A proof of concept on how to draw a line and have it act as a physical body. I owe MichaelAssadi a huge debt of gratitude for posting his example, it was what got me started.

The problem with other examples is that it basically draws a box at the intersection of each segment which I couldn't come up with a way of making it smooth. Also, as the move event fires (it looks like) every frame when drawing if you're swiping across the screen fast there are large gaps which objects may fall through (easy to do in Michael's example.)

I took inspiration from that and from the fact that you can use custom polygons to create physical bodies on a displayObject and am just laying a polygon on each segment. I think this should perform fairly well (would think as well as the boxes as it's just a 3-sided poly per segment and it gives 100% smooth coverage along the line. The one possible point of failure is that it is literally a flat poly, the poly is just (from line segment perspective) x1,y1 -> x2, y2 -> x1, y1. I can't imagine any cases where that would break but it's the biggest question about it in my mind. You could also have a problem if you have a fat line with the poly being it the middle. The interacting object won't "touch" until the center of the line (to see what I'm talking about set line_width = 25 or something. Could probably cheat this by doing radius on the ball's physics body but haven't played with it.

One way I'd like to try improving this if I get time to spend is to apply the smoothCurve (catmull splines) code that's being used in the Flight Path example. Anybody? Anybody? ; )

I'm not a neophyte developer but am new to Lua and Corona and did this very sloppy so forgive the lack of comments (should be fairly understandable though). Also, if anybody sees anything desperately wrong or has any style/code/design suggestions please let me know (you know, like mixing snake and camel case ; ).

NOTES:
There are two boxes on the bottom, the box on the left will drop a new ball (not collecting balls, so potential memory leak of seriously abused), and the box on the right will clear all lines.

The colors are the way they are because my 4 y/o daughter started playing with it on my iPad and I did that as a little treat for her.

It does seem to chug along pretty seriously at like the bottom of a U type curve and I haven't been able to figure that one out at all, any suggestions would be greatly appreciated. Set DrawMode to hybrid to see it clearly.

Thanks to all for sharing your code gems here and for any feedback and/or improvements!

Gerald

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require("physics")
physics.start()
physics.setDrawMode("normal")
 
local HEIGHT = display.contentHeight
local WIDTH = display.contentWidth
 
 
local lines = {}
local line_number = 1
local line_width = 5
local prev_x, prev_y, ball
 
local function draw_line(e)
 
  if e.phase == "began" then
    prev_x = e.x
    prev_y = e.y
 
  elseif e.phase == "moved" then
    lines[line_number] = display.newLine(prev_x, prev_y, e.x, e.y) 
    lines[line_number]:setColor(math.random(255), math.random(255), math.random(255))
    lines[line_number].width = line_width
    dist_x = e.x - prev_x
    dist_y = e.y - prev_y
    -- Add a physics body that's a flat polygon that follows each segment of the line
    physics.addBody(lines[line_number], "static", { density = 1, friction = 0.5, bounce = 0, shape = {0, 0, dist_x, dist_y, 0, 0} } )
    prev_x = e.x
    prev_y = e.y
    line_number = line_number + 1
  elseif e.phase == "ended" then
  end
end
 
 
new_ball_button = display.newRect(10, HEIGHT - 75, 150, 75)
delete_lines_button = display.newRect(WIDTH - 150, HEIGHT - 75, 150, 75)
 
-- Create a new ball
function new_ball_button:touch(e)
  if e.phase == "ended" then
    ball = display.newCircle(50, 50, 25) 
    physics.addBody(ball, {density = 5000, friction = 0.5, bounce = 0.1, radius = 25} )
    ball:setFillColor(math.random(255), math.random(255), math.random(255))
  end
end
 
-- Delete all existing lines
function delete_lines_button:touch(e)
 
  if e.phase == "ended" then
    for i = #lines, 1, -1 do
      if (lines[i]) then
        lines[i].parent:remove(lines[i])
        lines[i] = nil
      end
    end
    lines = {}
    line_number = 1
  end
end
 
 
Runtime:addEventListener("touch", draw_line)
new_ball_button:addEventListener("touch", new_ball_button)
delete_lines_button:addEventListener("touch", delete_lines_button)