Different principle of physics working with the same code

Questions
#1

UPD: I just gave up and increased the size of the go to six times the size of the first project

Note: I apologize if the question title was worded incorrectly. I didn’t know how to properly describe my problem.

Tried to learn how to make a platformer from the guides. In the first project, the character moves and jumps very smoothly and pleasing to the eye.

local GRAVITY = 100

function init(self)
	self.velocity = vmath.vector3()
	msg.post(".", "acquire_input_focus")
	self.move = true
end

function fixed_update(self, dt)
	self.velocity.y = self.velocity.y - GRAVITY * dt

	if self.ground_contact then
		self.velocity.y = 0
	end
		
	local position = go.get_position()
	position = position + self.velocity 
	go.set_position(position)
	self.velocity.x = 0 
end

function on_message(self, message_id, message, sender)
	if message_id == hash("contact_point_response")
	and message.group == hash("earth") then
		self.ground_contact = true;
	end
	
	if message_id == hash("contact_point_response")
	and message.group == hash("ledge") then
		self.ground_contact = true
		self.jumping = true
	end
end

function on_input(self, action_id, action)
	if action_id ~= hash("jump") then
		if action_id == hash("right") then
			self.velocity.x = self.velocity.x + 20
			if self.jumping then
				self.ground_contact = false
				self.jumping = false
			end
		elseif action_id == hash("left") then
			self.velocity.x = self.velocity.x - 20
			if self.jumping then
				self.ground_contact = false
				self.jumping = false
			end
		end
	elseif action.pressed and self.ground_contact == true and action_id == hash("jump") then
		self.ground_contact = false
		self.velocity.y = 35
	end
end

In the second, the code (if you ignore the if-else) is identical, but the character jerks rather unpleasantly instead of jumping smoothly

local GRAVITY = 100
local JUMP = 20 
local MOVE_SPEED = 100
function init(self)
	msg.post(".", "acquire_input_focus")

	self.velocity = vmath.vector3()

	self.downfall = true -- True - падение | False - удержание на месте

end

function fixed_update(self, dt)

	self.velocity.y = self.velocity.y - GRAVITY * dt

	if not self.downfall then
		self.velocity.y = 0
	end

	self.velocity.x = self.velocity.x * dt
	
	local position = go.get_position()
	position = position + self.velocity
	go.set_position(position)

	self.velocity.x = 0
	
end

function on_message(self, message_id, message, sender)
	if message_id == hash("contact_point_response")
	and message.own_group == hash("hands") then
		self.downfall = false 
	end
end

function on_input(self, action_id, action)
	
	if action_id == hash("right") then
		self.velocity.x = self.velocity.x + MOVE_SPEED
	end

	if action_id == hash("left") then
		self.velocity.x = self.velocity.x - MOVE_SPEED
	end

	--Прыжок
	if action_id == hash("jump") 
	and self.downfall == false 
	and action.pressed then
		self.velocity.y = JUMP
		self.downfall = true 
	end

end

The only difference between them is that in the first one I zoomed all go objects, and in the second one I tried to realize zooming through zoom (I use an orthographic camera))

(first project) TestPlatformerProject.zip (90.8 KB)
(second project) RiseToMountin.zip (259.9 KB)

#2

What I see is that the 2 code are much different.
For example this one doesn’t exist in the first code.

What do you mean of that?

#3

Yes, there are definitely differences, but they have to do with logic. When I did the first project it was according to the guide, but I wanted to write the logic myself. The physics is pretty much identical.

About self.velocity.x, I thought that’s the way to do it so that movement works the same regardless of the number of fps, maybe I misunderstood the principle of multiplication by dt

#4

You would do it here:

position = position + self.velocity * dt

Multiplying by dt changes the logic from “add this amount of velocity per frame” to “add this amount of velocity per second”.

You’re maybe confused by this line:

self.velocity.y = self.velocity.y - GRAVITY * dt

That one is fine because it says “reduce y velocity by this amount of gravity per second”.

Multiplying self.velocity.x by dt is meaningless.

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