Fix an issue where the mixer lagging too much behind CEF would cause us to display...

[nageru] / theme.lua

-- 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 transition_start = -2.0
local transition_end = -1.0
local transition_type = 0
local transition_src_signal = 0
local transition_dst_signal = 0

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

local live_signal_num = 0
local 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 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)
        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.
-- (The information is used for whether right-click on the channel should bring up
-- an input selector or not.)
-- 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 transition_type ~= NO_TRANSITION then
                if channel_involved_in(channel, transition_src_signal) or
                   channel_involved_in(channel, transition_dst_signal) then
                        return "#f00"
                end
        else
                if channel_involved_in(channel, live_signal_num) then
                        return "#f00"
                end
        end
        if channel_involved_in(channel, 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
                neutral_colors[channel - 2 + 1] = { red, green, blue }
        end
end

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

function in_transition(t)
       return t >= transition_start and t <= 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 live_signal_num == preview_signal_num then
                -- No transitions possible.
                return {}
        end

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

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

        return {"Cut"}
end

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

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

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

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

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

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

                -- Fade.
                if (live_signal_num ~= preview_signal_num) and
                   (is_plain_signal(live_signal_num) or
                    live_signal_num == STATIC_SIGNAL_NUM) and
                   (is_plain_signal(preview_signal_num) or
                    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)
        preview_signal_num = num
end

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

        if num == 0 then  -- Live.
                finish_transitions(t)
                if transition_type == ZOOM_TRANSITION then
                        -- Transition in or out of SBS.
                        local chain = get_sbs_chain(signals, t, width, height, input_resolution)
                        prepare = function()
                                prepare_sbs_chain(chain, calc_zoom_progress(t), transition_type, transition_src_signal, transition_dst_signal, width, height, input_resolution)
                        end
                        return chain.chain, prepare
                elseif transition_type == NO_TRANSITION and live_signal_num == SBS_SIGNAL_NUM then
                        -- Static SBS view.
                        local chain = get_sbs_chain(signals, t, width, height, input_resolution)
                        prepare = function()
                                prepare_sbs_chain(chain, 0.0, NO_TRANSITION, 0, SBS_SIGNAL_NUM, width, height, input_resolution)
                        end
                        return chain.chain, prepare
                elseif transition_type == FADE_TRANSITION then
                        return get_fade_chain(signals, t, width, height, input_resolution)
                elseif is_plain_signal(live_signal_num) then
                        local input_type = get_input_type(signals, live_signal_num)
                        local input_scale = needs_scale(signals, live_signal_num, width, height)
                        local chain = simple_chains[input_type][input_scale][true]
                        prepare = function()
                                chain.input:connect_signal(live_signal_num)
                                set_scale_parameters_if_needed(chain, width, height)
                                set_neutral_color_from_signal(chain.wb_effect, live_signal_num)
                        end
                        return chain.chain, prepare
                elseif live_signal_num == STATIC_SIGNAL_NUM then  -- Static picture.
                        prepare = function()
                        end
                        return static_chain_hq, prepare
                else
                        assert(false)
                end
        end
        if num == 1 then  -- Preview.
                num = 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]
                prepare = function()
                        chain.input:connect_signal(signal_num)
                        set_scale_parameters_if_needed(chain, width, height)
                        set_neutral_color(chain.wb_effect, 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]
                prepare = function()
                        prepare_sbs_chain(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
                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(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, neutral_colors[1])
        set_neutral_color(chain.input1.wb_effect, 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(effect, signal)
        if is_plain_signal(signal) then
                set_neutral_color(effect, neutral_colors[signal - INPUT0_SIGNAL_NUM + 1])
        end
end

function calc_zoom_progress(t)
        if t < transition_start then
                return 0.0
        elseif t > transition_end then
                return 1.0
        else
                local tt = (t - transition_start) / (transition_end - 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