Subpixel accurate triangle rasterizer prototype for Pico 8

triangle.p8

pico-8 cartridge // http://www.pico-8.com
version 18
__lua__
--[[function spline(x0,y0,x1,y1,c)
 local dx = x1 - x0
 local dy = y1 - y0
 local step = dx/dy
 
 local x = x0 - (y0 % 1) * step
 local y = flr(y0)
 local ymax = flr(y1)
 
 pset(x0, y0, 8)
 
 while y < ymax do
  pset(x, y, c)
 	y += 1
 	x += step
 end
end]]

function getline(x0,y0,x1,y1)
 local dx = x1 - x0
 local dy = y1 - y0
 local step = dx/dy
 
 local x = x0 - (y0 % 1) * step
 --local y = flr(y0)
 --local ymax = flr(y1)
 
 return {
  x = x,
  --y = y,
  --ymax = ymax,
  step = step
 }
end

function drawpx(x,y,c)
 pset(x, y, 
 bxor(pget(x, y), c))
end

function drawtri(tri,c)
 local dy = tri[2].y - tri[1].y
 local left, right
 
 if dy == 0 then
  left = getline(
   tri[1].x, tri[1].y,
   tri[3].x, tri[3].y)
  right = getline(
   tri[2].x, tri[2].y,
   tri[3].x, tri[3].y)
 else
  left = getline(
   tri[1].x, tri[1].y,
   tri[2].x, tri[2].y)
  right = getline(
   tri[1].x, tri[1].y,
   tri[3].x, tri[3].y)
 end
 
 local y = flr(tri[1].y)
 local ymax = flr(tri[3].y)
 
 y+=1
 left.x += left.step
 right.x += right.step
  
 while y <= ymax do
  local x=flr(left.x)
  local maxx=flr(right.x)
  while x < maxx do
   -- using pset here for debugging, use rectfill
   -- in the real world for performance!!
   drawpx(x,y,c)
   x += 1
  end
   y += 1
   left.x += left.step
   right.x += right.step
 end
end

function maketri(x0,y0,x1,y1,x2,y2)
 local top, middle, bottom, tmp
 local tris={}
 
 top = { x=x0, y=y0 }
 middle = { x=x1, y=y1 }
 bottom = { x=x2, y=y2 }
 
 if top.y > middle.y then
  tmp = top
  top = middle
  middle = tmp
 end
 
 if top.y > bottom.y then
  tmp = top
  top = bottom
  bottom = tmp
 end
 
 if middle.y > bottom.y then
  tmp = middle
  middle = bottom
  bottom = tmp
 end
 
 if top.y == middle.y then
  if top.x < middle.x then
   add(tris,{top,middle,bottom})
  else
   add(tris,{middle,top,bottom})
  end
 elseif bottom.y == middle.y then
  if middle.x < bottom.x then
   add(tris,{top,middle,bottom})
  else
   add(tris,{top,bottom,middle})
  end
 else
  local step = 
   (bottom.x - top.x) / 
   (bottom.y - top.y)
  local dist = middle.y - top.y
  local mid2 = {
   x = top.x + step * dist,
   y = middle.y
  }
  
  if middle.x < mid2.x then
   add(tris,{top,middle,mid2})
   add(tris,{middle,mid2,bottom})
  else
   add(tris,{top,mid2,middle})
   add(tris,{mid2,middle,bottom})
  end
 end
 
 return tris
end

xadd=0
yadd=0

function _draw()
 cls()
 local tris = {}
 add(tris,maketri(
  60 + xadd, 32 + yadd,
  31 + xadd, 97 + yadd,
  93 + xadd, 81 + yadd))
 add(tris,maketri(
  60 + xadd, 32 + yadd,
  93 + xadd, 81 + yadd,
  100 + xadd, 28 + yadd))
 add(tris,maketri(
  31 + xadd, 97 + yadd,
  93 + xadd, 81 + yadd,
  105 + xadd, 108 + yadd))
 add(tris,maketri(
  93 + xadd, 81 + yadd,
  105 + xadd, 108 + yadd,
  100 + xadd, 28 + yadd))
	  
 local c=1
 for i=1,#tris do
  for j=1,#tris[i] do
   local tri=tris[i][j]
   drawtri(tri,c)
  end
  c *= 2
 end
	
 --xadd += 0.1
 --yadd += 0.1
	
 xadd = sin(t() / 12) * 10
 yadd = cos(t() / 12) * 10
end