Beams Example

Beams are used internally for rendering cords, such as Gello's umbilical cord. Note that this only handles rendering and not physics.

Beams are best demonstrated by looking at the Ball and Chain enemies found in Gehenna, which have a chain that dynamically moves and stretches depending on how far away the spiked ball is from where it is chained.

Beams are comprised of any number of Points.

Code

You can download the full mod and its assets by clicking here.

Video Demonstration

A laser Beam of death incinerating enemies.

Example

local mod = RegisterMod("Repentogon Beam Example", 1)

-- Some constants.
local DAMAGE_MULTIPLIER = 0.01
local ITEM_ID = Isaac.GetItemIdByName("Life-Drain Gem")

-- First make a sprite.
local sprite = Sprite()
sprite:Load("gfx/chain_beam.anm2", true)
sprite:Play("Idle", true)

-- Handles the beam and damaging of the closest enemy to the player.
function mod:HandleLifeSteam(player)
    local data = player:GetData()

    -- Check if the player has our item, first.
    if not player:HasCollectible(ITEM_ID) then
        return
    end

    -- Get the beam stored in player:GetData(), or create one if it doesn't exist.
    local beam = data.LifeStealBeam
    if not beam then
        -- Create the beam. We don't need to use the overlay here.
        data.LifeStealBeam = Beam(sprite, "chain", false, false)
        beam = data.LifeStealBeam
    end

    -- Get the closest enemy
    local closestDistance, closestEnemy
    for _, enemy in ipairs(Isaac.GetRoomEntities()) do
        if enemy:IsActiveEnemy() and enemy:IsVulnerableEnemy() then
            local distanceToPlayer = enemy.Position:Distance(player.Position)
            if not closestDistance or closestDistance > distanceToPlayer then
                closestEnemy = enemy:ToNPC()
                closestDistance = distanceToPlayer
            end
        end
    end

    -- If there's no enemy in the room, end the function early.
    if not closestEnemy then
        return
    end

    -- Deal damage every frame.
    closestEnemy:TakeDamage(player.Damage * DAMAGE_MULTIPLIER, 0, EntityRef(player), 0)

    -- Render the beam. The position is in screen coordinates.
    -- The second argument, SpritesheetCoordinate, is what the Y position of the spritesheet is by the time this Point is reached.
    -- So for example, two points of 0 and 64 SpritesheetCoordinate will render the sprite starting from y0 to 64.
    -- An additional Point with a SpritesheetCoordinate of 0 will draw it in reverse from y64 to y0.
    -- Width is an optional third argument that is a multiplier used to determine the width of the Beam.
    beam:Add(Isaac.WorldToScreen(player.Position), 0)
    beam:Add(Isaac.WorldToScreen(closestEnemy.Position), 64) -- 64 is the height of the spritesheet.

    -- Render the beam. By default, this will clear all points.
    beam:Render()
end

mod:AddCallback(ModCallbacks.MC_POST_PLAYER_RENDER, mod.HandleLifeSteam)