Tutorials · Physics and Mechanisms · Chapter 13

The grapple

The belfry

1. Select knight and set his Position to 392, 224: the belfry’s floor, where chapter 11’s climb leaves him. Press F5 to play from there, and F5 again to stop. Afterwards press Ctrl+Z until it reads 48, 1792 again, or type 48, 1792 back.
2. Select tower, click + Add node and choose Node. Rename it belfry. The rings, the beam, the clock, the ledge and the bell go under it.
3. Keep Collision, Triggers, Forces and Bodies ticked in the Gizmos menu under the Viewport, as chapter 9 left them. The pictures on this page have all four on.

The grapple key

1. Choose View → Input map…. The Input window opens on its Bindings tab, with the six actions the tower has had since the starter.
2. Type grapple into the New action box and press Add.
3. With grapple selected, press Record a control and press F.
4. Press Save, and close the window.

Rings, a beam and a ledge

1. Select belfry, click + Add node and choose Sprite. Rename it ring_1 and set its Position to 160, 47. Drag art/iron-ring.png onto Texture, and set Size to 20, 26, Anchor to 0.5, 0.615 and Sort Layer to 1: the node on the ring’s middle, where a rope ties on, its plate against the roof.
2. In its Node fold, type rings into the Groups box, where it reads Add group, and press Enter.
3. Right-click ring_1 and choose Duplicate. Rename the copy ring_2 and set its Position to 224, 47. It keeps the group rings.
4. Select belfry and add a Sprite, beam, at Position 224, 60. Drag art/scaffold-plank.png onto Texture and set Size to 48, 8. Add a ColliderComponent with Size 48, 8, and tick One Way: a plank from x 200 to 248, its top at y 56, under ring_2.
5. Select belfry and add a Sprite, belfry_clock, at Position 152, 128. Drag art/clock-face.png onto Texture, chapter 7’s clock, and set Size to 128, 128 and Sort Layer to -2: on the back wall, under the rings.
6. Select belfry_clock and add a Sprite, belfry_clock_hand; its Position stays 0, 0. Drag art/clock-hand.png onto Texture, and set Size to 8, 64, Anchor to 0.5, 0.75 and Sort Layer to -1: the hand, straight up.
7. Select belfry and add a Sprite, bell_ledge, at Position 16, 96. Drag art/bell-ledge.png onto Texture, and set Size to 64, 16, Anchor to 0, 0 and Sort Layer to 1. Add a ColliderComponent with Size 64, 12 and Offset 32, 6: its cap and face, x 16 to 80 on the west wall, its top at y 96, 128 over the floor.
8. Select belfry and add a Sprite, bell, at Position 64, 32. Drag art/bell.png onto Texture, and set Size to 32, 34, Anchor to 0.5, 0 and Sort Layer to -1: hung from the roof over the ledge’s east half, x 48 to 80. It has no Collider.

The rope: a joint and a line

1. Select knight, click Add component and choose JointComponent. Set Kind to Target, Anchor to 0, -20, his hand, Stiffness to 1 and Damping to 0, and untick Enabled.
2. In its Limits fold, tick Use Limits; leave Min at 0 and set Max to 100.

Kind › Target, as a rope

The Joint’s Kind that ties a body to a point in the world, its Target Point. Chapter 9’s claw pulls its body to the point; with Use Limits ticked a Target is a rope instead: it holds the Anchor within Max of the point, lets it anywhere nearer, and holds nothing back while slack. Taut, it turns his motion along his swing and takes away only the part heading out. Unity’s Distance Joint 2D with Max Distance Only ticked and no connected body is the same rope.

Anchor

The Joint’s Anchor: where on him the rope ties, in his own units from his feet. 0, -20 is his hand, 20 up. The Line’s first point and the script’s Hand are the same place.

Target Point

The Joint’s Target Point: the ring the rope is on, in world units. Saved 0, 0, and not read while the joint is off; grapple.rgs sets it to where the ring hangs as it switches the joint on. The claw’s was walked by its script; the rope’s is set once a throw.

Stiffness

The Joint’s Stiffness, on the rope: 1, the whole way back each pass, so the rope never stretches past its Max. The claw’s 0.3 let it trail its point.

Damping

The Joint’s Damping, on the rope: 0, and dim, since a Target with Use Limits ticked is a rope, and a rope does not damp while slack, nor a body it alone swings. Hover it and the Inspector says so. The swing keeps its speed.

Enabled, saved off

The Joint’s Enabled: unticked, the rope holds nothing, coiled. grapple.rgs ticks it when F hooks a ring, and unticks it when he lets go.

Use Limits, Min and Max

The Joint’s limits, on a Target: how far its Anchor may be from its Target Point. Min 0, as near as it likes; Max 100, the rope’s length. Without Use Limits a Target pulls its Anchor onto its point, as chapter 9’s claw is pulled.

1. With knight selected, click Add component and choose LineComponent.
2. Click the + beside Points twice. Set [0] and [1] both to 0, -20, his hand.
3. Set Width to 2. Click Color; if the button at the top left of the picker reads U8, press it so it reads F, and set red 0.55, green 0.42, blue 0.25 and alpha 1: hemp.
4. Set Joint to RoundCaps, leave Closed unticked, and set Sort Layer to 6. Press Ctrl+S.

Points

The Line’s Points: the list it draws through, in order, in the node’s own units. The rope’s two are both 0, -20, his hand: a Line whose points are one draws nothing, the rope coiled. The script moves the second to the ring every step it is hooked, and back to his hand when it lets go. Godot’s Line2D is the same idea.

Width

The Line’s Width: how thick the stroke is, in world units on an unscaled node. 2 for a rope; a new Line’s is 4.

Color

The Line’s Color: its tint, red, green, blue and alpha, each 0 to 1 in the picker’s F mode. The rope’s 0.55, 0.42, 0.25, alpha 1 is hemp; a new Line’s is white.

Joint

The Line’s Joint: how its corners and ends are finished, a choice of the stroke’s and nothing to do with the Joint component. RoundCaps rounds its ends and corners, a rope’s. Flat, as it comes, is chapter 14’s lamp rail’s, and Round its crate puzzle’s fence’s.

Closed

The Line’s Closed: whether its last point joins back to its first. A rope is open, unticked as it comes; chapter 14’s fence is closed.

Sort Layer

The Line’s Sort Layer: where it draws among the pictures, lower first. 6 is over his picture’s 5, so the rope runs over his hand, not behind him. A new Line’s is 0.

His Rope Jump

1. With knight selected, in his Character Body’s Jumping fold, set Rope Jump to 680, his Jump Speed.

Rope Jump

The Character Body’s Rope Jump, in its Jumping fold, in world units a second: the upward speed a press of jump sets while he hangs on a rope, held by an enabled joint, off the ground and outside his Coyote Time. It is a jump: let go of Space early and his Jump Cut cuts it, and it spends no air jump. It does not let go of the rope; the script holding it does. Only a press made while he hangs counts: one pressed before F does not carry onto the rope. 0, as it comes: a press on a rope is nothing to his body.

The grapple

1. In Assets, click Create, choose New script, type grapple and press Enter. In the Code tab, replace everything in grapple.rgs with the script below, and press Ctrl+S.
2. Select knight, click Add component and choose Grapple. Its Rope reads 100, the joint’s Max, and its Hand 0, -20, the joint’s Anchor. Press Ctrl+S.

/// The knight's grapple. The `grapple` key (F) throws the rope at the
/// nearest iron ring he can see, if it is within the rope's reach of his
/// hand. Held, the rope holds and he swings on it. It comes off when the key
/// is let go of, when he jumps off it, and when he lands.
///
/// The rope is two components on this node. A Target joint, switched off,
/// with Use Limits from 0 to the rope's length, at full Stiffness and no
/// Damping, anchored at his hand: switched on with its Target Point where
/// the ring is, it keeps his hand within the rope's length of the ring, and
/// he swings on it. And a Line, whose first point is his hand and whose
/// second this script draws to the ring every step it is hooked.
///
/// A ring is found by where it is and by its group, never by its name.
component Grapple {
    /// How long the rope is: the joint's Max. A ring further than this from
    /// his hand is out of reach.
    rope: f32 = 100.0,
    /// Where the rope is tied on, from his feet: the joint's Anchor and the
    /// Line's first point.
    hand: Vec2 = vec2(0.0, -20.0),
    /// Whether the rope is on a ring.
    #[readonly]
    hooked: bool = false,
    /// Where the ring it is on hangs.
    #[readonly]
    hook: Vec2 = vec2(0.0, 0.0),

    fn loop() {
        if hooked {
            if held("grapple") && !pressed("jump") && !grounded() {
                let far_end = hook - position();
                set_point("LineComponent", "points.1", far_end);
            } else {
                hooked = false;
                set_flag("JointComponent", "enabled", false);
                set_point("LineComponent", "points.1", hand);
            }
        } else if pressed("grapple") {
            let ring = nearest_seen("rings", -1);
            let at = position_of(ring);
            if ring != node(0) && at.distance(position() + hand) <= rope {
                hooked = true;
                hook = at;
                let far_end = at - position();
                set_point("JointComponent", "target_point", at);
                set_flag("JointComponent", "enabled", true);
                set_point("LineComponent", "points.1", far_end);
            }
        }
    }
}

Rope and Hand

The Grapple’s two fields to type: the rope’s reach, 100, the joint’s Max, so the rope only hooks a ring already within its length of his hand; and where it ties on, 0, -20, the joint’s Anchor. Hooked and Hook are #[readonly] (attributes): the script sets them, and the Inspector shows them dim.

loop, pressed and held

loop runs every step. pressed("grapple") is true on the one step F goes down; held("grapple") every step it stays down. pressed("jump") is Space’s, the project’s jump.

nearest_seen

The nearest node in the group rings with no wall between this node and it, as the step began; -1 asks about walls on every collision layer. Nobody, node(0), when none can be seen. nearest is the same without the walls.

position_of, position and node

position_of(ring) is where the ring hangs, in the world; position() is where the knight’s feet are. node(0) is nobody, what nearest_seen answers when it finds no ring.

at.distance(…) and vec2

How far apart two points are, a method of every Vec2. position() + hand is his hand in the world: a ring further from it than Rope is out of reach, and F throws nothing.

set_point and set_flag

Write a point, or true or false, onto another component of this node. "target_point" and "enabled" are the Joint’s; "points.1" is the Line’s second point, from his feet, so hook - position() draws it to the ring wherever he is that step.

grounded

Whether his Character Body stood on something as the step began. Standing, the rope lets go: a rope holds a man off the ground, and on the ground it has nothing to do.

The swing

1. Set the knight’s Position to 392, 224. With knight selected, so his rope is drawn bright, press F5.
2. Hold A, and keep it held to the end. He walks west. On the rope it pushes his swing west, and off it, it is his walk west onto the ledge. A stride before he comes under the beam, press and hold Space: a jump.
3. At the top of the jump, press and hold F and let go of Space. The rope goes to ring_1, slack at first, and draws taut east of the ring.
4. He swings west under the ring, faster than he walks. As he passes under it and starts up, press and hold Space. He jumps off the rope, the rope coils back into his hand, and he flies up and west over the ledge’s edge, lands on it, and walks to the wall, west of the bell. Press F5 to stop, and Ctrl+Z back to 48, 1792.

Try the rope

1. Select knight and set Rope Jump to 0, what he had before this chapter. Set his Position to 392, 224.
2. Press F5 and swing as above. Space on the rope only lets go of it, as letting go of F does, and he drops to the floor. Held on the rope with A held, the swing alone never lifts his feet to the ledge’s top. Press F5 to stop.
3. Set Rope Jump back to 680 and the knight’s Position back to 48, 1792.

1. In grapple.rgs, write nearest("rings") for nearest_seen("rings", -1) and press Ctrl+S. Set the knight’s Position to 392, 224, and with knight selected press F5. Jump and press F as above, and keep F held: the rope goes to ring_2, through the beam, and he hangs under it, far east of the ledge. Press F5 to stop.
2. With nearest still in, write can_see(at, -1) && after ring != node(0) &&, and press Ctrl+S. Press F5 and jump as above: F at the top of the jump throws nothing now, and he comes down on the floor. Press F5 to stop.
3. Put nearest_seen("rings", -1) back, take can_see(at, -1) && out, press Ctrl+S, and set the knight’s Position back to 48, 1792.

1. Select knight. Set his Joint’s Max to 140 and his Grapple’s Rope to 140, and his Position to 392, 224.
2. Press F5 and swing as above, pressing Space as he passes under the ring. The swing hangs him 40 lower, and the jump off it no longer reaches the ledge: he meets its face and comes down on the floor under it. Press F5 to stop.
3. Set Max and Rope back to 100, and the knight’s Position back to 48, 1792.

The climb’s end

The tower, exported

1. Save with File → Save all: Export copies the files as they are on disk, the tilemap and input.ron with the scene.
2. Click the Build tab, or choose View → Panels → Build if it is not there.
3. In its Export section, the line under Export says where the game will be written, ending in physics-and-mechanisms/dist/physics-and-mechanisms/physics-and-mechanisms: your project’s folder, then dist, then a folder and a file both named after it.
4. Press Export. When it is done, the editor opens the new folder in your file manager.
5. Double-click physics-and-mechanisms, or physics-and-mechanisms.exe on Windows. On Linux, if double-clicking does not start it, run ./physics-and-mechanisms in a terminal in that folder.

The belfry and the rope

Check it works