Simple physics question

Questions
1

Why doesn’t the box drawn during draw follow the updated position of the polygon? Box.x and box.y show that the polygon is affected by its initial velocity and gravity, but no updates to the drawn position.
Thanks in advance

-- Use this function to perform your initial setup
function setup()
    print("Hello World!")
    ox= WIDTH/2
    oy = HEIGHT/2
    box = physics.body(POLYGON, vec2(ox,oy),vec2(ox,oy+20),vec2(ox+20,oy+20),vec2(ox+20,oy) )
    box.linearVelocity=vec2(20,0)
    box.type = DYNAMIC
     
    
end

-- This function gets called once every frame
function draw()
    -- This sets a dark background color 
    background(40, 40, 50)
   physics.gravity()
    -- This sets the line thickness
    strokeWidth(5)

    -- Do your drawing here
    if box.shapeType == POLYGON then
        strokeWidth(10)
        points = box.points
        for j = 1,#points do
            a = points[j]
            b = points[(j % #points)+1]
            line(a.x, a.y, b.x, b.y)
        end
    end
    print(box.x)
    print(box.y)
end
2

I believe it is because the points are in the local space of the body (and fixed in that space) not in the world space of the viewer. See PhysicsDebugDraw:draw() in the Physics Lab example project, and note the initial translate and rotate that move the view so that it is aligned with the local space.

3

Hi @Jdrj, few days ago I had a similar problem. Check the way I solved it following this link

http://twolivesleft.com/Codea/Talk/discussion/comment/11491#Comment_11491

It can give you perhaps a clue.

Saludos, Victor
~O)

4

I have edited your code and added comments to it to explain further my first comment:


function setup()
    ox= WIDTH/2
    oy = HEIGHT/2
    box = physics.body(POLYGON, --<< Relative to centre of mass
        vec2(-10, -10),         --<< and set counter-clockwise
        vec2(10, -10),
        vec2(10, 10),
        vec2(-10, 10))
    box.x = ox --<< Position of centre of mass
    box.y = oy --<< Position of centre of mass
    box.linearVelocity = vec2(20, 0)
    box.type = DYNAMIC -- The type is DYNAMIC by default
end

function draw()
    background(0)
    physics.gravity() -- Has no effect, without an argument
    strokeWidth(5)    -- Has no effect, as strokeWidth is reset to 10 below
    pushMatrix()      --<< Preserve the matrix
    translate(box.x, box.y) --<< Move view over the centre of mass
    rotate(box.angle)       --<< Rotate view for the body's rotation
    if box.shapeType == POLYGON then -- Will always be true
        strokeWidth(10)
        points = box.points -- These are in the local space of the box,
                            -- relative to the centre of mass and with no rotation
        for j = 1, #points do
            a = points[j]
            b = points[(j % #points) + 1]
            line(a.x, a.y, b.x, b.y)
        end
    end
    popMatrix() -- Restore the matrix
end
5

Thanks for that. I do appreciate it.

6

There is an explanation of the Physics Lab example project on the wiki here. I have extended it to cover in more detail the subject of this discussion.