Bilinear gaussian blur using texture2D lookups

bilineargaussianblur.lua

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+--[[
+The MIT License (MIT)
+
+Copyright (c) 2017 Tim Moore
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+]]--
+
+-- 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, sigma)
+	taps = math.floor(taps)
+	sigma = sigma >= 1 and sigma or (taps - 1) * offset / 6
+
+	if taps < 3 or taps % 2 ~= 1 then
+	    error(('Taps must be >=3 and odd. Was %d.'):format(taps))
+	end
+
+	local steps = (taps + 1) / 2
+
+	-- Calculate gaussian function.
+	local g_offsets = {}
+	local g_weights = {}
+	for i = 1, steps, 1 do
+		local offset = i - 1
+		g_offsets[i] = offset
+
+		-- 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 * (offset - 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 = i ~=2 and wB or wB / 2 -- On final tap the middle is getting sampled twice so half weight.
+		local weight = wA + wB
+		offsets[#offsets + 1] = (oA * wA + oB * wB) / weight
+		weights[#weights + 1] = weight
+	end
+
+	local code = {[[
+extern vec2 direction;
+uniform sampler2D tex0;
+vec4 effect(vec4 color, Image texture, vec2 tc, vec2 sc) {]]}
+
+    local norm = 0
+	if #g_weights % 2 == 0 then
+		code[#code+1] =  'vec4 c = vec4( 0.0f );'
+	else
+		local weight = g_weights[1]
+		norm = norm + weight
+		code[#code+1] = ('vec4 c = %f * texture2D( tex0, tc ).xyzw;'):format(weight)
+	end
+
+	local tmpl = 'c += %f * ( texture2D( tex0, tc + %f * direction ).xyzw + texture2D( tex0, tc - %f * direction ).xyzw );\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 {
+
+description = "Bilinear Gaussian blur shader (http://rastergrid.com/blog/2010/09/efficient-gaussian-blur-with-linear-sampling/)",
+
+new = function(self)
+	self.canvas_h, self.canvas_v = love.graphics.newCanvas(), love.graphics.newCanvas()
+	self.taps, self.offset, self.sigma = 7, 1, -1
+	self.shader = build_shader(self.taps, self.offset, self.sigma)
+
+	self.shader:send("direction", {1.0, 0.0} )
+end,
+
+draw = function(self, func, ...)
+	local c = love.graphics.getCanvas()
+	local s = love.graphics.getShader()
+	local co = {love.graphics.getColor()}
+
+	-- draw scene
+	self:_render_to_canvas(self.canvas_h, func, ...)
+
+	love.graphics.setColor(co)
+	love.graphics.setShader(self.shader)
+
+	local b = love.graphics.getBlendMode()
+	love.graphics.setBlendMode('alpha', 'premultiplied')
+
+	-- first pass (horizontal blur)
+	self.shader:send('direction', {1 / love.graphics.getWidth(), 0})
+	self:_render_to_canvas(self.canvas_v, love.graphics.draw, self.canvas_h, 0,0)
+
+	-- second pass (vertical blur)
+	self.shader:send('direction', {0, 1 / love.graphics.getHeight()})
+	love.graphics.draw(self.canvas_v, 0,0)
+
+	-- restore blendmode, shader and canvas
+	love.graphics.setBlendMode(b)
+	love.graphics.setShader(s)
+	love.graphics.setCanvas(c)
+end,
+
+set = function(self, key, value)
+	if key == "taps" then
+		-- 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.
+		self.taps = tonumber(value)
+	elseif key == "offset" then
+		-- 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.
+		self.offset = tonumber(value)
+	elseif key == "sigma" then
+		-- Sigma value for gaussian distribution. You don't normally need to set this.
+	    self.sigma = tonumber(value)
+	else
+		error("Unknown property: " .. tostring(key))
+	end
+
+	self.shader = build_shader(self.taps, self.offset, self.sigma)
+	return self
+end
+}