trajectory2.p8

pico-8 cartridge // http://www.pico-8.com
version 29
__lua__
-- point to point trajectory and animation
-- by @apa64

function _init()
  p1 = { x = 10, y = 10 }
  p2 = { x = 100, y = 10 }
  pk = { speed = 2, c = 11 }
end

function _update()  
  pk.x = pk.x and (pk.x + pk.speed) or 0
  --[[
  if (dx == 0) then
    -- vertical line
    return xk
  end
  if (dy == 0) then
    -- horizontal line
  end
  --]]
  pk.y = pointslope(p1.x, p1.y, p2.x, p2.y, pk.x)

  if (pk.x > 127 or pk.y > 127) then
    pk.x = 0
    p2.y += 10
  end
end

function _draw()
  cls(1)
  -- line1
  line(p1.x, p1.y, p2.x, p2.y, 5)
  pset(pk.x, pk.y, pk.c)
end

function pointslope(x1, y1, x2, y2, xk)
  local dx = x2 - x1
  -- can't handle vertical lines
  assert(dx != 0)
  local dy = y2 - y1
  -- slope
  local m = dy / dx
  -- y = mx + b | b = y - mx
  local b = y2 - m * x2
  local yk = m * xk + b
  -- local xk = (yk - b)/m
  -- oneliner: yk = ((y2 - y1)/(x2 - x1)) * xk + (y2 - ((y2 - y1) / (x2 - x1)) * x2)
  return yk
end

__gfx__
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