Tutorials · Physics and Mechanisms · Chapter 12

The flooded cellar

Under the ground floor

1. Select tower, click + Add node and choose Node. Rename it cellar. The hatch, the water and the crate in this chapter go under it; the cellar’s backdrop goes under backdrop, with the others.
2. 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 cellar, painted

1. Select stone in the Hierarchy and open the Tilemap tab beside the Inspector. Under Layers, click stone.
2. Click Pick, or press I over the Viewport, and drag over the west wall’s two cells just over the ground floor: column 0 of rows 110 and 111, x 0, y 1760 to 1792.
3. Click Stamp, or press S, and click column 0 of row 114, x 0, y 1824. Click again at rows 116, 118, 120, 122, 124 and 126, y 1856, 1888, 1920, 1952, 1984 and 2016: the west wall, down to y 2048.
4. With Pick, drag over the east wall’s two cells the same way: column 39 of rows 110 and 111, x 624.
5. With Stamp, click column 39 of row 114, x 624, y 1824, and again at rows 116, 118, 120, 122, 124 and 126.
6. With Pick, drag over the ground floor’s floor, both its rows, 112 and 113, from column 1 to column 38: x 16 to 624, y 1792 to 1824.
7. With Stamp, click column 1 of row 126, x 16, y 2016: the cellar’s base, its top 224 under the ground floor’s.
8. With Pick, drag over columns 18 to 25 of the same floor, both rows: x 288 to 416.
9. With Stamp, click column 18 of row 117, x 288, y 1872: the pier’s moss-capped top.
10. With Pick, drag over the storeroom’s inner wall, the one over the doorway its portcullis shuts: columns 16 and 17 of rows 87 to 93, x 256 to 288, y 1392 to 1504.
11. With Stamp, click row 119, y 1904, at columns 18, 20, 22 and 24, x 288, 320, 352 and 384: the pier’s stone, from under its top down to the cellar’s floor.
12. Click Erase, or press E, and drag over columns 6 to 9 of rows 112 and 113: x 96 to 160, y 1792 to 1824. That is H0.
13. Click Save at the top of the Tilemap panel.

Erase

The tilemap painter’s Erase, E: clears every cell the drag passes over, on the chosen layer. A cleared cell of a solid layer stops nothing, so H0 is a hole through the slab’s stone.

The camera and the backdrop

1. Select confiner and set its Position to 320, 1024 and its Bounds to 640, 2048: the whole tower as it now is, from y 0 to 2048.
2. Select backdrop, click + Add node and choose Sprite. Rename it backdrop_cellar and set its Position to 0, 1792. Drag art/old-dark-castle-interior-background.png onto Texture, set Size to 640, 224, Anchor to 0, 0 and Sort Layer to -10, and in its Draw mode fold set Draw Mode to Tiled and Tile Size to 960, 304, as the eight backdrops above it.

The Confiner’s Bounds

The Confiner’s Bounds: the rectangle the camera’s view is kept inside, its width and height in world units, centred on the Confiner’s Position. 640 by 2048 at 320, 1024 runs from the tower’s top to the cellar’s bottom.

A hatch over H0

1. Select cellar, click + Add node and choose Sprite. Rename it hatch and set its Position to 128, 1796. Drag art/hatch.png onto Texture, and set Size to 32, 8, Anchor to 1, 0.5 and Sort Layer to 1: the west leaf, from x 96 to 128.
2. Click Add component and choose ColliderComponent. Set its Size to 64, 8 and tick One Way: one plank over the whole of H0, its top at y 1792, flush with the floor.
3. Select hatch, click + Add node and choose Sprite. Rename it hatch_east; its Position stays 0, 0. Drag art/hatch.png onto Texture, set Size to 32, 8, Anchor to 0, 0.5 and Sort Layer to 1, and tick Flip › X: the east leaf, from x 128 to 160, hinged at H0’s east face.

The water

1. Select cellar, click + Add node and choose Effector. Rename it water and set its Position to 320, 1858.
2. In its Area, open Shape and set Size to 608, 192 and Offset to 0, 62: the cellar room, x 16 to 624, y 1824 to 2016. Click the button beside Mask, untick World, and tick Knight and Crate, as every effector’s in the tower.
3. In its Effector, set Kind to Buoyancy, Waterline to 2, Density to 1.5 and Drag to 5. Leave Priority at 0 and Enabled ticked.
4. Click Add component and choose SpriteComponent. Drag art/water-surface.png onto Texture, set Size to 608, 8, Anchor to 0.5, 0 and Sort Layer to 6, and in Draw mode choose Tiled with a Tile Size of 16, 8: the foam along the water’s top.
5. Select water, click + Add node and choose Sprite. Rename it water_depth and set its Position to 0, 8. Drag art/water.png onto Texture, set Size to 608, 150, Anchor to 0.5, 0 and Sort Layer to 6, and choose Tiled with a Tile Size of 16, 16: the water, down to the floor.

Kind › Buoyancy

The Effector’s Kind for water. Every step it lifts each body in its Area by its Density times the body’s own Gravity times the share of the body under its Waterline, and slows the body’s fall or rise by its Drag times the same share. A body in the Area wholly over the line is neither lifted nor slowed. It reads Waterline, Density and Drag, and dims the rows of the other kinds. Unity’s Buoyancy Effector 2D is the same idea, its Surface Level this Waterline; Godot has no water of its own.

Waterline

The Effector’s Waterline: where the water’s top is, in world units under the node’s own Position, not the Area’s. The node is at y 1858, so 2 puts the top at y 1860, 36 under the cellar’s ceiling. The Forces gizmo draws it as a line across the Area. The pictures do not follow it: they hang from the node. A new Effector’s is 0.

Density

The Effector’s Density: the lift on a body wholly under, as a multiple of that body’s own Gravity; a body partly under gets that share of it. A body floats still where the lift is its weight, with one part in Density of it under: at 1.5, two-thirds. A crate, 24 tall, floats with 16 under; the knight, 30 tall, with 20. Since it counts against each body’s own Gravity, his 2000 and a crate’s 1800 float alike. A new Effector’s is 1.2.

Drag

The Effector’s Drag: how much of a body’s motion the water takes away each second, times the share of it under; at 5, a body all under loses a twelfth of it each step. It takes the share of the motion the world gave: the body’s fall and rise, and its Drift, such as a shove’s glide. It never takes what the body’s own keys or script drive, so in the water he walks, and floats along, at his full Speed. Drag is what stops his plunge short of the floor and stills his bob. A new Effector’s is 4.

A crate on the pier

1. Right-click crate_5 in the Hierarchy and choose Duplicate. Drag the copy onto cellar, rename it crate_11, and set its Position to 392, 1872: on the pier’s east end.

Down the hatch

1. Select water, so its waterline is drawn bright, and press F5.
2. Hold D for about half a second, until he stands on the hatch, and let go. He stands on it as on the floor.
3. Press S and Space together. He drops through the hatch, through H0, and the camera follows him down into the water.
4. Move the pointer over the Viewport and press F: the Viewport frames water, the whole cellar, as in the pictures. Watch him plunge, bob back up past where he comes to rest, and settle. The plunge is over in a second: to see it step by step, press Pause or F6 as he drops, then Step or F7, and watch the Play bar’s step counter. Press F5 to stop.

A stroke

1. Press F5 to stop if the game is running. Select knight, and in his Character Body’s Jumping fold set Swim Stroke to 680, the same as his Jump Speed.

Swim Stroke

The Character Body’s Swim Stroke, in its Jumping fold, in world units a second: the upward speed a press of jump sets while the middle of his box is under a Buoyancy waterline and he is neither on the ground nor within his Coyote Time. Once a press: held, it strokes once; let go and pressed again, it strokes again. Letting go does not cut it short, as it cuts a jump, and it spends no air jump. On the ground a press is a jump, as ever. With his middle out of the water, at the top of a stroke’s rise, a press is nothing. His 680, his Jump Speed, lifts him from his line out through H0. 0, as it comes, cannot swim.

1. Select water and press F5. Walk him onto the hatch with D, drop through with S and Space, and press F over the Viewport.
2. Wait until he floats still under the hatch, about six seconds after the drop. Press Space once. He rises out of the water, up through H0 and past the hatch, which lets him through from below, and lands on it, where he went down. Press F5 to stop.

1. Select knight and set Swim Stroke to 0, what he had before this chapter.
2. Select water, press F5, walk onto the hatch and drop through. Press Space as often as you like, in his plunge and once he floats: nothing lifts him, and he floats under the hatch for good. Press F5 to stop.
3. Set Swim Stroke back to 680.

The pier and the crate

1. Select water and press F5. Walk onto the hatch, drop through, press F over the Viewport, and wait until he floats still.
2. Hold D. He floats east at his walk until the pier’s face stops him, its top 8 over his feet.
3. Keep D held and press Space: a stroke lifts him over the pier’s top, and he lands on it, the water to his knees.
4. Keep D held. He wades east at his full walk, meets the crate and shoves it off the pier’s east end. Let go of D once the crate is off, before he follows it. The crate glides on through the deep water, slows, bobs and floats still. Press F5 to stop.

Step Height, afloat

The Character Body’s Step Height, in its Slopes fold, which chapter 3 set to 12: the highest step he walks up as he walks, from the ground (or on the step he lands). Afloat he is neither, so it climbs nothing, however small the step.

Try the water’s fields

1. Select water and set Drag to 0.
2. Press F5, walk onto the hatch, drop through and press F over the Viewport. Nothing slows his plunge: he goes down to the cellar’s floor, the water throws him back up out of it into H0, and down he goes again, deep into the water. Press F5 to stop.
3. Set Drag back to 5.

1. Select water and set Waterline to 40. The gizmo’s line drops 38; the foam does not move, since the pictures hang from the node.
2. Press F5, walk onto the hatch, drop through and press F over the Viewport. He floats 38 lower, at the new line. Once he floats still, press Space: the stroke lifts him into H0, but from there his feet stay short of the hatch’s top, and he falls back into the water. Press F5 to stop.
3. Set Waterline back to 2.

1. Select water and set Density to 2.5.
2. Press F5, walk onto the hatch, drop through and press F over the Viewport. He floats higher, with two-fifths of him under, his middle over the water. Press Space: nothing, for a stroke asks for his middle under. Press F5 to stop.
3. Set Density back to 1.5.

The way back up

The cellar

Check it works