GLua experiment to take a screenshot and generate an SSTV signal from it

sstv.lua

-- SSTV experiment (CC0 Public Domain, free to use in whatever you want)
-- Created by: Zachary Blystone ( [email protected] )

local rate = 44100
local image = {}

local function co()

	local dt = 1 / rate
	local p = 0

	local function tone( t, freq )
		for i=1, math.Round(t * rate) do
			coroutine.yield( math.sin( p * math.pi * 2 ) * .2 )
			p = p + dt * freq
		end
	end
	local function bit( b )
		tone(.03, b == 1 and 1100 or 1300) -- 1100 = 1, 1300 = 0
	end
	local function brt( b )
		return 1500 * (1-b) + 2300 * (b)
	end

	-- I don't know if this first part is correct, I'm just guessing by ear
	-- Couldn't find any documentation on it
	tone(.1, 1900)
	tone(.1, 1500)
	tone(.1, 1900)
	tone(.1, 1500)
	tone(.1, 2300)
	tone(.1, 1500)
	tone(.1, 2300)
	tone(.1, 1500)
	tone(.1, 1900)

	tone(.3, 1900) -- Leader tone
	tone(.01, 1200) -- break
	tone(.3, 1900) -- Leader tone

	local mode = 8
	tone(.03, 1200) -- VIS Start bit
	bit(0) -- bit 0
	bit(0)
	bit(1)
	bit(1)
	bit(0)
	bit(0)
	bit(0) -- bit 6
	bit(0) -- Parity (0 = even, 1 = odd)
	tone(.03, 1200) -- VIS Stop bit

	-- Samples an RGB pixel from the image
	local function sampleRGB(x,y)

		local col = image[y * 256 + x]
		return col.r/255, col.g/255, col.b/255

	end

	-- Converts RGB to Y, R-Y, B-Y
	local function sampleYRB(x,y)

		local r,g,b = sampleRGB(x,y)
		local Y = 16 + (65.738 * r + 129.057 * g + 25.064 * b)
		local R = 128 + (112.439 * r -94.154 * g - 18.285 * b)
		local B = 128 + (-37.945 * r -74.494 * g + 112.439 * b)
		return Y/255,R/255,B/255

	end

	for i=0, 239 do

		tone(0.009, 1200) -- Sync
		tone(0.003, 1500) -- Porch
		for j=0, 255 do local y,r,b = sampleYRB(j, i)  tone(0.138 / 256, brt(y)) end -- Y scan
		tone(0.0045, 1500) -- Separator pulse
		tone(0.0015, 1900) -- Porch

		for j=0, 127 do local y,r,b = sampleYRB(j*2, i)  tone(0.069 / 128, brt(r)) end -- R-Y scan
		tone(0.0045, 2300) -- Separator pulse
		tone(0.0015, 1500) -- Porch
		for j=0, 127 do local y,r,b = sampleYRB(j*2, i)  tone(0.069 / 128, brt(b)) end -- B-Y scan

	end

	-- Couldn't find documentation on the footer signal

end

local capturedFrame = false
local doCapture = false

hook.Add("PostRender", "SSTV_Capture", function()

	if doCapture and not capturedFrame then

		render.CapturePixels()

		-- Save pixels into image
		for y=0, 240-1 do
			for x=0, 256-1 do
				local u = x/256
				local v = y/240
				local r,g,b = render.ReadPixel(u * ScrW(), v * ScrH())
				image[y * 256 + x] = Color(r,g,b)
			end
		end

		doCapture = false
		capturedFrame = true

	end

end)

concommand.Add("sstv", function()

	capturedFrame = false
	doCapture = true

	LocalPlayer():ConCommand("stopsound")

	timer.Simple(.2, function()

		if capturedFrame then
			sstv_id = (sstv_id or 0) + 1
			local sstv_file = "sstvz_" .. sstv_id

			local c = coroutine.create(co)
			local function data(t)

				local b, x = coroutine.resume(c)
				if b and x then return x end
				return 0

			end

			sound.Generate(sstv_file, rate, 78, data)
			surface.PlaySound(sstv_file)
		else
			print("NO FRAME")
		end

	end)

end)