Emanuele Feronato

Talking about Platform game game, Actionscript 2, Flash and Game development.

In this 3rd step I introduced the gravity.

Now the hero starts on the top of the screen and he can walk “downhill” thanks to gravity.

During next step I’ll introduce jumps, then a massive explication will follow.

And AS3 version, of course.

I have to say, making a platform engine is funny and a little harder than I expected.

tile_size = 20;
ground_acceleration = 1;
ground_friction = 0.8;
air_acceleration = 0.1;
air_friction = 0.7;
ice_acceleration = 0.15;
ice_friction = 0.95;
treadmill_speed = 2;
max_speed = 3;
xspeed = 0;
yspeed = 0;
falling = false;
gravity = 0.5;
level = new Array();
level[0] = [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
level[1] = [1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1];
level[2] = [1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1];
level[3] = [1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1];
level[4] = [1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 1, 3, 3, 3, 3, 1, 1, 4, 4, 4, 4, 1, 1];
player = [1, 0];
function create_level(l) {
	_root.createEmptyMovieClip("level_container", 1);
	level_height = l.length;
	level_width = l[0].length;
	for (y=0; ymax_speed) {
		xspeed = max_speed;
	}
	if (xspeed<-max_speed) {
		xspeed = -max_speed;
	}
	if (falling) {
		yspeed += gravity;
	}
	xspeed += bonus_speed;
	x_pos += xspeed;
	y_pos += yspeed;
	check_collisions();
	level_container.hero._x = x_pos;
	level_container.hero._y = y_pos;
	xspeed -= bonus_speed;
};
function ground_under_feet() {
	bonus_speed = 0;
	left_foot_x = Math.floor((x_pos-6)/tile_size);
	right_foot_x = Math.floor((x_pos+5)/tile_size);
	foot_y = Math.floor((y_pos+9)/tile_size);
	left_foot = level[foot_y][left_foot_x];
	right_foot = level[foot_y][right_foot_x];
	if (left_foot != 0) {
		current_tile = left_foot;
	} else {
		current_tile = right_foot;
	}
	switch (current_tile) {
	case 0 :
		speed = air_acceleration;
		friction = air_friction;
		falling = true;
		break;
	case 1 :
		over = "ground";
		speed = ground_acceleration;
		friction = ground_friction;
		break;
	case 2 :
		over = "ice";
		speed = ice_acceleration;
		friction = ice_friction;
		break;
	case 3 :
		over = "treadmill";
		speed = ground_acceleration;
		friction = ground_friction;
		bonus_speed = -treadmill_speed;
		break;
	case 4 :
		over = "treadmill";
		speed = ground_acceleration;
		friction = ground_friction;
		bonus_speed = treadmill_speed;
		break;
	}
}
function check_collisions() {
	get_edges();
	// collision to the bottom 
	if (yspeed>0) {
		if ((level[bottom][right] != 0) or (level[bottom][left] != 0)) {
			y_pos = bottom*tile_size-9;
			yspeed = 0;
			falling = false;
			get_edges();
		}
	}
	// collision to the left 
	if (xspeed<0) {
		if ((level[top][left] != 0) or (level[bottom][left] != 0)) {
			x_pos = (left+1)*tile_size+6;
			xspeed = 0;
		}
	}
	// collision to the right          
	if (xspeed>0) {
		if ((level[top][right] != 0) or (level[bottom][right] != 0)) {
			x_pos = right*tile_size-6;
			xspeed = 0;
		}
	}
}
function get_edges() {
	// right edge
	right = Math.floor((x_pos+5)/tile_size);
	// left edge   
	left = Math.floor((x_pos-6)/tile_size);
	// bottom edge
	bottom = Math.floor((y_pos+8)/tile_size);
	// top edge
	top = Math.floor((y_pos-9)/tile_size);
}

Download source code and enjoy.