[SOLVED] Bezier Curves, how to trace a line?

[unixfreak]

How can i make a point traverse across a bezier curve?
I whipped up a small example which hopefully will show what i want to achieve.

I'm trying to get an object to travel along a defined bezier curve, but having trouble figuring out how i would do this.
Basically, i want to plot 2 points, then move an object between them.

Using https://love2d.org/wiki/BezierCurve:render
I attached an example below, you can press space to reset/random points.

bezier_test.love

(762 Bytes) Downloaded 185 times

I have also tried fiddling around with this:
https://love2d.org/wiki/BezierCurve:evaluate
Which mentions about "move objects along paths", but i do not understand it.

Here's the code in the attached love file:

Code: Select all

function love.load()
	setRandomPoints()
end


function setRandomPoints()
	--source point
	x=love.math.random(200,love.graphics.getWidth()-200)
	y=love.math.random(200,love.graphics.getHeight()-200)

	--destination point
	dx=love.math.random(200,love.graphics.getWidth()-200)
	dy=love.math.random(200,love.graphics.getHeight()-200)

	curve = love.math.newBezierCurve(
		x,
		y,
		x*1.5- x/1.5,	
		dx,
		dy
	)

	ball = {}
	ball.x = x
	ball.y = y

end

function love.draw()
	love.graphics.setColor(150,150,150,255)
	love.graphics.line(curve:render())

	love.graphics.setColor(0,160,100,255)
	love.graphics.circle("fill", ball.x, ball.y, 5)
end

function love.keypressed(key)
	if key == "space" then
		setRandomPoints()
	end

	if key == "escape" then
		love.event.quit()
	end
end

[Ref]

May be something like you want. (old love with extraneous code)
Click on control points to see effect,

[MasterLee]

And now another episode of Two and a Half coordinates:

Code: Select all

	curve = love.math.newBezierCurve(
		x,
		y,
		x*1.5- x/1.5,	
		dx,
		dy
	)

[unixfreak]

May be something like you want. (old love with extraneous code)
Click on control points to see effect,

Thanks, i'll have a look at the code later, that should be useful.

And now another episode of Two and a Half coordinates:

Or i just clobbered together a snippet in about 30 seconds and ran it once... The love file works to visualize what should happen, i don't see how that matters.

[MasterLee]

I changed your code a little bit to make the circle move along the curve, but be warned that the circle will not move at uniform speed

Code: Select all

function love.load()
	setRandomPoints()
end


function setRandomPoints()
	--source point
	x=love.math.random(200,love.graphics.getWidth()-200)
	y=love.math.random(200,love.graphics.getHeight()-200)

	--destination point
	dx=love.math.random(200,love.graphics.getWidth()-200)
	dy=love.math.random(200,love.graphics.getHeight()-200)

	curve = love.math.newBezierCurve(
		x,
		y,
		x*1.5- x/1.5,	
		dx*1.5 - dy/1.5,
		dx,
		dy
	)

	ball = {}
	ball.x = x
	ball.y = y

end

t=0

function love.draw()
	love.graphics.setColor(150,150,150,255)
	love.graphics.line(curve:render())

	love.graphics.setColor(0,160,100,255)
	local x,y=curve:evaluate(t)
	love.graphics.circle("fill", x, y, 5)
end

function love.keypressed(key)
	if key == "w" then
		t=(t+0.03)%1
	end
	if key == "a" then
		t=(t-0.03)%1
	end
	if key == "space" then
		setRandomPoints()
	end

	if key == "escape" then
		love.event.quit()
	end
end

[Ref]

Was able to fix constant speed issue for spline but not for Bezier curve.
For what it's worth (drag points to see that speed remains constant)
Best

[unixfreak]

Thank you both, very much this has helped alot.

I think i understand this more now. I only stumbled upon this function for bezier curves yesterday, and it's not something i'm familiar with.
Using your examples i managed to implement something roughly that works... although my implementation seems to cause an issue with the "t" value MasterLee provided in their example.

What is the best way to determine when the object has traversed to the destination value?
Printing the "t" value there, only ever shows something like "0.99961439483113" as it's max value.

How would i determine when the object has reached the opposite point of the bezier curver?

Here's a snippet of my code how it's being used, i tried to comment it so it's understandable. This works sometimes, but every now and again causes an error: invalid evaluation parameter: must be between 0 and 1.

Code: Select all

function missiles.update(dt)

	-- missiles are added to a table in another function
	for i, missile in ipairs(missiles.launched) do
		
		-- increase t value of missile, so it traverse bezier curve
		missile.t= missile.t+missile.speed*dt
		
		-- evaluate new mx/my values
		missile.mx,missile.my=missile.curve:evaluate(missile.t)
		
		--when the missile reaches the destination
		--using this value works most of the time, but sometimes causes the error
		if t > 0.99 then
		
			-- add an impact target to the targets table
			targets.add(map.nations[missile.target].x,map.nations[missile.target].y,missile.attacker,missile.target)
			
			--remove the trajectory path of the launched missile
			table.remove(missiles.launched,i)
		end
	end
end

Also this should probably be more helpful showing how i am trying to use this;


The actual curve placement needs a bit of work, but the missiles following the bezier curve works to an extent, however it rarely causes the error as mentioned above.

[MasterLee]

Code: Select all

function missiles.update(dt)

	-- missiles are added to a table in another function
	for i, missile in ipairs(missiles.launched) do
		
		-- increase t value of missile, so it traverse bezier curve
		missile.t= missile.t+missile.speed*dt
		
	
		--when the missile reaches the destination
		--using this value works most of the time, but sometimes causes the error
		if t > 0.99 then
		
			-- add an impact target to the targets table
			targets.add(map.nations[missile.target].x,map.nations[missile.target].y,missile.attacker,missile.target)
			
			--remove the trajectory path of the launched missile
			table.remove(missiles.launched,i)
		else
			-- evaluate new mx/my values
			missile.mx,missile.my=missile.curve:evaluate(missile.t)
		end		
	end
end

Use the else clause. Also you should read:
https://love2d.org/forums/viewtopic.php ... me#p210479
Because the way you use table.remove enables an missile to move far beyond the target.

[unixfreak]

Thanks that makes sense now, i changed it a little so that the target is always added at the exact end of the bezier curve. Many thanks.
I understand the ipairs function, i don't see what you mean there though. But this is working great nicely now.

Code: Select all

function missiles.update(dt)
	for i, missile in ipairs(missiles.launched) do

		missile.t = missile.t+missile.speed*dt
		
		if missile.t > 1 then
			targets.add(map.nations[missile.target].x,map.nations[missile.target].y,missile.attacker,missile.target)
			table.remove(missiles.launched,i)
		else
			missile.mx,missile.my=missile.curve:evaluate(missile.t)
		end
	end
end

[MasterLee]

Read the following post it covers the problem
https://love2d.org/forums/viewtopic.php ... me#p210511