Tutorials · Physics and Mechanisms · Chapter 11

Belts and magnets

Start on this floor

1. Select knight and set his Position to 149, 448. Press F5: he stands on the loft’s floor, east of H6. Press F5 to stop.
2. Set his Position back to 48, 1792, or press Ctrl+Z until it reads so.
3. Select tower, click + Add node and choose Node. Rename it loft. Every node in this chapter goes under it.
4. In the Gizmos menu, keep Collision, Forces, Bodies and Triggers ticked, as chapter 10 left them. As there, the pictures leave Collision off: the Colliders’ green outlines are on your screen and not in them.

The belfry’s floor

1. Select stone in the Hierarchy, open the Tilemap tab, and under Layers click stone.
2. Choose 50% from the zoom readout under the Viewport, so the upper hall’s floor and the rows over the loft are both in view, as in the picture below; or scroll the Viewport between the Pick and the Stamp.
3. With Pick, drag over the upper hall’s floor, both its rows, from x 16, column 1, at the west wall, to x 416, column 25. The bar along the bottom of the window says “Lifted 25x2”.
4. With Stamp, click at x 16, y 224: column 1 of row 14.
5. With Pick, drag over the same floor from column 6 to column 12, seven cells, both rows.
6. With Stamp, click at x 512, y 224: column 32 of row 14. The slab runs on to the east wall.
7. Click Save at the top of the Tilemap panel.

The plank and the gantry

1. Select loft, click + Add node and choose Sprite. Rename it scaffold_7, set its Position to 424, 396, drag art/scaffold-plank.png onto Texture and set Size to 48, 8. Click Add component, choose ColliderComponent, set its Size to 48, 8 and tick One Way.
2. Select loft, add a Sprite and rename it gantry. Set its Position to 432, 340, drag art/scaffold-plank.png onto Texture, set Size to 96, 8, and choose Tiled with a Tile Size of 48, 8. Add a ColliderComponent, Size 96, 8, One Way ticked.

The floor belt

1. In the Hierarchy, right-click crate_1 and choose Duplicate. Drag the copy onto loft, rename it crate_8 and set its Position to 420, 448: on the loft’s floor, west of the belt.
2. Select loft, click + Add node and choose Effector. Rename it belt and set its Position to 552, 448: on the floor.
3. In its Area, open Shape and set Size to 144, 2 and Offset to 0, 1. Set Mask to Knight and Crate.
4. In its Effector, set Kind to Surface, Surface Speed to 90 and Grip to 20.
5. Click Add component and choose SpriteComponent: art/belt.png, Size 144, 12, Anchor 0.5, 0, Sort Layer 1, Tiled with a Tile Size of 16, 12.
6. Select loft, add a Sprite and rename it belt_roller: Position 480, 448, art/belt-roller.png, Size 12, 12, Anchor 0.5, 0, Sort Layer 2. Press Ctrl+S.

The Effector’s Kind › Surface

The Effector’s Kind for a belt: it carries every body touching its Area along the node’s own x at its Surface Speed, closing the gap to it at its Grip. What it gives is the body’s Drift, so the body walks on it as on the floor, the belt’s speed added. A body counts as touching up to 1 unit outside the Area, so a body standing on the strip is on the belt.

The Effector’s Surface Speed

The Effector’s Surface Speed, for a Surface belt: how fast it carries what is on it, in world units a second, along the node’s own x, east on an unturned node. Negative runs it the other way. A new Effector’s is 120. The floor belt’s is 90, east; the gantry’s, below, -60, west.

The Effector’s Grip

The Effector’s Grip, for a Surface belt: how much of the gap between a body’s speed and the belt’s it closes each second. A rate, not a share a step, so a belt feels the same at any frame rate. The floor belt’s 20 closes a third of the gap a step at sixty steps a second: a crate let go of at 103.8 is within 1 of the belt’s 90 seven steps later. The gantry’s 4 closes a fifteenth a step. 0 carries nothing. A new Effector’s is 10.

Two scripts for the magnets

1. In Assets, click Create, choose New script, type magnet and press Enter. In the Code tab, replace everything in magnet.rgs with the first script below, and press Ctrl+S. Make magnet_lever.rgs the same way, with the second.

/// A magnet. The Effector on this node pulls what its Area holds toward the
/// node while it is switched on. The lever's `on` switches it on; its `off`
/// switches it off, and what it held falls. A step that hears both ends
/// switched off.
component Magnet {
    fn on_signal() {
        if heard("on") {
            set_flag("EffectorComponent", "enabled", true);
        }
        if heard("off") {
            set_flag("EffectorComponent", "enabled", false);
        }
    }
}

/// The magnets' lever. The `action` key (E) throws it over while the knight
/// is in its Area's reach: it turns its handle and says `on` or `off` to
/// whoever its Signal wires them to.
component MagnetLever : Area {
    /// How many things its Area notices: one while the knight is in its
    /// reach, none while he is not.
    #[readonly]
    near: i32 = 0,
    /// Whether it is thrown on.
    #[readonly]
    on: bool = false,

    fn on_enter() {
        near += 1;
    }

    fn on_exit() {
        near -= 1;
    }

    fn loop() {
        if near > 0 && pressed("action") {
            on = !on;
            set_flag("SpriteComponent", "flip.x", on);
            if on {
                emit("on");
            } else {
                emit("off");
            }
        }
    }
}

component Magnet

A script with no base, like chapter 10’s Vent: on a step it hears on, set_flag ticks its own Effector’s Enabled, and on off unticks it, and what the magnet held falls. heard says which word arrived. A step that hears both ends switched off.

component MagnetLever : Area

The lever’s script, with an Area base: Near counts the Areas in its reach, On whether it is thrown on. Each pressed action in its reach flips it, turns the handle with set_flag("SpriteComponent", "flip.x", on), and emits on or off.

The crane

1. Select loft and add a Sprite, crane_arm: Position 555, 256, art/crane-arm.png, Size 64, 28, Anchor 0, 0, Sort Layer 1. The jib hangs under the ceiling, its cable’s end at x 613.
2. Select loft, click + Add node and choose Effector. Rename it magnet and set its Position to 613, 294: where the magnet’s middle hangs from the cable.
3. In its Area, open Shape and set Size to 26, 139 and Offset to 0, 84.5. Set Mask to Knight and Crate.
4. In its Effector, set Kind to Point, Strength to 8000 and Radius to 192. Leave Falloff at Linear, and untick Enabled.
5. Click Add component and choose SpriteComponent: art/magnet.png, Size 32, 30, Sort Layer 2; its Anchor stays 0.5, 0.5, so it hangs by its eye from the cable.
6. Click Add component and choose ColliderComponent, Size 32, 30: the magnet itself, which stops what it pulls.
7. Click Add component and choose Magnet. Press Ctrl+S. To see it as in the picture, press F over the Viewport with magnet selected, and choose 100% from the zoom readout under the Viewport.

The Effector’s Kind › Point

The Effector’s Kind for a magnet: it pulls every body in its Area toward its own node, by its Strength weakened by its Falloff at the body’s distance from the node. A negative Strength shoves away instead. Only the Area says who is pulled; the Radius says only how hard.

The Effector’s Strength

The Effector’s Strength, for a Point magnet: its pull at the node itself, in world units a second squared. Negative shoves away. A new Effector’s is 800. The crane’s 8000 is 2083 at a crate on the floor, by its Falloff; the rack’s are 2400 and 4800.

The Effector’s Falloff

The Effector’s Falloff, for a Point magnet: how its pull weakens with the distance d from the node. Constant: the Strength wherever the Area holds the body; the Radius is not read and dims. Linear, as it comes: Strength × (1 − d ÷ Radius), full at the node and nothing at the Radius and beyond. InverseSquare: Strength ÷ (1 + (d / Radius)²), full at the node, half at the Radius, and never nothing.

The Effector’s Radius

The Effector’s Radius, for a Point magnet with a Falloff other than Constant: the distance its Falloff is measured over, in world units. It is not a second shape: the Area still says who is pulled. A new Effector’s is 128. The crane’s 192 leaves 2083 of its 8000 at the floor, 142 out. The Forces gizmo draws it as a circle round the node.

The gantry’s belt

1. Select loft, click + Add node and choose Effector. Rename it gantry_belt and set its Position to 552, 336.
2. In its Area: Size 144, 2, Offset 0, 1, Mask Knight and Crate. In its Effector: Kind Surface, Surface Speed -60, Grip 4.
3. Add a SpriteComponent: art/belt.png, Size 144, 12, Anchor 0.5, 0, Sort Layer 1, Tiled with a Tile Size of 16, 12, and in its Flip fold tick X, so its chevrons point west.
4. Add a ColliderComponent: Size 144, 12, Offset 0, 6, One Way ticked. Its top is flush with the gantry’s planks.
5. Select loft and add a Sprite, gantry_roller: Position 480, 336, art/belt-roller.png, Size 12, 12, Anchor 0.5, 0, Sort Layer 2. Press Ctrl+S.

The magnet rack

1. Select loft and add a Sprite, rack_shelf_1: Position 200, 340, art/scaffold-plank.png, Size 48, 8, Tiled with a Tile Size of 48, 8. Add a ColliderComponent, Size 48, 8. Press Ctrl+D, rename the copy rack_shelf_2 and set its Position to 264, 340.
2. Duplicate crate_1 twice more, as crate_8: drag each copy onto loft, and name them crate_9 at Position 200, 336 and crate_10 at 264, 336, one on each shelf.
3. Select loft, add an Effector, rack_magnet_1, at Position 200, 268. Its Area: Size 32, 53, Offset 0, 41.5, Mask Knight and Crate. Its Effector: Kind Point, Strength 2400, Falloff Constant; untick Enabled. Add the SpriteComponent (art/magnet.png, Size 32, 30, Sort Layer 2), the ColliderComponent (Size 32, 30) and Magnet, as the crane’s.
4. Select loft, add an Effector, rack_magnet_2, at Position 264, 268. Its Effector: Kind Point, Strength 4800, Falloff InverseSquare, Radius 56; untick Enabled. Add the Sprite, the Collider and Magnet as on the first.
5. In its Area, open Shape and click the + beside Outline four times. Type the points: [0] -16, 15; [1] 16, 15; [2] 24, 68; [3] -24, 68. Set Mask to Knight and Crate. Press Ctrl+S.

The Area’s Shape › Outline, traced

The Area’s Outline, in its Shape: points, in the node’s units, that replace the box, its Size no longer read. An Area notices by its Outline exactly, as it is drawn, never cut into steps. The second rack magnet’s four points, -16, 15 to 16, 15 across its face and 24, 68 to -24, 68 across the shelf, make a fan its crate rises up inside. Empty, as on every other Area here, the box is the shape.

The lever

1. Select loft and add a Sprite, crane_lever: Position 452, 448, on the floor west of the belt, art/lever.png, Size 20, 24, Anchor 0.5, 1, Sort Layer 1.
2. Click Add component and choose AreaComponent: Size 464, 192, Offset -60, -96, Mask Knight alone.
3. Click Add component and choose MagnetLever.
4. Click Add component and choose SignalComponent. Click the + beside Connections six times. Give [0], [1] and [2] the Signal on, and [3], [4] and [5] the Signal off; pick magnet, rack_magnet_1 and rack_magnet_2 for the To of the first three, and the same three, in order, for the last three. Press Ctrl+S.

The crane at work

1. Set the knight’s Position to 149, 448. Then select belt, so its arrow is drawn at full strength, as in the pictures, and press F5.
2. Hold D: he walks to the crate and shoves it east. Let go as its middle reaches the belt’s west end, where its drum is: he stands off the belt, in the lever’s reach. The belt carries the crate to the wall.
3. Once the crate is against the wall, press E. The crane lifts it up through the gantry to its face and holds it there.
4. Press E again. The crate falls onto the gantry’s belt, which carries it west, out from under the magnet and off its end onto the planks, where it stops under H7. Press F5 to stop, and set his Position back to 48, 1792.

The magnet takes him too

1. Set the knight’s Position to 149, 448 and press F5. Shove the crate onto the belt as before, wait for it to reach the wall, and press E: the crane lifts it to its face.
2. Hold D: he walks onto the belt, which carries him on east, into the crane’s column. The magnet lifts him off the belt and holds him up against the crate it holds. Let go of D.
3. Press E: the magnets let go, he falls onto the belt and the crate onto the gantry. Hold A to walk back off the belt. Press F5 to stop, and set his Position back to 48, 1792.

Try it

1. Select magnet and set its Effector’s Strength to 6000.
2. Set the knight’s Position to 149, 448, press F5, shove the crate onto the belt, and press E once it is at the wall. The crate stays on the belt: at the floor the pull is less than its Gravity. The rack’s crates rise as before.
3. Press F5 to stop. Set Strength back to 8000 and the knight’s Position back to 48, 1792.

1. Select rack_magnet_2 and set its Effector’s Radius to 28.
2. Set the knight’s Position to 149, 448, press F5, select loft and press F over the Viewport, and throw the lever as before. The Constant magnet lifts its crate; the InverseSquare one leaves its crate on its shelf.
3. Press F5 to stop. Set Radius back to 56 and the knight’s Position back to 48, 1792.

1. Select belt and set its Effector’s Grip to 4, the gantry belt’s.
2. Set the knight’s Position to 149, 448, select belt again, press F5, and shove the crate onto the belt as before. It comes up to the belt’s speed slowly, still faster than the belt for most of the way to the wall.
3. Press F5 to stop. Set Grip back to 20 and the knight’s Position back to 48, 1792.

The way up

1. Set the knight’s Position to 149, 448 and press F5. Shove the crate onto the belt; once it is at the wall, press E, and once it is at the magnet’s face, E again: it drops onto the gantry’s belt, which takes it west.
2. Hold A for a moment, until he is under the plank’s east end, and jump up onto it with Space held; jump again, onto the gantry.
3. Once the crate has stopped on the gantry under H7, jump straight up, hold D once his feet are over the crate’s top, and let go of D over its east end: he lands on the crate.
4. Hold A and Space together: he jumps up through H7 and lands on the belfry’s floor, west of the opening. Let go. Press F5 to stop, and set his Position back to 48, 1792.

The loft

Check it works