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Cart #41917 | 2017-06-24 | Code ▽ | Embed ▽ | License: CC4-BY-NC-SA
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While working on my 3d engine I've written a function that I think would be generally useful and I haven't seen it done before. It draws horizontal dithered lines such that odd and even rows have an alternating checkerboard pattern. Code is obviously in the cart but I post below as well.

The code runs just as fast the native LINE function but with the bonus of optional dithered-e-ness

 
function dither(sx,ex,y,draw_colour,shade_colour)
  if (sx<0 and ex<0) or (sx>127 and ex>127) then return end
  sx = max(0,min(sx,127))
	 ex = max(0,min(ex,127))
	 if ex < sx then
	  sx,ex = ex,sx
	 end
	 local sos = sx % 2
	 local eos = ex % 2
	 local yind = y*64+0x6000
	 local ctu = -1
	 if y%2 == 0 then
	  ctu = draw_colour
	 else
	  ctu = shade_colour
	 end

	  if sos == 1 then
	   sx -= 1	   
	   poke(yind+sx/2, bor(band(peek(yind+sx/2),0x0f), band(0xf0,ctu)))	   
	   sx += 2
	  end 

	  if eos == 0 then		      
	   poke(yind+flr(ex/2), bor(band(peek(yind+ex/2),0xf0),band(0x0f, ctu)))
	   ex -= 1
	  end

	  memset(yind +sx/2,ctu, (ex/2-sx/2)+1)
end

The tricky bit being to deal with the start and end of the line when they do no perfectly align with the byte boundaries.

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