, which shops them in self.x and self.y — the place self is the vec being created.
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That iadd factor is a technique, a particular operate that I can name on a vec. It’s like each vec carries round its personal little bag of capabilities wherever it seems — and since they’re particular to vec, I don’t have to fret about reusing names. (In actual fact, reusing names will be very useful, as we’ll see later!)
The identify iadd is (very!) quick for “in-place add”, suggesting that the primary vector provides the second vector to itself relatively than creating a brand new third vector. That’s one thing I anticipate to be doing rather a lot, and making a technique for it saves me some valuable tokens.
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Lastly, these humorous __add and __sub strategies are particular to Lua (if enchanted accurately, which is a part of what the obj gobbledygook does) — they let me use + and - on my vecs similar to they have been numbers.
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That is the core concept of objects. A vec has some information — x and y — and a few code — for including one other vec to itself. If I later uncover some new factor I desire a vec to have the ability to do, I can add one other technique right here, and it’ll be obtainable on each vec all through my recreation. I can repeat myself just a little bit much less, and I can maintain these associated concepts collectively, separate from every little thing else.
Get the essential jist? I hope so, as a result of I’ve actually gotta get a transfer on right here.
Now that I’ve a option to outline objects, I can flip Star Anise into one.
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What a mouthful! However for probably the most half, this is identical code as earlier than, simply rearranged. For instance, the brand new anise:draw() technique has principally been lower and pasted from my previous _draw() — all besides the cls() name, since that has nothing to do with drawing Star Anise.
I’ve mixed the px and py variables right into a single vector, pos (quick for “place”), which I now need to consult with as self.pos — that’s so PICO-8 is aware of whose pos I’m speaking about. In any case, it’s theoretically doable for me to create a couple of Star Anise now. I gained’t, however PICO-8 doesn’t know that!
A Star Anise object is created and assigned to participant when the sport begins, after which _update() calls participant:replace() and _draw() calls participant:draw() to get the identical results as earlier than.
I did make one reasonably dramatic change on this code. The wordy code I had for studying buttons has turn into rather more compact and inscrutable, and the shifting variable is gone. An enormous a part of the rationale for that is that I contemplate Star Anise’s motion to be a part of himself, however studying enter to be a part of the recreation, so I wished to separate them up. Meaning shifting is a bit awkward, since I beforehand up to date it as a part of studying enter. As a substitute, I’ve turned Star Anise’s motion into one other vector, which I set in _update() utilizing this mouthful:
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The b2n() operate turns a button right into a number, and I solely use it right here. It turns true into 1 and false into 0. Consider it as measuring “how a lot” the button is held down, from 0 to 1, besides in fact there can’t be any reply within the center.
Unpacking {that a} bit additional, b2n(btn(➡️)) - b2n(btn(⬅️)) means “how a lot we’re holding proper, minus how a lot we’re holding left”. If the participant is just holding the best button, that’s 1 – 0 = 1. In the event that they’re solely holding the left button, that’s 0 – 1 = -1. In the event that they’re holding each or neither, that’s 0. The outcomes are the identical as earlier than, however the code is smaller.
As soon as Star Anise’s transfer is about, the remaining works equally to earlier than: I replace left based mostly on horizontal motion (however depart it alone when there isn’t anyway), I alter his place (now utilizing :iadd()), and I exploit the stroll animation when he’s shifting in any respect. And that’s it!
I like to make use of the time period “actor” to consult with a definite factor within the recreation world; it conjures an enthralling and concrete picture of assorted characters acting on a stage. I believe I picked it up from the Doom supply code. “Entity” is extra frequent and is used closely in Unity, however will be confused with an “entity–part–system” setup, which Unity additionally helps. After which there are heretics who consult with recreation issues as “objects” regardless that that’s additionally a programming time period.
This code is a high quality begin, however it’s not fairly what I need. There’s nothing right here truly referred to as an actor, for starters. My setup nonetheless solely works for Star Anise!
I’d higher repair that. The notion of an “actor” is fairly imprecise, so a generic actor gained’t do a lot by itself, however it’s good to outline one as a template for a way I anticipate actual actors to work.
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How does a clean actor replace or draw itself? By doing nothing.
(I do assume that each actor has a place; this will likely not essentially be the case in video games with very broad concepts about what an “actor” is, however it’s cheap sufficient for my functions.)
Now, to hyperlink this with Star Anise, I’ll have anise inherit from actor. Meaning he’ll turn into a specialised type of actor, and specifically, all of the strategies on actor will even seem on anise. Chances are you’ll discover that anise was beforehand a specialised type of obj (like actor and vec) — in actual fact, the one purpose I can name vec(x, y) like a operate is that it inherits some magic stuff from obj. Shock!
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I can now delete anise:init(), because it’s an identical to actor:init(). I nonetheless have anise:replace() and anise:draw(), which override the strategies on actor, so these don’t want altering.
Every thing nonetheless solely works for Star Anise, however I’m getting nearer! I solely want another change. As a substitute of getting solely participant, I’ll make a checklist of actors.
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This does just about what it reads like. The add() operate, particular to PICO-8, provides an merchandise to the top of a listing. The all() operate, additionally particular to PICO-8, helps undergo a listing. And the for blocks imply, for every factor on this checklist, run this code.
Now, eventually, I’ve one thing that might work for actors apart from Star Anise. All I must do is outline them and add them to the actors checklist, and so they’ll robotically be up to date and drawn, similar to him!
Admittedly, this hasn’t gotten me wherever concrete. The sport nonetheless performs precisely the identical because it did after I began. I’m betting that I’ll finally have a couple of actor, although, so I’d as properly lay the groundwork for that now whereas it’s straightforward. It doesn’t take a lot effort, and I discover that if I give myself little early inroads like this, it seems like much less of a slog to later come again and broaden on the concepts. That is the form of factor I meant by extra construction revealing itself — as soon as I’ve one actor, a pure subsequent step is to permit for a number of actors.
I’ve put it off lengthy sufficient. I can’t keep away from it any longer. Nevertheless it’s difficult sufficient to deserve its personal put up, so I don’t fairly wish to do it but.
As a substitute, I’ll write as a lot code as doable apart from the precise collision detection. There’s a bit extra work to do to plug it in.
For instance: what am I going to collide with? The one factor within the universe, at the moment, is Star Anise himself. It might be good to have, say, some floor. And that’s an incredible excuse to toodle round a bit within the sprite editor.
I went by means of a number of iterations earlier than touchdown on this. Star Anise lives on a moon, in order that was my tenet. The moon is grey and dusty and pitted, so at first I attempted drawing a tile with tiny craters in it. Sadly, that was a busy mess to have a look at when tiled, and I didn’t suppose I’d have sufficient tile house for having completely different variants of tiles. I’m already utilizing 9 tiles right here simply to have neat edges.
And so I landed on this easy sample with simply sufficient texture to be paying homage to one thing, which is all you really want with low-res sprite artwork. It labored out properly sufficient to outlive, almost unchanged, all the best way to the ultimate recreation. It was impressed by a imprecise reminiscence of Starbound’s moondust tiles, which I used to be fairly certain had diagonal striping, although I didn’t truly take a look at them to be certain.
Chances are you’ll discover I drew these on the second tab of sprites. I need to have the ability to discover tiles shortly when drawing maps, so I assumed I’d put “terrain” on a devoted tab and reserve the primary one for Star Anise, different actors, particular results, and different less-common tiles. That turned out to be a superb concept.
Chances are you’ll additionally discover that a type of dots on the center proper is lit up. How mysterious! We’ll get to that subsequent time.
With a couple of easy tiles drawn, I can sprinkle a pair within the map tab. I do know I need Metroid-style discrete screens, so I’m not apprehensive about digicam scrolling but; the top-left nook (16×16 tiles) is sufficient to play with for now.
I draw two rows of tiles on the backside of that display screen. It’s just a little onerous to gauge for the reason that toolbar and standing bar get in the best way, however the backside row of the display screen might be at y = 15. You may as well maintain Spacebar to get a grid, with squares indicating each half-screen.
Lastly, to make this seem within the recreation, I want solely ask PICO-8 to attract the map earlier than I draw actors on high of it.
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The PICO-8 map() operate takes (no less than) six arguments: the top-left nook of the map to begin drawing from, measured in tiles; the top-left nook on the display screen to attract to, measured in pixels; and the width/top of the rectangle to attract from the map, measured in tiles. This can draw a 32×32 block of tiles from the top-left nook of the map to the top-left nook of the display screen.
After all, with no collision detection, these tiles are nothing greater than background pixels, and the sport treats them as such.
No drawback. I can repair that. Type of.
I’m not going into collision detection but, however I can provide you a style, to offer you an concept of the objectives.
The core of it comes right down to this line, from the top of anise:replace().
That strikes Star Anise by one pixel in every course the participant is holding. What I wish to do is cease him when he hits one thing stable.
Hm, sounds onerous. Let’s suppose for a second a couple of less complicated drawback: how can I cease him falling by means of the bottom, within the dumbest method doable?
The bottom is flat, and it takes up the bottow two rows of tiles. Meaning its high edge is 14 tiles, or 112 pixels, beneath the highest of the display screen. Thus, Star Anise shouldn’t be in a position to transfer beneath that line.
However wait! Star Anise’s place is a single level at his high left, not even inside his helmet. What I actually need is for his ft to not go beneath that line, and the underside of his ft is three tiles (24 pixels) beneath his place. Thus, his place shouldn’t go beneath y = 112 – 24 = 88.
That sounds doable.
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And certain sufficient, it works!
This isn’t going to get us very far, in fact. He nonetheless walks by means of the air, he can nonetheless stroll off the display screen, and if I alter the terrain then the code gained’t be proper any extra. I’m additionally fairly certain I didn’t truly write this in observe. However hopefully it offers you the teeniest concept of the issue we’re going to resolve subsequent time.
Half 2: Collision → (coming quickly!)
Actually, actually, actually shortly, right here’s how that obj snippet works.
Lua’s major information construction is the desk. It may be used to make ordered lists of issues, as I did above with actors, however it can be used for arbitrary mappings. I can assign some worth to a specific key, then shortly look that key up once more later. Type of like a Rolodex.
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Notice that the values (and keys!) don’t need to be strings; they are often something you want, even different tables. However for string keys, you are able to do one thing particular:
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In every single place you see a dot (or colon) utilized in Lua, that’s truly wanting up a string in a desk.
With me to this point? Hope so.
Any Lua desk can be assigned a metatable, which is one other desk full of assorted magic stuff that impacts the primary desk’s habits. Many of the magic stuff takes the type of a particular key, beginning with two underscores, whose worth is a operate that might be referred to as specifically circumstances. That operate is then referred to as a metamethod. (There’s a whole section on this in the Lua book, and a summary of metamethods on the Lua wiki.)
One frequent use for metamethods is to make regular Lua operators work on tables. For instance, you may make a desk that may be referred to as like a operate by offering the __call metamethod.
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One particularly helpful metamethod is __index, which is known as if you attempt to learn a key from the desk, however the important thing doesn’t exist.
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As a substitute of a operate, __index can be one other (third!) desk, by which case the important thing might be seemed up in that desk as a substitute. And if that desk has a metatable with an __index, Lua will comply with that too, and carry on going till it will get an reply.
That is basically what’s referred to as prototypical inheritance, as seen in JavaScript (and extra subtly in Python): an object consists of its personal values plus a prototype, and if code tries to fetch one thing from the thing that doesn’t exist, the prototype is checked as a substitute. For the reason that prototype may need its personal prototype, the entire sequence is known as the prototype chain.
That’s all you must know to comply with the obj snippet, so right here it’s once more.
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The concept is that sorts are used each as metatables and prototypes — they’re at all times their very own __index. At first, we’ve got solely obj, which seems like this:
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Now we use obj:prolong{} to create a brand new sort. Observe alongside and see what occurs. Lua solely seems for metamethods like __call straight within the metatable and ignores __index, so I copy them into the brand new prototype. Then I make the prototype its personal __index, as with obj, and likewise keep in mind the “superclass” as __super (although I by no means find yourself utilizing it). Lastly I set the “superclass” because the prototype’s metatable.
(Oh, by the best way: in Lua, in case you name a operate with solely a single desk or string literal as its argument, you possibly can depart off the parentheses. So foo{} simply means foo({}).)
That produces one thing like the next, noting that this isn’t fairly actual Lua syntax:
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Bear in mind this syntax?
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That’s precisely equal to:
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So in spite of everything is alleged and performed, we have:
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Now for the magic half. After I name vec(), Lua checks the metatable. (The __call in the primary desk does nothing!) The metatable is obj, which does have a __call, so Lua calls that operate and inserts vec as the primary argument. Then obj.__call creates an empty desk, assigns self (which is the primary argument, so vec) because the empty desk’s metatable, and calls the brand new desk’s init technique.
Ah, however the brand new desk is empty, so it doesn’t have an init technique. No drawback: it has a metatable with an __index, so Lua consults that as a substitute. The metatable’s __index is vec, and vec does include an init, in order that’s what will get referred to as. (If there have been no vec.init, then Lua would see that vec additionally has a metatable with an __index, and continued alongside. That’s why I didn’t want an anise.init.)
That’s additionally why defining vec:__add works — it places the __add metamethod into vec, which turns into the metatable for all vector objects, thus robotically making + work on them.
That’s all there may be to it. It’s doable to get rather more elaborate with this in a variety of methods, however that is the naked minimal — and it might nonetheless be trimmed down additional.
Notice that you may’t truly name obj itself. Pop quiz: why not?