Gravity and jumping

Gravity is the single most important force in a platformer. Getting it right makes the game feel tight and responsive. Getting it wrong — even slightly — makes everything feel floaty, sticky, or unfair.

Gravity as acceleration

In the real world, gravity accelerates objects downward at 9.8 m/s². In a game, the unit is pixels per second per second, and the exact value is chosen for feel rather than realism. A typical pygame platformer uses something in the range of 800–1200 px/s².

The implementation is simple: every frame, add a downward velocity delta:

vel.y += GRAVITY * dt
pos.y += vel.y   * dt

The ball starts falling slowly, then faster, then faster still — exactly the parabolic arc players expect. The critical detail is that you also need a ground check: when the player's feet touch a platform, zero vel.y and stop moving downward.

if feet_on_ground:
    vel.y = 0

Without that reset the player sinks through the floor as accumulated velocity continues driving them downward.

Jump as an impulse

A jump is not a continuous force — it is an impulse: a one-time change to velocity. When the player presses the jump key, set vel.y to a large negative value (negative because pygame's y-axis points downward):

if jump_key_pressed and on_ground:
    vel.y = -JUMP_SPEED

From that point on, gravity decelerates the upward motion, brings it to a momentary stop at the apex, then accelerates the player back down. The jump arc is entirely determined by the ratio of JUMP_SPEED to GRAVITY.

Coyote time

Coyote time is a small grace period that lets the player jump for a few frames after walking off a ledge. Without it, a player who walks past the edge by a single pixel is denied a jump that feels — to them — like it should have worked.

The implementation: maintain a coyote_frames counter. While the player is on the ground, reset it to a small value (e.g. 6). Each frame the player is in the air, decrement it. A jump is allowed if coyote_frames > 0, not just if on_ground.

Players never consciously notice coyote time when it is present. They do notice when it is absent.

Jump buffering

Jump buffering is the mirror of coyote time: when the player presses jump slightly before landing, the press is buffered and fires as soon as the feet touch ground. Without buffering, pressing jump a frame too early results in nothing happening, which feels unresponsive.

Implementation: maintain a jump_buffer counter. When the jump key is pressed, set it to a small value (e.g. 8). Decrement it each frame. On the frame the player lands, check if jump_buffer > 0 and fire the jump immediately.

Together, coyote time and jump buffering make the jump feel generous and fair — a major factor in whether a platformer is enjoyable to play.

Where to go next

Next: gravity in code — adding all of this to a running pygame player class, including ground collision and the coyote time counter.