-- The theme is what decides what's actually shown on screen, what kind of
-- transitions are available (if any), and what kind of inputs there are,
-- if any. In general, it drives the entire display logic by creating Movit
-- chains, setting their parameters and then deciding which to show when.
--
-- Themes are written in Lua, which reflects a simplified form of the Movit API
-- where all the low-level details (such as texture formats) are handled by the
-- C++ side and you generally just build chains.

local state = {
	transition_start = -2.0,
	transition_end = -1.0,
	transition_type = 0,
	transition_src_signal = 0,
	transition_dst_signal = 0,

	neutral_colors = {
		{0.5, 0.5, 0.5},  -- Input 0.
		{0.5, 0.5, 0.5}   -- Input 1.
	},

	live_signal_num = 0,
	preview_signal_num = 1
}

-- Valid values for live_signal_num and preview_signal_num.
local INPUT0_SIGNAL_NUM = 0
local INPUT1_SIGNAL_NUM = 1
local SBS_SIGNAL_NUM = 2
local STATIC_SIGNAL_NUM = 3

-- Valid values for transition_type. (Cuts are done directly, so they need no entry.)
local NO_TRANSITION = 0
local ZOOM_TRANSITION = 1  -- Also for slides.
local FADE_TRANSITION = 2

-- Last width/height/frame rate for each channel, if we have it.
-- Note that unlike the values we get from Nageru, the resolution is per
-- frame and not per field, since we deinterlace.
local last_resolution = {}

-- Utility function to help creating many similar chains that can differ
-- in a free set of chosen parameters.
function make_cartesian_product(parms, callback)
	return make_cartesian_product_internal(parms, callback, 1, {})
end

function make_cartesian_product_internal(parms, callback, index, args)
	if index > #parms then
		return callback(unpack(args))
	end
	local ret = {}
	for _, value in ipairs(parms[index]) do
		args[index] = value
		ret[value] = make_cartesian_product_internal(parms, callback, index + 1, args)
	end
	return ret
end

function make_sbs_input(chain, signal, deint, hq)
	local input = chain:add_live_input(not deint, deint)  -- Override bounce only if not deinterlacing.
	input:connect_signal(signal)

	local resample_effect = nil
	local resize_effect = nil
	if (hq) then
		resample_effect = chain:add_effect(ResampleEffect.new())
	else
		resize_effect = chain:add_effect(ResizeEffect.new())
	end
	local wb_effect = chain:add_effect(WhiteBalanceEffect.new())

	local padding_effect = chain:add_effect(IntegralPaddingEffect.new())

	return {
		input = input,
		wb_effect = wb_effect,
		resample_effect = resample_effect,
		resize_effect = resize_effect,
		padding_effect = padding_effect
	}
end

-- The main live chain.
function make_sbs_chain(input0_type, input1_type, hq)
	local chain = EffectChain.new(16, 9)

	local input0 = make_sbs_input(chain, INPUT0_SIGNAL_NUM, input0_type == "livedeint", hq)
	local input1 = make_sbs_input(chain, INPUT1_SIGNAL_NUM, input1_type == "livedeint", hq)

	input0.padding_effect:set_vec4("border_color", 0.0, 0.0, 0.0, 1.0)
	input1.padding_effect:set_vec4("border_color", 0.0, 0.0, 0.0, 0.0)

	chain:add_effect(OverlayEffect.new(), input0.padding_effect, input1.padding_effect)
	chain:finalize(hq)

	return {
		chain = chain,
		input0 = input0,
		input1 = input1
	}
end

-- Make all possible combinations of side-by-side chains.
local sbs_chains = make_cartesian_product({
	{"live", "livedeint"},  -- input0_type
	{"live", "livedeint"},  -- input1_type
	{true, false}           -- hq
}, function(input0_type, input1_type, hq)
	return make_sbs_chain(input0_type, input1_type, hq)
end)

function make_fade_input(chain, signal, live, deint, scale)
	local input, wb_effect, resample_effect, last
	if live then
		input = chain:add_live_input(false, deint)
		input:connect_signal(signal)
		last = input
	else
		input = chain:add_effect(ImageInput.new("bg.jpeg"))
		last = input
	end

	-- If we cared about this for the non-main inputs, we would have
	-- checked hq here and invoked ResizeEffect instead.
	if scale then
		resample_effect = chain:add_effect(ResampleEffect.new())
		last = resample_effect
	end

	-- Make sure to put the white balance after the scaling (usually more efficient).
	if live then
		wb_effect = chain:add_effect(WhiteBalanceEffect.new())
		last = wb_effect
	end

	return {
		input = input,
		wb_effect = wb_effect,
		resample_effect = resample_effect,
		last = last
	}
end

-- A chain to fade between two inputs, of which either can be a picture
-- or a live input. In practice only used live, but we still support the
-- hq parameter.
function make_fade_chain(input0_live, input0_deint, input0_scale, input1_live, input1_deint, input1_scale, hq)
	local chain = EffectChain.new(16, 9)

	local input0 = make_fade_input(chain, INPUT0_SIGNAL_NUM, input0_live, input0_deint, input0_scale)
	local input1 = make_fade_input(chain, INPUT1_SIGNAL_NUM, input1_live, input1_deint, input1_scale)

	local mix_effect = chain:add_effect(MixEffect.new(), input0.last, input1.last)
	chain:finalize(hq)

	return {
		chain = chain,
		input0 = input0,
		input1 = input1,
		mix_effect = mix_effect
	}
end

-- Chains to fade between two inputs, in various configurations.
local fade_chains = make_cartesian_product({
	{"static", "live", "livedeint"},  -- input0_type
	{true, false},                    -- input0_scale
	{"static", "live", "livedeint"},  -- input1_type
	{true, false},                    -- input1_scale
	{true}                            -- hq
}, function(input0_type, input0_scale, input1_type, input1_scale, hq)
	local input0_live = (input0_type ~= "static")
	local input1_live = (input1_type ~= "static")
	local input0_deint = (input0_type == "livedeint")
	local input1_deint = (input1_type == "livedeint")
	return make_fade_chain(input0_live, input0_deint, input0_scale, input1_live, input1_deint, input1_scale, hq)
end)

-- A chain to show a single input on screen.
function make_simple_chain(input_deint, input_scale, hq)
	local chain = EffectChain.new(16, 9)

	local input = chain:add_live_input(false, input_deint)
	input:connect_signal(0)  -- First input card. Can be changed whenever you want.

	local resample_effect, resize_effect
	if input_scale then
		if hq then
			resample_effect = chain:add_effect(ResampleEffect.new())
		else
			resize_effect = chain:add_effect(ResizeEffect.new())
		end
	end

	local wb_effect = chain:add_effect(WhiteBalanceEffect.new())
	chain:finalize(hq)

	return {
		chain = chain,
		input = input,
		wb_effect = wb_effect,
		resample_effect = resample_effect,
		resize_effect = resize_effect
	}
end

-- Make all possible combinations of single-input chains.
local simple_chains = make_cartesian_product({
	{"live", "livedeint"},  -- input_type
	{true, false},          -- input_scale
	{true, false}           -- hq
}, function(input_type, input_scale, hq)
	local input_deint = (input_type == "livedeint")
	return make_simple_chain(input_deint, input_scale, hq)
end)

-- A chain to show a single static picture on screen (HQ version).
local static_chain_hq = EffectChain.new(16, 9)
local static_chain_hq_input = static_chain_hq:add_effect(ImageInput.new("bg.jpeg"))
static_chain_hq:finalize(true)

-- A chain to show a single static picture on screen (LQ version).
local static_chain_lq = EffectChain.new(16, 9)
local static_chain_lq_input = static_chain_lq:add_effect(ImageInput.new("bg.jpeg"))
static_chain_lq:finalize(false)

-- Used for indexing into the tables of chains.
function get_input_type(signals, signal_num)
	if signal_num == STATIC_SIGNAL_NUM then
		return "static"
	elseif signals:get_interlaced(signal_num) then
		return "livedeint"
	else
		return "live"
	end
end

function needs_scale(signals, signal_num, width, height)
	if signal_num == STATIC_SIGNAL_NUM then
		-- We assume this is already correctly scaled at load time.
		return false
	end
	assert(is_plain_signal(signal_num))
	return (signals:get_width(signal_num) ~= width or signals:get_height(signal_num) ~= height)
end

function set_scale_parameters_if_needed(chain_or_input, width, height)
	if chain_or_input.resample_effect then
		chain_or_input.resample_effect:set_int("width", width)
		chain_or_input.resample_effect:set_int("height", height)
	elseif chain_or_input.resize_effect then
		chain_or_input.resize_effect:set_int("width", width)
		chain_or_input.resize_effect:set_int("height", height)
	end
end

-- API ENTRY POINT
-- Returns the number of outputs in addition to the live (0) and preview (1).
-- Called only once, at the start of the program.
function num_channels()
	return 4
end

function is_plain_signal(num)
	return num == INPUT0_SIGNAL_NUM or num == INPUT1_SIGNAL_NUM
end

-- Helper function to write e.g. “720p60”. The difference between this
-- and get_channel_resolution_raw() is that this one also can say that
-- there's no signal.
function get_channel_resolution(signal_num)
	local res = last_resolution[signal_num]
	if (not res) or not res.is_connected then
		return "disconnected"
	end
	if res.height <= 0 then
		return "no signal"
	end
	if not res.has_signal then
		if res.height == 525 then
			-- Special mode for the USB3 cards.
			return "no signal"
		end
		return get_channel_resolution_raw(res) .. ", no signal"
	else
		return get_channel_resolution_raw(res)
	end
end

-- Helper function to write e.g. “60” or “59.94”.
function get_frame_rate(res)
	local nom = res.frame_rate_nom
	local den = res.frame_rate_den
	if nom % den == 0 then
		return nom / den
	else
		return string.format("%.2f", nom / den)
	end
end

-- Helper function to write e.g. “720p60”.
function get_channel_resolution_raw(res)
	if res.interlaced then
		return res.height .. "i" .. get_frame_rate(res)
	else
		return res.height .. "p" .. get_frame_rate(res)
	end
end

-- API ENTRY POINT
-- Returns the name for each additional channel (starting from 2).
-- Called at the start of the program, and then each frame for live
-- channels in case they change resolution.
function channel_name(channel)
	local signal_num = channel - 2
	if is_plain_signal(signal_num) then
		return "Input " .. (signal_num + 1) .. " (" .. get_channel_resolution(signal_num) .. ")"
	elseif signal_num == SBS_SIGNAL_NUM then
		return "Side-by-side"
	elseif signal_num == STATIC_SIGNAL_NUM then
		return "Static picture"
	end
end

-- API ENTRY POINT
-- Returns, given a channel number, which signal it corresponds to (starting from 0).
-- Should return -1 if the channel does not correspond to a simple signal
-- (one connected to a capture card, or a video input). The information is used for
-- whether right-click on the channel should bring up a context menu or not,
-- typically containing an input selector, resolution menu etc.
--
-- Called once for each channel, at the start of the program.
-- Will never be called for live (0) or preview (1).
function channel_signal(channel)
	if channel == 2 then
		return 0
	elseif channel == 3 then
		return 1
	else
		return -1
	end
end

-- API ENTRY POINT
-- Called every frame. Returns the color (if any) to paint around the given
-- channel. Returns a CSS color (typically to mark live and preview signals);
-- "transparent" is allowed.
-- Will never be called for live (0) or preview (1).
function channel_color(channel)
	if state.transition_type ~= NO_TRANSITION then
		if channel_involved_in(channel, state.transition_src_signal) or
		   channel_involved_in(channel, state.transition_dst_signal) then
			return "#f00"
		end
	else
		if channel_involved_in(channel, state.live_signal_num) then
			return "#f00"
		end
	end
	if channel_involved_in(channel, state.preview_signal_num) then
		return "#0f0"
	end
	return "transparent"
end

function channel_involved_in(channel, signal_num)
	if is_plain_signal(signal_num) then
		return channel == (signal_num + 2)
	end
	if signal_num == SBS_SIGNAL_NUM then
		return (channel == 2 or channel == 3)
	end
	if signal_num == STATIC_SIGNAL_NUM then
		return (channel == 5)
	end
	return false
end

-- API ENTRY POINT
-- Returns if a given channel supports setting white balance (starting from 2).
-- Called only once for each channel, at the start of the program.
function supports_set_wb(channel)
	return is_plain_signal(channel - 2)
end

-- API ENTRY POINT
-- Gets called with a new gray point when the white balance is changing.
-- The color is in linear light (not sRGB gamma).
function set_wb(channel, red, green, blue)
	if is_plain_signal(channel - 2) then
		state.neutral_colors[channel - 2 + 1] = { red, green, blue }
	end
end

function finish_transitions(t)
	if state.transition_type ~= NO_TRANSITION and t >= state.transition_end then
		state.live_signal_num = state.transition_dst_signal
		state.transition_type = NO_TRANSITION
	end
end

function in_transition(t)
       return t >= state.transition_start and t <= state.transition_end
end

-- API ENTRY POINT
-- Called every frame.
function get_transitions(t)
	if in_transition(t) then
		-- Transition already in progress, the only thing we can do is really
		-- cut to the preview. (TODO: Make an “abort” and/or “finish”, too?)
		return {"Cut"}
	end

	finish_transitions(t)

	if state.live_signal_num == state.preview_signal_num then
		-- No transitions possible.
		return {}
	end

	if (is_plain_signal(state.live_signal_num) or state.live_signal_num == STATIC_SIGNAL_NUM) and
	   (is_plain_signal(state.preview_signal_num) or state.preview_signal_num == STATIC_SIGNAL_NUM) then
		return {"Cut", "", "Fade"}
	end

	-- Various zooms.
	if state.live_signal_num == SBS_SIGNAL_NUM and is_plain_signal(state.preview_signal_num) then
		return {"Cut", "Zoom in"}
	elseif is_plain_signal(state.live_signal_num) and state.preview_signal_num == SBS_SIGNAL_NUM then
		return {"Cut", "Zoom out"}
	end

	return {"Cut"}
end

function swap_preview_live()
	local temp = state.live_signal_num
	state.live_signal_num = state.preview_signal_num
	state.preview_signal_num = temp
end

function start_transition(type_, t, duration)
	state.transition_start = t
	state.transition_end = t + duration
	state.transition_type = type_
	state.transition_src_signal = state.live_signal_num
	state.transition_dst_signal = state.preview_signal_num
	swap_preview_live()
end

-- API ENTRY POINT
-- Called when the user clicks a transition button.
function transition_clicked(num, t)
	if num == 0 then
		-- Cut.
		if in_transition(t) then
			-- Ongoing transition; finish it immediately before the cut.
			finish_transitions(state.transition_end)
		end

		swap_preview_live()
	elseif num == 1 then
		-- Zoom.
		finish_transitions(t)

		if state.live_signal_num == state.preview_signal_num then
			-- Nothing to do.
			return
		end

		if is_plain_signal(state.live_signal_num) and is_plain_signal(state.preview_signal_num) then
			-- We can't zoom between these. Just make a cut.
			io.write("Cutting from " .. state.live_signal_num .. " to " .. state.live_signal_num .. "\n")
			swap_preview_live()
			return
		end

		if (state.live_signal_num == SBS_SIGNAL_NUM and is_plain_signal(state.preview_signal_num)) or
		   (state.preview_signal_num == SBS_SIGNAL_NUM and is_plain_signal(state.live_signal_num)) then
			start_transition(ZOOM_TRANSITION, t, 1.0)
		end
	elseif num == 2 then
		finish_transitions(t)

		-- Fade.
		if (state.live_signal_num ~= state.preview_signal_num) and
		   (is_plain_signal(state.live_signal_num) or
		    state.live_signal_num == STATIC_SIGNAL_NUM) and
		   (is_plain_signal(state.preview_signal_num) or
		    state.preview_signal_num == STATIC_SIGNAL_NUM) then
			start_transition(FADE_TRANSITION, t, 1.0)
		else
			-- Fades involving SBS are ignored (we have no chain for it).
		end
	end
end

-- API ENTRY POINT
function channel_clicked(num)
	state.preview_signal_num = num
end

function get_fade_chain(state, signals, t, width, height, input_resolution)
	local input0_type = get_input_type(signals, state.transition_src_signal)
	local input0_scale = needs_scale(signals, state.transition_src_signal, width, height)
	local input1_type = get_input_type(signals, state.transition_dst_signal)
	local input1_scale = needs_scale(signals, state.transition_dst_signal, width, height)
	local chain = fade_chains[input0_type][input0_scale][input1_type][input1_scale][true]
	local prepare = function()
		if input0_type == "live" or input0_type == "livedeint" then
			chain.input0.input:connect_signal(state.transition_src_signal)
			set_neutral_color_from_signal(state, chain.input0.wb_effect, state.transition_src_signal)
		end
		set_scale_parameters_if_needed(chain.input0, width, height)
		if input1_type == "live" or input1_type == "livedeint" then
			chain.input1.input:connect_signal(state.transition_dst_signal)
			set_neutral_color_from_signal(state, chain.input1.wb_effect, state.transition_dst_signal)
		end
		set_scale_parameters_if_needed(chain.input1, width, height)
		local tt = calc_fade_progress(t, state.transition_start, state.transition_end)

		chain.mix_effect:set_float("strength_first", 1.0 - tt)
		chain.mix_effect:set_float("strength_second", tt)
	end
	return chain.chain, prepare
end

-- SBS code (live_signal_num == SBS_SIGNAL_NUM, or in a transition to/from it).
function get_sbs_chain(signals, t, width, height, input_resolution)
	local input0_type = get_input_type(signals, INPUT0_SIGNAL_NUM)
	local input1_type = get_input_type(signals, INPUT1_SIGNAL_NUM)
	return sbs_chains[input0_type][input1_type][true]
end

-- API ENTRY POINT
-- Called every frame. Get the chain for displaying at input <num>,
-- where 0 is live, 1 is preview, 2 is the first channel to display
-- in the bottom bar, and so on up to num_channels()+1. t is the
-- current time in seconds. width and height are the dimensions of
-- the output, although you can ignore them if you don't need them
-- (they're useful if you want to e.g. know what to resample by).
--
-- <signals> is basically an exposed InputState, which you can use to
-- query for information about the signals at the point of the current
-- frame. In particular, you can call get_width() and get_height()
-- for any signal number, and use that to e.g. assist in chain selection.
--
-- You should return two objects; the chain itself, and then a
-- function (taking no parameters) that is run just before rendering.
-- The function needs to call connect_signal on any inputs, so that
-- it gets updated video data for the given frame. (You are allowed
-- to switch which input your input is getting from between frames,
-- but not calling connect_signal results in undefined behavior.)
-- If you want to change any parameters in the chain, this is also
-- the right place.
--
-- NOTE: The chain returned must be finalized with the Y'CbCr flag
-- if and only if num==0.
function get_chain(num, t, width, height, signals)
	local input_resolution = {}
	for signal_num=0,1 do
		local res = {
			width = signals:get_width(signal_num),
			height = signals:get_height(signal_num),
			interlaced = signals:get_interlaced(signal_num),
			is_connected = signals:get_is_connected(signal_num),
			has_signal = signals:get_has_signal(signal_num),
			frame_rate_nom = signals:get_frame_rate_nom(signal_num),
			frame_rate_den = signals:get_frame_rate_den(signal_num)
		}

		if res.interlaced then
			-- Convert height from frame height to field height.
			-- (Needed for e.g. place_rectangle.)
			res.height = res.height * 2

			-- Show field rate instead of frame rate; really for cosmetics only
			-- (and actually contrary to EBU recommendations, although in line
			-- with typical user expectations).
			res.frame_rate_nom = res.frame_rate_nom * 2
		end

		input_resolution[signal_num] = res
	end
	last_resolution = input_resolution

	-- Make a (semi-shallow) copy of the current state, so that the returned prepare function
	-- is unaffected by state changes made by the UI before it is rendered.
	local state_copy = {}
	for key, value in pairs(state) do
		state_copy[key] = value
	end
	state_copy.neutral_colors = { unpack(state.neutral_colors) }

	if num == 0 then  -- Live.
		finish_transitions(t)
		if state.transition_type == ZOOM_TRANSITION then
			-- Transition in or out of SBS.
			local chain = get_sbs_chain(signals, t, width, height, input_resolution)
			local prepare = function()
				prepare_sbs_chain(state_copy, chain, calc_zoom_progress(state_copy, t), state_copy.transition_type, state_copy.transition_src_signal, state_copy.transition_dst_signal, width, height, input_resolution)
			end
			return chain.chain, prepare
		elseif state.transition_type == NO_TRANSITION and state.live_signal_num == SBS_SIGNAL_NUM then
			-- Static SBS view.
			local chain = get_sbs_chain(signals, t, width, height, input_resolution)
			local prepare = function()
				prepare_sbs_chain(state_copy, chain, 0.0, NO_TRANSITION, 0, SBS_SIGNAL_NUM, width, height, input_resolution)
			end
			return chain.chain, prepare
		elseif state.transition_type == FADE_TRANSITION then
			return get_fade_chain(state_copy, signals, t, width, height, input_resolution)
		elseif is_plain_signal(state.live_signal_num) then
			local input_type = get_input_type(signals, state.live_signal_num)
			local input_scale = needs_scale(signals, state.live_signal_num, width, height)
			local chain = simple_chains[input_type][input_scale][true]
			local prepare = function()
				chain.input:connect_signal(state_copy.live_signal_num)
				set_scale_parameters_if_needed(chain, width, height)
				set_neutral_color_from_signal(state_copy, chain.wb_effect, state_copy.live_signal_num)
			end
			return chain.chain, prepare
		elseif state.live_signal_num == STATIC_SIGNAL_NUM then  -- Static picture.
			local prepare = function()
			end
			return static_chain_hq, prepare
		else
			assert(false)
		end
	end
	if num == 1 then  -- Preview.
		num = state.preview_signal_num + 2
	end

	-- Individual preview inputs.
	if is_plain_signal(num - 2) then
		local signal_num = num - 2
		local input_type = get_input_type(signals, signal_num)
		local input_scale = needs_scale(signals, signal_num, width, height)
		local chain = simple_chains[input_type][input_scale][false]
		local prepare = function()
			chain.input:connect_signal(signal_num)
			set_scale_parameters_if_needed(chain, width, height)
			set_neutral_color(chain.wb_effect, state_copy.neutral_colors[signal_num + 1])
		end
		return chain.chain, prepare
	end
	if num == SBS_SIGNAL_NUM + 2 then
		local input0_type = get_input_type(signals, INPUT0_SIGNAL_NUM)
		local input1_type = get_input_type(signals, INPUT1_SIGNAL_NUM)
		local chain = sbs_chains[input0_type][input1_type][false]
		local prepare = function()
			prepare_sbs_chain(state_copy, chain, 0.0, NO_TRANSITION, 0, SBS_SIGNAL_NUM, width, height, input_resolution)
		end
		return chain.chain, prepare
	end
	if num == STATIC_SIGNAL_NUM + 2 then
		local prepare = function()
		end
		return static_chain_lq, prepare
	end
end

function place_rectangle(resample_effect, resize_effect, padding_effect, x0, y0, x1, y1, screen_width, screen_height, input_width, input_height)
	local srcx0 = 0.0
	local srcx1 = 1.0
	local srcy0 = 0.0
	local srcy1 = 1.0

	padding_effect:set_int("width", screen_width)
	padding_effect:set_int("height", screen_height)

	-- Cull.
	if x0 > screen_width or x1 < 0.0 or y0 > screen_height or y1 < 0.0 then
		if resample_effect ~= nil then
			resample_effect:set_int("width", 1)
			resample_effect:set_int("height", 1)
			resample_effect:set_float("zoom_x", screen_width)
			resample_effect:set_float("zoom_y", screen_height)
		else
			resize_effect:set_int("width", 1)
			resize_effect:set_int("height", 1)
		end
		padding_effect:set_int("left", screen_width + 100)
		padding_effect:set_int("top", screen_height + 100)
		return
	end

	-- Clip.
	if x0 < 0 then
		srcx0 = -x0 / (x1 - x0)
		x0 = 0
	end
	if y0 < 0 then
		srcy0 = -y0 / (y1 - y0)
		y0 = 0
	end
	if x1 > screen_width then
		srcx1 = (screen_width - x0) / (x1 - x0)
		x1 = screen_width
	end
	if y1 > screen_height then
		srcy1 = (screen_height - y0) / (y1 - y0)
		y1 = screen_height
	end

	if resample_effect ~= nil then
		-- High-quality resampling.
		local x_subpixel_offset = x0 - math.floor(x0)
		local y_subpixel_offset = y0 - math.floor(y0)

		-- Resampling must be to an integral number of pixels. Round up,
		-- and then add an extra pixel so we have some leeway for the border.
		local width = math.ceil(x1 - x0) + 1
		local height = math.ceil(y1 - y0) + 1
		resample_effect:set_int("width", width)
		resample_effect:set_int("height", height)

		-- Correct the discrepancy with zoom. (This will leave a small
		-- excess edge of pixels and subpixels, which we'll correct for soon.)
		local zoom_x = (x1 - x0) / (width * (srcx1 - srcx0))
		local zoom_y = (y1 - y0) / (height * (srcy1 - srcy0))
		resample_effect:set_float("zoom_x", zoom_x)
		resample_effect:set_float("zoom_y", zoom_y)
		resample_effect:set_float("zoom_center_x", 0.0)
		resample_effect:set_float("zoom_center_y", 0.0)

		-- Padding must also be to a whole-pixel offset.
		padding_effect:set_int("left", math.floor(x0))
		padding_effect:set_int("top", math.floor(y0))

		-- Correct _that_ discrepancy by subpixel offset in the resampling.
		resample_effect:set_float("left", srcx0 * input_width - x_subpixel_offset / zoom_x)
		resample_effect:set_float("top", srcy0 * input_height - y_subpixel_offset / zoom_y)

		-- Finally, adjust the border so it is exactly where we want it.
		padding_effect:set_float("border_offset_left", x_subpixel_offset)
		padding_effect:set_float("border_offset_right", x1 - (math.floor(x0) + width))
		padding_effect:set_float("border_offset_top", y_subpixel_offset)
		padding_effect:set_float("border_offset_bottom", y1 - (math.floor(y0) + height))
	else
		-- Lower-quality simple resizing.
		local width = round(x1 - x0)
		local height = round(y1 - y0)
		resize_effect:set_int("width", width)
		resize_effect:set_int("height", height)

		-- Padding must also be to a whole-pixel offset.
		padding_effect:set_int("left", math.floor(x0))
		padding_effect:set_int("top", math.floor(y0))
	end
end

-- This is broken, of course (even for positive numbers), but Lua doesn't give us access to real rounding.
function round(x)
	return math.floor(x + 0.5)
end

function lerp(a, b, t)
	return a + (b - a) * t
end

function lerp_pos(a, b, t)
	return {
		x0 = lerp(a.x0, b.x0, t),
		y0 = lerp(a.y0, b.y0, t),
		x1 = lerp(a.x1, b.x1, t),
		y1 = lerp(a.y1, b.y1, t)
	}
end

function pos_from_top_left(x, y, width, height, screen_width, screen_height)
	local xs = screen_width / 1280.0
	local ys = screen_height / 720.0
	return {
		x0 = round(xs * x),
		y0 = round(ys * y),
		x1 = round(xs * (x + width)),
		y1 = round(ys * (y + height))
	}
end

function prepare_sbs_chain(state, chain, t, transition_type, src_signal, dst_signal, screen_width, screen_height, input_resolution)
	chain.input0.input:connect_signal(0)
	chain.input1.input:connect_signal(1)
	set_neutral_color(chain.input0.wb_effect, state.neutral_colors[1])
	set_neutral_color(chain.input1.wb_effect, state.neutral_colors[2])

	-- First input is positioned (16,48) from top-left.
	-- Second input is positioned (16,48) from the bottom-right.
	local pos0 = pos_from_top_left(16, 48, 848, 477, screen_width, screen_height)
	local pos1 = pos_from_top_left(1280 - 384 - 16, 720 - 216 - 48, 384, 216, screen_width, screen_height)

	local pos_fs = { x0 = 0, y0 = 0, x1 = screen_width, y1 = screen_height }
	local affine_param
	if transition_type == NO_TRANSITION then
		-- Static SBS view.
		affine_param = { sx = 1.0, sy = 1.0, tx = 0.0, ty = 0.0 }   -- Identity.
	else
		-- Zooming to/from SBS view into or out of a single view.
		assert(transition_type == ZOOM_TRANSITION)
		local signal, real_t
		if src_signal == SBS_SIGNAL_NUM then
			signal = dst_signal
			real_t = t
		else
			assert(dst_signal == SBS_SIGNAL_NUM)
			signal = src_signal
			real_t = 1.0 - t
		end

		if signal == INPUT0_SIGNAL_NUM then
			affine_param = find_affine_param(pos0, lerp_pos(pos0, pos_fs, real_t))
		elseif signal == INPUT1_SIGNAL_NUM then
			affine_param = find_affine_param(pos1, lerp_pos(pos1, pos_fs, real_t))
		end
	end

	-- NOTE: input_resolution is not 1-indexed, unlike usual Lua arrays.
	place_rectangle_with_affine(chain.input0.resample_effect, chain.input0.resize_effect, chain.input0.padding_effect, pos0, affine_param, screen_width, screen_height, input_resolution[0].width, input_resolution[0].height)
	place_rectangle_with_affine(chain.input1.resample_effect, chain.input1.resize_effect, chain.input1.padding_effect, pos1, affine_param, screen_width, screen_height, input_resolution[1].width, input_resolution[1].height)
end

-- Find the transformation that changes the first rectangle to the second one.
function find_affine_param(a, b)
	local sx = (b.x1 - b.x0) / (a.x1 - a.x0)
	local sy = (b.y1 - b.y0) / (a.y1 - a.y0)
	return {
		sx = sx,
		sy = sy,
		tx = b.x0 - a.x0 * sx,
		ty = b.y0 - a.y0 * sy
	}
end

function place_rectangle_with_affine(resample_effect, resize_effect, padding_effect, pos, aff, screen_width, screen_height, input_width, input_height)
	local x0 = pos.x0 * aff.sx + aff.tx
	local x1 = pos.x1 * aff.sx + aff.tx
	local y0 = pos.y0 * aff.sy + aff.ty
	local y1 = pos.y1 * aff.sy + aff.ty

	place_rectangle(resample_effect, resize_effect, padding_effect, x0, y0, x1, y1, screen_width, screen_height, input_width, input_height)
end

function set_neutral_color(effect, color)
	effect:set_vec3("neutral_color", color[1], color[2], color[3])
end

function set_neutral_color_from_signal(state, effect, signal)
	if is_plain_signal(signal) then
		set_neutral_color(effect, state.neutral_colors[signal - INPUT0_SIGNAL_NUM + 1])
	end
end

function calc_zoom_progress(state, t)
	if t < state.transition_start then
		return 0.0
	elseif t > state.transition_end then
		return 1.0
	else
		local tt = (t - state.transition_start) / (state.transition_end - state.transition_start)
		-- Smooth it a bit.
		return math.sin(tt * 3.14159265358 * 0.5)
	end
end

function calc_fade_progress(t, transition_start, transition_end)
	local tt = (t - transition_start) / (transition_end - transition_start)
	if tt < 0.0 then
		return 0.0
	elseif tt > 1.0 then
		return 1.0
	end

	-- Make the fade look maybe a tad more natural, by pumping it
	-- through a sigmoid function.
	tt = 10.0 * tt - 5.0
	tt = 1.0 / (1.0 + math.exp(-tt))

	return tt
end
