Making a Game Boy recreation in 2017: A « Sheep It Up! » Submit-Mortem (allotment half of of)

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Everyone has childhood dreams. Mine became as soon as to ruin a recreation for my fist console: the Nintendo Game Boy. This day, I fulfilled this dream, by releasing my first Game Boy recreation on a real cartridge: Sheep It Up!

In this article, I could present the tools I old, and some pitfalls a newcomer love me had to beat to ruin this challenge a actuality! Attributable to it’s length, this article can be split in two formula:

• Fragment 1 : Instruments old / Technical challenges / Art challenges
• Fragment 2 : Audio Constraints / Making Cartridges / Player Feedback (Upcoming)

« Sheep It Up! » is an arcade recreation where a sheep has to climb up by striking himself to flying velcro straps. The view is easy, but the sport impulsively gets moderately nice looking: how high are you able to climb with out falling down?

As a recreation collector myself, I essential this recreation to be produced with out sacrificing any fundamental Game Boy titles. So the total lot is manufactured particularly for this recreation: the pcb, the rom, the shell, the protective case and even the sticker! We furthermore tried to withhold the worth cheap, so all americans can revel within the sport: $15 (+shipping). It can perhaps well elope on any Game Boy model, from the first one to the GBA SP, collectively with the Huge Game Boy.

While you happen to proceed to own a Game Boy, you have to well well steal a cartridge on the writer internet internet internet page:

https://catskullgames.com/sheep-it-up

In the ninety’s, growing recreation for a 8-bit recreation console became as soon as a proper internet internet page, and required a crew of highly professional mavens. Designers, artists and builders from this period are proper heroes to me: to be capable to arrangement such very unbiased appropriate video games with the shrimp tools they had compared the ones we own on the present time.

In 2017, it’s tranquil different labor to ruin a recreation for a 8-bit console. But, thanks to the very unbiased appropriate homebrew communities, now we own different tools to ruin our life more uncomplicated! Without them, an hobbyist love me could well well well under no instances be in a position arrangement a Game Boy recreation on his own. So, what are these tools?

For starter, the programming language. Encourage within the day, all dedicated gaming hardware became as soon as programmed in Assembly. It be tranquil that you have to well well factor in (and even instructed) to expend Assembly on the present time. But it’s now no longer the ideal option, as many devkits for 8-bits and sixteen-bits consoles are per the C programming language. For the Game Boy, this suitable tool is believed as Game Boy Builders Kit (GBDK).

Then, to ruin art work and level arrangement, you now no longer want to expend graph paper and a hand scanner. With a chunk of of luck, you have to well well now expend two complementary allotment of intention:

• Game Boy Tile Designer (GBTD). It can perhaps well allow you design sprites and tiles, and to export them in a binary structure that could well well furthermore be read by a Game Boy.
• Game Boy Arrangement Builder (GBMB).  This one will mean you have to well well arrangement level and background image the expend of the tiles drawn with GBTD (it’s more or much less love Tiled, but for Game Boy).

Final but no longer least, we furthermore desire a capability to test our recreation. This is where modernity is important. While within the ninety’s builders had to expend dear ICE containers, on the present time we own highly efficient intention emulators that can elope on any computer. To ascertain your own recreation, the most though-provoking substitute is BGB. It be a extraordinarily staunch Game Boy emulator that comes with a highly efficient debugger, the one tool you have to well well want to arrangement a working recreation!

The sport is in prime-moral nook, the entire other scary home windows are the many debugger tools!

But to be with out a doubt sure your recreation is working, you have to well well furthermore want to test it on proper hardware. In the ninety’s, other folks were burning their program on a EPROM chip, and old some special cartridges to drag this chip into an real Game Boy. While efficient, it became as soon as moderately a prolonged and dear process. This day, we own what are known as « Flashcarts »: cartridges where you have to well well drag a SD card alongside with your recreation ROM to play it on an real Game Boy. The view is an identical, but the tools are more simply accessible and sooner to expend. A variety of Game Boy Flashcarts exists, but the most though-provoking one is arguably the one built by Krikzz: The Everdrive Game Boy. A more contemporary and improved model became as soon as launched this summer season, but I old the frail version that I sold some time within the past.

   
A flashcart from the ninety’s vs one from the 2010’s

No matter these monumental tools accessible in 2017, making a recreation on the present time for a 1989 recreation console is tranquil a internet internet page. Especially for americans which were professional to expend « stylish tools », similar to Unity, Unreal Engine or Godot. Listed below are the most significant challenges I confronted at some level of the arrangement of Sheep It Up!, collectively with the things that surprised me about how a Game Boy is de facto working.

Size limitation

Let’s start with the glaring one: Sheep It Up! is a somewhat easy recreation. One ingredient that explains this fact is that the entire recreation weights handiest 32KB. Yup, the code, the pictures, and even the sounds are all fitting in a minute 32KB condominium. To present you a level of comparability, 32KB is the size of the Wikipedia logo in a extraordinarily minute resolution:

                    
           Beefy Game: 32KB                            160x146px (PNG-24): 32KB 

No doubt, no longer every Game Boy recreation fits in 32KB. For me, it became as soon as a technical limitation to be capable to release the sport on an real cartridge (more essential capabilities in allotment 2). The most though-provoking and most noteworthy Game Boy titles are with out a doubt capability larger than that :

• Pokemon Red / Blue are 1024KB (so broad!)
• Wario Land and Zelda Link’s Awakening are 512KB
• Kirby Dream Land is 256KB
• Gargoyle’s Quest is 128KB

In level of fact, few Game Boy titles are handiest 32KB. Most of them are from the youth of the console. As an illustration, both Alleyway (one amongst the Game Boy initiate title in Japan) and Tetris (initiate title in US and Europe) are 32KB video games. These are both ravishing titles, but as a end result of their minute cartridge condominium, they’re furthermore considerably « shrimp » in scope: single show, few diversified graphical and audio property, and loads others.

      
Alleyway and Tetris are 32KB video games, love Sheep It Up!

CPU: fiddling with Moveable Energy!

The Game Boy is powered by a 4MHz CPU, custom built for the console (it’s some distance a combination between Zilog’s Z80 and Intel’s 8080 processors). All in all, Gameboy processing energy is similar to a NES, and even a chunk of more highly efficient due the reduced show dimension and colors to command (more on that latter). No matter a mere 4Mhz tempo, the entire Game Boy video games are displayed in fixed 60fps. Address stop that PS4 Skilled and Xbox One X!

But, for a programmer new out of college, the biggest constraints can be that it’s some distance a 8-bits processor. As you furthermore could can unbiased know, internally the entire computers are the expend of Zero and 1 to process data. A single digit, that could well well also unbiased own either Zero or 1 as mark, is believed as a « bit » (short for « binary digit », because it ought to handiest own two diversified values). A processor is defined as « 8-bits » if it ought to process 8 bits of data in a single operation.

How does it own an affect on the making of video video games?

Neatly, the much less « bits » the CPU can process, the more shrimp will your gameplay variables be. As an illustration, in video games we expend different integers numbers to represent values: life, tempo, ranking, and loads others.

With 8 bits of data, an integer can retailer smaller numbers than a sixteen bits variable :

• 8 bits integer variable: -127 to 128 (or Zero to 255 if no longer the expend of signal)
• sixteen bits integer variable: -32768 to 32767 (or Zero to 65535 if no longer the expend of signal)

If that’s tranquil obscure for you, let’s expend a concrete example from Sheet It Up!

As you have to well well look in this image, the ranking variable has 5 digits, and can fluctuate from Zero to 99999. Sadly, it’s capability larger than what a 8 bits integer variable can retailer. I mean, even a sixteen bits integer variable is no longer sufficient to expend care of such a « monumental » ranking!

So, in tell to be capable to notice ranking in Sheep It Up!, I had to expend no longer one, but 5 diversified 8 bits integer variables to retailer the participant ranking. I mean you have to well well guess how « enjoyable » it became as soon as to take care of all these variables, particularly after I added the capability to withhold a high ranking, thus I had to evaluate two values saved staunch thru 5 diversified variables. Programming on a 8 bits intention will power you to come up with complex solutions love that always, whereas stylish programs will simply expend 32 bits or Sixty 4 bits variables and under no instances pose such a internet internet page to their programmers.

(N.B. for experienced recreation builders: I do know that I’m able to were more efficient than the expend of a full int8 to retailer a single digit ranging from Zero to 9. But I with out a doubt essential every variable to be capable to head from Zero to 255, as with out a doubt I ruin no longer retailer the ranking digit staunch now, but the tile identification old to command this quantity on the show)

One world, two planes

Any image displayed on the Game Boy contains two formula: a background layer (BKG) and several other bright objects, assuredly known as sprites (OBJ). You is presumably shrimp to a most on 40 sprites onscreen, with the extra limitation that the Game Boy can’t present larger than 10 sprites on the same line.

              Background (BKG)                                      Sprites (OBJ)

In level of fact, it’s some distance a chunk of more complex than that, as there is a further « window » layer than can scroll in a different way from the background. It be assuredly old to ruin the buyer interface (ranking, and loads others.).  But this residue is « non-transparent »: this could occasionally perhaps well conceal any graphical data on the background layer within the lend a hand of it. So, for the sake of simplicity, you have to well well withhold in suggestions that you’ve got got a single « background » layer on a Game Boy, but that you have to well well ruin allotment of it scrolling independently for parents that’ve to.

Game Boy sees 4 colors!

Let’s introduce the elephant within the room: the fresh Game Boy model can handiest present 4 diversified colors.

One can argue that it’s no longer with out a doubt Gentle Grey and Darkish Grey, but somewhat Gentle Inexperienced and Darkish Inexperienced, but that’s no longer the level: you have to well well handiest expend 4 colors to design our art work. On the least, as prolonged as you are drawing the background, since the case is diversified with sprites!

Indeed, as you have to well well look in this image, the Sheep is drawn with three colors handiest. Why is that? Truly, I drew the sprite with 4 colors. But the 4th coloration, here « Dim », is simply no longer displayed since it’s old as « transparent coloration » by the Game Boy. That implies, the sprites are no longer all « sq. pictures », but they’ll furthermore be a huge quantity of shapes allowing to observe the background image within the lend a hand of them.

Does it mean that I’m able to no longer expend sad to design sprites?

No doubt no longer, as you have to well well look under with the « velcro » strap sprite. For this sprite, I with out a doubt old a special palette of three+1 colors: « White » is old as transparent coloration, and « Dim » is displayed on the sprite.

Fun with palettes

No matter being shrimp to 4 colors handiest, the Game Boy with out a doubt uses three diversified palettes to command pictures, and that is the explanation very frosty!

As you have to well well look, there could be a single « 4 colors » palette for the background layer, and two « three colors + 1 transparent coloration » palettes that the sprites can expend (every sprite can expend one palette or the opposite). The frosty thing about these palettes is that you have to well well freely place any coloration you’ll want to any palette slot. This allow you ruin some fantastic effects, such because the travel-in / travel-out viewed in many video games.

To ruin a travel-in on a Game Boy, you unbiased need alter the colors on the three palettes. In the 1st step, the entire palette slots are filled with white coloration. Then, in Step 2, the darkest coloration slot (sad) is filled with gentle gray, to step by step present the image. In Step three, Gentle Gray becomes Darkish Gray, etc, except the 4 colors are displayed on every palette.

The expend of an identical capability, we can furthermore ruin a character « blink », by switching the colors on its palette every frame. As an illustration, this capability is old when Mario gain a huge name and change into invincible in Huge Mario Land.

Tiling the field!

There could be one final oddity within the capability a Game Boy present pictures, no longer lower than for a recreation developer professional on contemporary-gen engines. Because the console does no longer own a « frame buffer », you cannot specify the color of every pixel on show in my view. Indeed, the entire on-show pictures are built by assembling quite loads of « tiles », i.e. 8×8 pixels squares. This is appropriate for both background and sprites :

This capability, designed to lower the amount of video RAM essential to command pictures, implies that you cannot design freehand traces on the Game Boy show. But it’s furthermore one amongst the secret that makes the Game Boy in a position to command such beautiful video games with a mere 4Mhz CPU and handiest 8KB of video RAM (sure, I wrote kilobytes and no longer megabytes or gigabytes).

On the opposite hand, one amongst the quandary of this tile-primarily based fully present is that our « sheep » and « velcro » sprites are with out a doubt fabricated from larger than one sprite. The Sheep is 16×16 pixels, so in belief it will need 4 8×8 sprites to be displayed. With a chunk of of luck the Game Boy furthermore own a 8×16 present mode for the sprites, meaning that with out a doubt every object in Sheep It Up! is handiest fabricated from two sprites, and no longer 4.

Stop you do no longer disregard that we are shrimp to 40 sprites onscreen, with the extra limitation that the Game Boy can’t present larger than 10 sprites on the same line?

In this case, it capability that we will no longer own more 5 sheep aspect by aspect on the same line, as an substitute of 10 as you can need belief originally (since the sheep is fabricated from 2 sprites from an hardware level of see).

Beefy-show tiles

I did no longer mention it yet, but the Game Boy show resolution is 160×144 pixels. It capability that you need 20×18 = 360 tiles to duvet the entire show.

Sadly, the Game Boy VRAM is shrimp (8KB), and it ought to retailer handiest 256 diversified tiles. It capability that, with out any coding trick, you cannot present a fullscreen image :

The bottom line is to reuse tiles, as you have to well well look, the tile image own different « empty condominium », meaning that a single « white tile » in RAM is positioned several times on the image. In level of fact, the title show image handiest uses 178 diversified tiles, meaning I tranquil had room for some extra essential capabilities if I essential to.

I ruin no longer know what capability they followed to re-expend tiles within the ninety’s, but on the present time we own a extraordinarily fantastic tool for that, known as Game Boy Tile Information Generator. You feed it with a PNG image (in 4 colors) and this could occasionally perhaps well automatically generate you the tiles and the tilemap essential to command it on an real Game Boy. Clearly, this could occasionally perhaps well furthermore automatically detect and reuse identical tiles to keep as remarkable video RAM as that you have to well well factor in on the console!

For data, different the 80s and ninety’s consoles work in an identical capability, with 8×8 tiles-primarily based fully present: Nes, Master Arrangement, PC-Engine, Huge Nintendo, Genesis, Game Boy, Game Instruments… The first mainstream home consoles to own a frame buffer were the PlayStation and Saturn, and the first Nintendo handled to form it became as soon as the Game Boy Attain (The Atari Lynx being the first handheld with in my view addressable pixels). So discovering out to « work with tiles » on the Game Boy is de facto a transferable capability for parents that later work on yet every other retro recreation console!

Guidelines from the mavens: making 4 colors sprites!

Let’s stop with somewhat tip coming from the proficient builders at Nintendo. Please notice at this screenshot from Wario Land :

Stop you look how the coin, a sprite, is de facto drawn with 4 colors? How did they compose such a feat, after I unbiased appropriate told you that the Game Boy can handiest present three colors per sprite ?

The answer is easy : every coin is fabricated from 2 sprites, and every sprite uses a special object palette. As you have to well well look, when we split the coin in two halves, it’s indeed fabricated from two 8×16 sprites. As repeatedly with Nintendo, there could be a monumental attention to detail: they did no longer simply merged two « sprites halves », but they design them a chunk of of tilted and they overlapped them by 1 pixels, so the coin with out a doubt feel more love a single ingredient!

This is the pinnacle of the first allotment of postmortem. Thanks for reading it this some distance! In allotment 2, we will talk about the audio challenges (lot of oddities here too!), and the design the cartridges were built with out destroying any beforehand existing Game Boy recreation. I could furthermore lumber over the feedback I purchased from players on Sheep It Up!

I am hoping you enjoyed this article! And, for parents that proceed to own a Game Boy laying spherical, ruin no longer disregard that you have to well well steal an resplendent Sheep It Up cartridge for a mere $15 from Catskull video games. Every cartridge can be assembled by hand earlier than being sent to you!

While you happen to’d resolve to study when the allotment 2 of this postmortem goes are residing, or after I release yet every other recreation for a retro console, you have to well well signal in to my e-newsletter the expend of the gain under!