Vital.sandbox

util.easing.back_in_out

Shared

Applies a back ease-in-out — pulls back at the start and overshoots at the end


Syntax

local result = util.easing.back_in_out(
    progress,
    overshoot = 1.70158
)

Back easing anticipates motion by briefly exceeding the [0, 1] range before settling.

  • overshoot parameter — controls how far the curve pulls back or overshoots.
  • Best for — playful UI elements, cartoon-style movement, and elastic button presses.

Parameters

TypeNameDescription
floatprogressInterpolation factor, typically in the range [0, 1]
floatovershootOvershoot magnitude applied to both ends.

Returns

TypeNameDescription
floatresultEased progress value

Examples

Sample output at key progress values
core.engine.print("info", util.easing.back_in_out(0.0))  -- 0.0
core.engine.print("info", util.easing.back_in_out(0.25)) -- near start
core.engine.print("info", util.easing.back_in_out(0.5))  -- midpoint
core.engine.print("info", util.easing.back_in_out(0.75)) -- near end
core.engine.print("info", util.easing.back_in_out(1.0))  -- 1.0
Pair with lerp to animate a value
local result = util.easing.lerp(0, 100, util.easing.back_in_out(0.5))

core.engine.print("info", result)
Animate a circle across the screen
local resolution = core.engine.get_resolution()
local duration = 1.5
local start_tick = core.engine.get_tick()

util.event.on("sandbox:draw", function()
    local elapsed = (core.engine.get_tick() - start_tick) / 1000.0
    local t = math.min(elapsed / duration, 1.0)
    local x = util.easing.lerp(0, resolution[1], util.easing.back_in_out(t))

    core.engine.draw_circle({x, resolution[2]*0.5}, 16, {1, 1, 1, 1})
end)

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SyntaxParametersReturnsExamples