Building a Bird from Boxes

The birds in this world have no 3D model. Each one is assembled at runtime from a few basic shapes and then animated with some simple rules. Up close they’re clearly low-poly, but in flight at a distance they read as birds, and that’s enough. They’re ambient — there to make the sky feel occupied, not to be inspected.

The shape

A bird is four parts:

• a small box for the body,
• two flat triangular wings, one on each side, each on its own pivot so it can rotate,
• a flat triangle for the tail.

The wings are built as a couple of triangles each and set to be visible from both sides, since they’re paper-thin and you often see them from below. That’s the entire model. Colour is rolled per bird — pale greys and whites for gulls, darker browns and slate for the higher land birds.

var bm := BoxMesh.new()
bm.size = Vector3(0.12, 0.1, 0.5)   # the body
# each wing is two triangles on a pivot node, so rotating the
# pivot flaps the wing

The flap

Flapping is one line of maths. A sine wave moves smoothly back and forth between -1 and 1 forever, so feeding the clock through a sine gives a value that rises and falls. Use that to set the wing angle and the wings beat up and down. The two wings use opposite signs so they mirror each other.

var flap := sin(time * flap_speed) * amplitude
wing_left.rotation.z = flap
wing_right.rotation.z = -flap

Each bird starts at a random point in the cycle so they don’t all flap in unison, and the flap speed varies per bird.

Making the motion believable

A bird that flew in a straight line at a constant beat would look mechanical. A few small touches fix that:

• Wandering. Each bird picks a random target point and steers toward it. When it gets close, it picks another. So it drifts around the map instead of flying off.
• Turning gradually. It doesn’t snap to face a new target; it rotates toward the desired heading a little each frame, giving smooth curves.
• Banking. When it turns, it rolls into the turn — leaning like a real bird or an aircraft. The roll amount is taken from how sharply it’s turning.
• Flapping vs gliding. It flaps harder when climbing and holds its wings still to glide when descending, which is what real birds do.
• Holding altitude. It aims to stay a set distance above whatever is below it — terrain or sea — so gulls skim low over the water while land birds circle higher.

None of these are complicated, but together they turn a flapping box into something that moves like a bird. There’s no flight simulation underneath — just a handful of rules layered up.

To keep things cheap, a bird that’s far from the player skips all of this and does nothing until you’re closer.

Next, a slower flier with a different feel: butterflies.