Jono Shields - Collisions, Music and More

GBAGameJam - Day 13

Collisions

This should be fun.

FYI I first learned collision logic making a similar platformer in pico8. I found these tutorials invaluable for that. And I think its a great resource that can be applied in any programming language even though the examples are lua & pico8 specific.

This should be easy now that we can get map data.

So first off I am making a simple method to apply gravity so that we can move better.

    bn::fixed_point apply_gravity(bn::fixed_point pos)
    {
        return bn::fixed_point(pos.x(), pos.y() +1);
    }

And I call it like this...

cat_sprite.set_position(apply_gravity(cat_sprite.position()));

It takes a position and adds 1 to the y value. Yes positive y is down in Butano, I am slowly getting used to it too.

Now we fall :D

Good start. Now to stop us falling when we hit the ground. Note that for now I am not bothering too much about the sprites exact position. You will see what I mean.

The next function we make is one to check for collisions and stop the falling.

    void check_collisions(bn::sprite_ptr& sprite, bn::affine_bg_ptr& map)
    {
        // gets the current map cell the sprite is on.
        bn::fixed current_cell = get_map_index(sprite.x(), sprite.y(), map.dimensions().width());

        // at the moment all I am doing is checking if the cell is not the sky cell (aka not zero)
        bool is_hit = map.map().cells_ref().value().at(current_cell.integer()) != 0;
        
        if(is_hit){
            // if it is hit then we undo gravity :D
            sprite.set_y(sprite.y() -1);
        }
    }

Things to note...

We are checking exactly where the centre of our sprite is, but we can change this to the bottom of our sprite fairly easily.
We are just checking that the cell isn't sky, somehow we need to know what tiles we can stand on and what we can't.

Being able to tell different tiles apart is hard though. Their index is determined by the image processor that creates the tiles and the map from our bmp. What I did notice though is that it kind of looks like the following...

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 01 02 03 00 00 00 00 00 00 00 00 00 00 00
00 00 00 04 05 06 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
07 08 08 07 09 10 00 00 00 00 00 00 00 00 00 00 00
11 11 11 11 11 11 07 08 07 08 07 08 07 08 08 07 09
11 11 12 11 11 11 11 11 12 11 11 11 11 11 11 12 12

So the tiles position in the tile array is determined by the order it appears in the map.

And since I don't plan on using the top of the map anyway, prehaps I can throw all the floor tiles into the first row. Like this...

00 01 02 03 04 05 06 07 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 08 09 10 00 00 00 00 00 00 00 00 00 00 00
00 00 00 11 12 13 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
01 02 01 03 04 05 00 00 00 00 00 00 00 00 00 00 00
14 14 14 14 14 14 07 04 07 02 07 01 07 01 02 07 03
14 14 15 14 14 14 14 14 12 14 14 14 14 14 14 15 14

Now my collision check could can just confirm if the tile is between 0 and 8, if it is then it is ground.

I mean, this should work. At least until it becomes complex enough to have different types of collisions and such.

I will mull on this for a little while haha.

Music

In the meantime I got inspired by someone making chiptunes for their game and decided to do the same.

I downloaded MilkyTracker, which at first was really confusing, but this youtuber has tons of awesome tutorials on it.

With a little effort we have some background music playing.

// music item import for .mod .xm .it and .s3m
#include "bn_music_items.h"

// to play your song on repeat at 50% volume
bn::bn_music_items::my_song.play(0.5);

NOTE: You can use sound_items in the same way for .wav files.

Probably a little too cyberpunk for the current game, but it was fun to make anyway.

You can listen to it here.

You can download the .gba here...

.gba