vision/shaders/fastgaussianblur.lua

--[[
The MIT License (MIT)

Copyright (c) 2017 Tim Moore
Adapted for new moonshine API by Matthias Richter

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
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copies or substantial portions of the Software.

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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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]]--

-- Bilinear Gaussian blur filter as detailed here: http://rastergrid.com/blog/2010/09/efficient-gaussian-blur-with-linear-sampling/
-- Produces near identical results to a standard Gaussian blur by using sub-pixel sampling,
-- this allows us to do ~1/2 the number of pixel lookups.

-- unroll convolution loop
local function build_shader(taps, offset, offset_type, sigma)
  taps = math.floor(taps)
  sigma = sigma >= 1 and sigma or (taps - 1) * offset / 6
  sigma = math.max(sigma, 1)

  local steps = (taps + 1) / 2

  -- Calculate gaussian function.
  local g_offsets = {}
  local g_weights = {}
  for i = 1, steps, 1 do
		g_offsets[i] = offset * (i - 1)

    -- We don't need to include the constant part of the gaussian function as we normalize later.
    -- 1 / math.sqrt(2 * sigma ^ math.pi) * math.exp(-0.5 * ((offset - 0) / sigma) ^ 2 )
    g_weights[i] = math.exp(-0.5 * (g_offsets[i] - 0) ^ 2 * 1 / sigma ^ 2 )
  end

  -- Calculate offsets and weights for sub-pixel samples.
  local offsets = {}
  local weights = {}
  for i = #g_weights, 2, -2 do
    local oA, oB = g_offsets[i], g_offsets[i - 1]
    local wA, wB = g_weights[i], g_weights[i - 1]
    wB = oB == 0 and wB / 2 or wB -- On center tap the middle is getting sampled twice so half weight.
    local weight = wA + wB
    offsets[#offsets + 1] = offset_type == 'center' and (oA + oB) / 2 or (oA * wA + oB * wB) / weight
    weights[#weights + 1] = weight
  end

  local code = {[[
    extern vec2 direction;
    vec4 effect(vec4 color, Image tex, vec2 tc, vec2 sc) {]]}

  local norm = 0
  if #g_weights % 2 == 0 then
    code[#code+1] =  'vec4 c = vec4( 0.0 );'
  else
    local weight = g_weights[1]
    norm = norm + weight
    code[#code+1] = ('vec4 c = %f * texture2D(tex, tc);'):format(weight)
  end

  local tmpl = 'c += %f * ( texture2D(tex, tc + %f * direction)+ texture2D(tex, tc - %f * direction));\n'
  for i = 1, #offsets, 1 do
    local offset = offsets[i]
    local weight = weights[i]
    norm = norm + weight * 2
    code[#code+1] = tmpl:format(weight, offset, offset)
  end
  code[#code+1] = ('return c * vec4(%f) * color; }'):format(1 / norm)

  local shader = table.concat(code)
  return love.graphics.newShader(shader)
end

return function(moonshine)
  local taps, offset, offset_type, sigma = 7, 1, 'weighted', -1
  local shader = build_shader(taps, offset, offset_type, sigma)

  local function draw(buffer)
    shader:send('direction', {1 / love.graphics.getWidth(), 0})
    moonshine.draw_shader(buffer, shader)

    shader:send('direction', {0, 1 / love.graphics.getHeight()})
    moonshine.draw_shader(buffer, shader)
  end

  local setters = {}

  -- Number of effective samples to take per pass. e.g. 3-tap is the current pixel and the neighbors each side.
  -- More taps = larger blur, but slower.
  setters.taps = function(v)
    assert(tonumber(v) >= 3, "Invalid value for `taps': Must be >= 3")
    assert(tonumber(v)%2 == 1, "Invalid value for `taps': Must be odd")
    taps = tonumber(v)
    shader = build_shader(taps, offset, offset_type, sigma)
  end

  -- Offset of each tap.
  -- For highest quality this should be <=1 but if the image has low entropy we
  -- can approximate the blur with a number > 1 and less taps, for better performance.
  setters.offset =  function(v)
    offset = tonumber(v) or 0
    shader = build_shader(taps, offset, offset_type, sigma)
  end

  -- Offset type, either 'weighted' or 'center'.
  -- 'weighted' gives a more accurate gaussian decay but can introduce modulation
  -- for high frequency details.
  setters.offset_type = function(v)
    assert(v == 'weighted' or v == 'center', "Invalid value for 'offset_type': Must be 'weighted' or 'center'.")
    offset_type = v
    shader = build_shader(taps, offset, offset_type, sigma)
  end

  -- Sigma value for gaussian distribution. You don't normally need to set this.
  setters.sigma =  function(v)
    sigma = tonumber(v) or -1
    shader = build_shader(taps, offset, offset_type, sigma)
  end

  return moonshine.Effect{
    name = "fastgaussianblur",
    draw = draw,
    setters = setters,
    -- no defaults here, as we dont want the shader to be built 3 times on startup
  }
end