AttachElementsOffsets

This article concerns the note left in attachElements.

Problem

The offset coordinates reflect the object space, not the world space. This means that you cannot simply visualize the attachment in the map editor and calculate the offsets between the 2 sets of world coordinates for "theElement" and "theAttachToObject".

For example, if "theAttachToElement" has XYZ rotations, then "theElement" will inherit these rotations. The specified rotation offsets will then be performed from these starting rotation points. Simply put, "theElement" will be rotated twice.

Solution

The following code shows how to use offsets calculated in the map editor with 'attachElements':

addEventHandler( "onResourceStart", resourceRoot,
    function()
        -- Postion and rotations from the map editor:
        local mainPos = { -756, 995, 14 }
        local mainRot = { 0, 0, 90 }            -- Two rotations are zero. See note in attachRotationAdjusted

        local subPos = { -756, 999, 24 }
        local subRot = { 89, 0, 177 }           -- One rotation is zero. See note in attachRotationAdjusted

        -- Create the objects
        mainObject = createObject ( 17050, mainPos[1], mainPos[2], mainPos[3], mainRot[1], mainRot[2], mainRot[3] )
        subObject = createVehicle ( 519, subPos[1], subPos[2], subPos[3], subRot[1], subRot[2], subRot[3] )

        -- Attach so they look like what they do in the map editor
        attachRotationAdjusted ( subObject, mainObject )
    end
)


function attachRotationAdjusted ( from, to )
    -- Note: Objects being attached to ('to') should have at least two of their rotations set to zero
    --       Objects being attached ('from') should have at least one of their rotations set to zero
    -- Otherwise it will look all funny

    local frPosX, frPosY, frPosZ = getElementPosition( from )
    local frRotX, frRotY, frRotZ = getElementRotation( from )
    local toPosX, toPosY, toPosZ = getElementPosition( to )
    local toRotX, toRotY, toRotZ = getElementRotation( to )
    local offsetPosX = frPosX - toPosX
    local offsetPosY = frPosY - toPosY
    local offsetPosZ = frPosZ - toPosZ
    local offsetRotX = frRotX - toRotX
    local offsetRotY = frRotY - toRotY
    local offsetRotZ = frRotZ - toRotZ

    offsetPosX, offsetPosY, offsetPosZ = applyInverseRotation ( offsetPosX, offsetPosY, offsetPosZ, toRotX, toRotY, toRotZ )

    attachElements( from, to, offsetPosX, offsetPosY, offsetPosZ, offsetRotX, offsetRotY, offsetRotZ )
end


function applyInverseRotation ( x,y,z, rx,ry,rz )
    -- Degress to radians
    local DEG2RAD = (math.pi * 2) / 360
    rx = rx * DEG2RAD
    ry = ry * DEG2RAD
    rz = rz * DEG2RAD

    -- unrotate each axis
    local tempY = y
    y =  math.cos ( rx ) * tempY + math.sin ( rx ) * z
    z = -math.sin ( rx ) * tempY + math.cos ( rx ) * z

    local tempX = x
    x =  math.cos ( ry ) * tempX - math.sin ( ry ) * z
    z =  math.sin ( ry ) * tempX + math.cos ( ry ) * z

    tempX = x
    x =  math.cos ( rz ) * tempX + math.sin ( rz ) * y
    y = -math.sin ( rz ) * tempX + math.cos ( rz ) * y

    return x, y, z
end