Day 13: Platforms & Collision

Create the Platform Sprite

Create a new sprite called platform. In the costume editor, draw a filled rectangle — about 100-150 pixels wide and 20 pixels tall. Position it somewhere above the ground on the stage.

You can draw multiple rectangles on the same costume to create several platforms at different heights. Since they are all part of one sprite, a single touching [platform v]? block detects all of them.

Land on Platforms

Right now, your gravity code only checks for the ground. We need the player to also land on platforms. This is where the or operator comes in — the player should stop falling when touching the ground or a platform.

We’ll also make two improvements to the code structure:

1. Move first, then check. Moving change y by (velocity) to the top of the loop means the player moves before we check what they’re touching. This is important for jumping — without it, the player would jump and immediately be detected as touching the ground again.
2. Use a gravity variable instead of a hardcoded -1. This makes it easy to adjust later.

Create a new variable called gravity. Then update your gravity code to match this:

when green flag clicked
set [gravity v] to [-1]
set [velocity v] to [0]
forever
  change y by (velocity)
  if <not <<touching [ground v]?> or <touching [platform v]?>>> then
    change [velocity v] by (gravity)
  else
    set [velocity v] to [0]
  end
end

The key change from yesterday is wrapping the condition with or:

<touching [ground v]?> or <touching [platform v]?>

This means: “if the player is touching the ground or touching a platform, stop falling.”

Test it now. Click the green flag and walk your player under the platform, then jump. You should land on the platform and stop falling. If the player falls through, double-check that your or block has both touching checks inside it.

Add Landing Bounce

Try jumping high and falling onto a platform. At high speeds, the player can sink into a platform because change y by (velocity) moves several pixels at once.

To fix this, we add a bounce inside the else branch. When the player lands while falling, we reverse and halve their velocity. This gently pushes them back out of the surface.

Update the else branch:

when green flag clicked
set [gravity v] to [-1]
set [velocity v] to [0]
forever
  change y by (velocity)
  if <not <<touching [ground v]?> or <touching [platform v]?>>> then
    change [velocity v] by (gravity)
  else
    if <(velocity) < (0)> then
      set [velocity v] to ((-0.5) * (velocity))
    else
      set [velocity v] to [0]
    end
  end
end

Here’s what the else branch does now:

• If velocity < 0 (the player was falling): set velocity to -0.5 * velocity. Since velocity is negative, this flips it to a small positive value, bouncing the player gently upward until they settle onto the surface. For example, if velocity is -8, then -0.5 * -8 = 4 — the player bounces up at half the speed they were falling.
• Otherwise (the player was moving up or standing still): set velocity to 0.

Test it. Jump from a high platform and land on a lower one. You should see a gentle bounce when you land instead of an instant stop.

Jump from Platforms

Create a new variable called jump-velocity and set it to 10 at the start. Then update your jump code — the player should be able to jump when touching the ground or a platform:

when green flag clicked
set [jump-velocity v] to [10]
set [gravity v] to [-1]
set [velocity v] to [0]
forever
  change y by (velocity)
  if <not <<touching [ground v]?> or <touching [platform v]?>>> then
    change [velocity v] by (gravity)
  else
    if <(velocity) < (0)> then
      set [velocity v] to ((-0.5) * (velocity))
    else
      set [velocity v] to [0]
    end
  end
end

when [space v] key pressed
if <<touching [ground v]?> or <touching [platform v]?>> then
  set [velocity v] to (jump-velocity)
end

Test it now. You should be able to jump onto the platform and jump again from on top of it. Try changing jump-velocity and gravity to see how they affect the feel of the game.

Wall Collision (Move-Check-Step Up)

Try walking into the side of a platform. You’ll walk right through it. To fix this, we need to check for walls after every horizontal move.

There’s a catch: when you’re standing on the ground or a platform, your player is already touching it. If we just check touching after moving sideways, the game would think you’re hitting a wall every time — even when you’re just walking on flat ground.

The fix is a step-up test:

1. Move the player sideways
2. If touching something, step up 5 pixels
3. If still touching something, it’s a real wall — undo the move
4. Step back down

If stepping up clears the overlap, you were just standing on a surface — no wall. If you’re still stuck after stepping up, there’s a wall in the way.

We’ll switch from arrow keys to A and D keys for left/right movement. This keeps the player’s hands in one area of the keyboard — A/D for movement and space for jumping — which is easier during gameplay.

Update your left/right movement code:

when green flag clicked
forever
  if <key [a v] pressed?> then
    change x by (-5)
    if <<touching [ground v]?> or <touching [platform v]?>> then
      change y by (5)
      if <<touching [ground v]?> or <touching [platform v]?>> then
        change x by (5)
      end
      change y by (-5)
    end
  end
  if <key [d v] pressed?> then
    change x by (5)
    if <<touching [ground v]?> or <touching [platform v]?>> then
      change y by (5)
      if <<touching [ground v]?> or <touching [platform v]?>> then
        change x by (-5)
      end
      change y by (-5)
    end
  end
end

Test it — your player should walk normally on flat ground, but stop when they hit the side of a platform.

Known Bug: Head Bump

Try jumping up into the bottom of a platform. You’ll notice the player can get stuck to the ceiling. This happens because when the player moves upward into a platform, velocity is set to 0 — but the player is now inside the platform, and there’s nothing to push them back out.

This is a tricky problem to solve cleanly, and we’ll fix it in a future lesson. For now, design your platforms so the player doesn’t need to jump directly into the bottom of one. Space your platforms far enough apart that the player lands on top rather than hitting from below.

Add More Platforms

To add more platforms, edit the platform sprite’s costume and draw more rectangles at different positions. Since they are all part of the same sprite, your code works for all of them automatically — no changes needed.

Try creating a layout with 3-4 platforms at different heights that the player can jump between.

Tip: Make your platforms at least 20 pixels thick. Thin platforms can cause the player to fall through them at high speeds.