BDR

This was added to the API in version 0.90. See SCN for older carts.

The BDR callback allows you to execute code between the rendering of each scan line. The primary reason to do this is to manipulate the palette. Doing so makes it possible to use a different palette for each scan line, and therefore more than 16 colors at a time.

Usage

function BDR(scanline)
  -- your code here
end

Parameters

• scanline : The scan line about to be drawn (0..143)

Details

+-----------+-----------------
| scanline  | display        |
+-----------+----------------+
| 0 - 3     | TOP BORDER     |
| 4         | ROW 0          |  equiv SCN(0)
| ...       | ...            |  equiv SCN(n - 4)
| 139       | ROW 135        |  equiv SCN(135)
| 140 - 143 | BOTTOM BORDER  |
+-----------+----------------+

Example Glitch

Glitch effect using BDR:

-- 'Change screen offset in every scanline' demo
-- author: Vadim
shake=0
d=4
function TIC()
	if btnp()~=0 then shake=30 end
	if shake>0 then
		poke(0x3FF9+1,math.random(-d,d))
		shake=shake-1
		if shake==0 then memset(0x3FF9,0,2) end
	end
	cls(12)
	print("PRESS ANY KEY TO GLITCH!",54,64)
end

function BDR(row)
	if shake>0 then
		poke(0x3FF9,math.random(-d,d))
	end
end

Example 256 Colors

The following example displays all 256 possible shades of gray. This can be done with any color.

-- title:  256 shades of gray
-- author: Marcuss2, fixed by nesbox
-- desc:   Showoff of grayscale
-- script: lua
-- input:  mouse

ADDR = 0x3FC0
palette = 0

function addLight()
 for i=0, 15 do
  for j=0, 2 do
   poke(ADDR+(i*3)+j, palette)
  end
  palette = palette + 1
 end
end

function BDR(scnline)
 if scnline % 8 == 0 then
  addLight()
 end
end

function init()
 for i=0, 16 do
  rect(i*15, 0, 15, 240, i)
 end
end

init()

function TIC()
 palette = 0
end