Sky.lua

-- The sky, generated rather than shipped.
--
-- The overworld's, on every VOXEL rung. Wherever the diorama is drawn the void
-- behind it is sky rather than a black plate: at 75 degrees the horizon is
-- genuinely in frame and the bands run down to meet it, and at the steeper rungs
-- the void that shows is the ground running out past the map edge, which gets
-- the same sky above the same haze. A battle's placed camera keeps the flat fill
-- it has always had -- its horizon is above the frame and its look is not this
-- rung's to change.
--
-- THE RECIPE is the 8-bit skybox one: a short palette of blues painted as flat
-- horizontal bands, deepest overhead, with a CHECKERBOARD of the next band
-- dithered into the bottom of each one. Alternating two colours on a pixel grid
-- is how a machine with four colours to a palette got a fifth, sixth and seventh
-- out of them, and it is what keeps four bands reading as a gradient rather than
-- as four stripes. No clouds, nothing moving.
--
-- NOTHING IS RESAMPLED, which is the whole of why it is drawn this way. There is
-- no baked 160x144 picture scaled up to the window and no downsized buffer blown
-- back up: one full-region rectangle through a shader that answers every pixel
-- from its own canvas coordinate. A pixel of sky is computed at the size it is
-- displayed at, so there is nothing for a filter to soften and nothing to go
-- stale when the window or the zoom changes. The shader does bind one texture,
-- but it is a palette rather than an image -- the bands, one texel each, sampled
-- nearest (see rampFor, and why it is not a uniform array).
--
-- THE PIXEL GRID follows the zoom for the same reason. Bands and dither cells
-- are measured in DIORAMA pixels -- the pass's own pixels-per-world-pixel, handed
-- in fresh every frame -- so a chunky sky at 4x is a chunky sky at 12x, band
-- edges land on the same grid the world's own texels do, and a ZOOM keypress is
-- reflected in the frame that follows it rather than whenever something else
-- happened to rebuild.
--
-- PALETTE ORDER, which is easy to get wrong. Stored LIGHTEST FIRST, because that
-- is shade order: a display mode transforms a four-colour palette by replacing it
-- outright (PaletteFX.effectiveColors hands back GRAYS or CLASSIC), and those are
-- written light to dark. So the sky reads the list backwards -- deepest shade
-- overhead, shade 1 at the horizon -- and GRAY gets greys the right way up for
-- nothing.
--
-- WHAT TIME IT IS decides the colours. The palette itself lives in DayNight
-- (four phase palettes, blended along the clock and re-quantised to the
-- lattice), and this file paints whatever the clock says: blue at noon, gold
-- and violet through the twilights -- warmed further around the low sun by a
-- dithered GLOW -- and deep navy under the moon. The sun and moon themselves
-- hang here too: cell-art discs on the same grid as the dither, scissored to
-- the sky's own region so a setting body slips below the horizon point and is
-- gone, never wandering under the map.

-- the mod namespace (see main.lua): V.require loads a sibling module
local V = ...

local DayNight = V.require("DayNight")
local PaletteFX = require("src.render.PaletteFX")

local Sky = {}

-- The most bands a phase palette may paint with. Eight leaves headroom over
-- DayNight's six-band ones without paying for more; the ramp the shader reads
-- them from is built at the width actually used, so the cap costs nothing.
Sky.MAX_BANDS = 8

-- The checkerboard between bands. DITHER_START is how far down a band it begins,
-- as a fraction of that band: lower is a wider blend, and 1 switches it off. 0.6
-- leaves the top of each band flat -- a band dithered all the way through reads
-- as one averaged colour instead of as a step with a soft bottom edge.
Sky.DITHER = true
Sky.DITHER_START = 0.6

-- How much of the frame the bands cover when the horizon is NOT in it, as a
-- fraction of the canvas height.
--
-- At the steeper rungs the camera looks down far enough that the ground plane's
-- vanishing line is above the top edge -- there is no horizon to hang the pale
-- end on, but there is still void up there where the map runs out, and it should
-- read as sky. So the bands take the same slice of the frame the top rung's own
-- horizon gives them, which keeps the sky looking like one sky across the whole
-- ladder instead of changing character rung by rung.
Sky.SPAN = 0.23

-- How much ELEVATION the gradient spans above the horizon, in radians, for
-- a caller that anchors the sky IN SPACE rather than to the frame (the VR
-- eyes -- see Voxel3D.beginScene). On the flat screen the bands run from
-- the top edge of the frame down to the horizon, which is right for a
-- camera whose pitch is the rung's: the frame IS the window on the sky.
-- A headset's frame is wherever the head points, so glueing the zenith
-- band to its top edge drags the whole gradient around with the head. An
-- anchored caller instead hangs the gradient over a fixed slice of sky --
-- horizon to ELEV_SPAN up -- and hands paint() the canvas row that span's
-- top lands on this frame (the `top` argument), so tilting the head slides
-- the frame across a sky that stays put.
Sky.ELEV_SPAN = math.rad(55)

-- ------- the bands
--
-- Top first, each a { r, g, b } in 0..1, as the display mode has them.
--
-- Memoised, because this runs once a frame and the answer only moves when the
-- mode does.
local cache = { bands = nil, key = {}, ramp = nil }

function Sky.bands()
  local pal = DayNight.palette()
  local shades = PaletteFX.effectiveColors(pal) or pal
  local n = math.min(#shades, #pal, Sky.MAX_BANDS)
  local key, k = cache.key, 0
  local same = cache.bands ~= nil and #cache.bands == n
  for i = 1, n do
    local c = shades[i]
    for ch = 1, 3 do
      k = k + 1
      if key[k] ~= c[ch] then same = false end
      key[k] = c[ch]
    end
  end
  if same then return cache.bands end

  -- the ramp is these bands as a texture (see rampFor); a new list is a new
  -- ramp, and the old one is nothing's to keep
  if cache.ramp and cache.ramp.release then pcall(cache.ramp.release, cache.ramp) end
  cache.ramp, cache.rampFor = nil, nil

  local bands = {}
  for i = 1, n do
    -- backwards: the palette's darkest rung is the top band
    local c = shades[n - i + 1]
    bands[i] = { c[1] / 255, c[2] / 255, c[3] / 255 }
  end
  cache.bands = bands
  return bands
end

-- The hour's haze -- the palest band, in 0..1 -- which is both the sky's
-- bottom edge and the right flat fill for any outdoor void that wants to
-- match the clock without painting bands (the battle arena's backdrop).
function Sky.haze()
  local bands = Sky.bands()
  return bands and bands[#bands] or nil
end

-- Put the sky onto a flat descriptor: the bands to paint, plus the flat fill
-- replaced by the palest of them. That fill is what the caller CLEARS to, so
-- making it the bottom band's own colour means the haze below the sky and the
-- bottom of the sky are one colour -- the join has no seam, and a frame that
-- cannot paint the bands is a hazy sky rather than a wrong one.
--
-- Mutates the descriptor, which is a fresh table per frame from its caller.
function Sky.dress(sky)
  local bands = Sky.bands()
  local haze = bands and bands[#bands]
  if not (sky and haze) then return sky end
  sky[1], sky[2], sky[3] = haze[1], haze[2], haze[3]
  sky.bands = bands
  return sky
end

-- Where the sky's bottom edge goes, in canvas pixels: the camera's own horizon
-- when that is in frame, and SPAN of the frame when it is not (see SPAN). nil
-- when there is no room for any of it.
function Sky.region(h, horizonY)
  if not (h and h > 0) then return nil end
  local edge = horizonY
  if not (edge and edge > 0) then edge = h * Sky.SPAN end
  edge = math.min(edge, h)
  if edge < 1 then return nil end
  return edge
end

-- ------- the pass
--
-- One rectangle, one shader. Every pixel answers for itself from its canvas
-- coordinate, so the sky is drawn at exactly the resolution it is displayed at
-- -- there is no image being scaled and so nothing to be soft. The one texture
-- bound is the band ramp, which is a PALETTE and not a picture: n texels wide,
-- sampled nearest, one lookup per pixel (see rampFor).
--
-- `cell` quantises BOTH the band edges and the dither: the y a pixel is judged
-- by is the top of its own cell row, so a whole cell row is one colour and every
-- edge in the sky lands on the diorama's pixel grid.
local SHADER_SRC = [[
uniform Image ramp;     // the bands, one texel each, top of the sky first
uniform float count;    // how many texels wide that ramp is
uniform float edge;     // the sky's bottom, in canvas pixels
uniform float top;      // where the deepest band begins, in canvas pixels --
                        // 0 glues the gradient to the frame (the flat
                        // screen); an anchored caller passes the row its
                        // fixed elevation span starts on, often negative
uniform float cell;     // the diorama's pixel size, in canvas pixels
uniform float start;    // where the checker begins inside a band
uniform float axisX;    // the "toward the ground" direction on the canvas:
uniform float axisY;    // (0,1) for a level camera; a rolled VR eye tips
                        // it, and edge/top are distances along it
uniform vec3 rayBase;   // the eye's ray fan (VRRig eyeCamera.skyRay): a
uniform vec3 rayDu;     // canvas point at fractions (u, v) looks along
uniform vec3 rayDv;     // base + u*du + v*dv, world axes -- so each pixel
                        // knows its TRUE elevation and the gradient is a
                        // real skybox, untouched by any head motion
uniform float raySpan;  // radians of elevation the gradient covers
uniform vec2 invSize;   // 1/w, 1/h: canvas pixels to fractions
uniform float useRay;   // 0 = the flat screen's frame-linear gradient
uniform float cellAng;  // one checker cell in RADIANS (ray path): the
                        // dither's own grid, laid on azimuth/elevation so
                        // the pattern is glued to the SKY -- a screen-cell
                        // parity flips under every head motion and the
                        // whole gradient shimmers
uniform float alpha;
uniform float glowAmt;  // twilight warmth around the low sun; 0 = none
uniform vec2 glowPos;   // the sun disc, in canvas pixels (flat path)
uniform float glowInvR; // 1 / the glow's reach in pixels (flat path)
uniform vec3 glowDir;   // the sun's world direction (ray path)
uniform float glowInvA; // 1 / the glow's reach in radians (ray path)
uniform vec3 glowColor;

// Band `i`, read from its own texel centre. The index is clamped rather than
// trusted: `pos` below can land exactly on `count` when the arithmetic is
// carried at mediump -- which is the fragment default on GLSL ES -- and a
// sample past the last band must be the last band, not whatever is off the
// end of the image.
vec3 bandAt(float i) {
  return Texel(ramp, vec2((clamp(i, 0.0, count - 1.0) + 0.5) / count, 0.5)).rgb;
}

vec4 effect(vec4 color, Image tex, vec2 tc, vec2 sc) {
  float tn;
  float parity;
  float glowD = 2.0;                                  // past the reach
  if (useRay > 0.5) {
    // A SKYBOX, computed instead of stored: the pixel's own ray lands in
    // a cell of the sky's angular grid (azimuth columns and elevation
    // rows, cellAng square), and EVERYTHING -- the band, the checker's
    // parity, the glow -- is answered from that cell's centre. The
    // screen grid quantises nothing here; that is the point. A screen
    // quantisation of similar pitch laid under the sky grid beats
    // against it (moire), and every subpixel head motion re-snaps the
    // beat -- the fizz. Sampled per pixel, the picture is exactly a
    // nearest-filtered texture on a dome: its cells slide smoothly with
    // the world and no motion of the head recomputes the pattern. The
    // one seam, where azimuth wraps behind the camera, is a single cell
    // column of a dither pattern.
    vec3 dir = rayBase + rayDu * (sc.x * invSize.x)
                       + rayDv * (sc.y * invSize.y);
    float elev = atan(dir.y, length(dir.xz));
    float ei = floor(elev / cellAng);                 // elevation row
    if (ei < 0.0) { discard; }                        // below the horizon
    float ai = floor(atan(dir.x, dir.z) / cellAng);   // azimuth column
    float elc = (ei + 0.5) * cellAng;                 // the row's centre
    tn = 1.0 - clamp(elc / max(raySpan, 0.001), 0.0, 1.0);
    parity = mod(ai + ei, 2.0);
    if (glowAmt > 0.0) {
      // the glow by the angle between the CELL's centre direction and
      // the sun's own, so its rings are pinned to the same sky grid
      float azc = (ai + 0.5) * cellAng;
      vec3 cd = vec3(cos(elc) * sin(azc), sin(elc), cos(elc) * cos(azc));
      glowD = acos(clamp(dot(cd, glowDir), -1.0, 1.0)) * glowInvA;
    }
  } else {
    vec2 cc0 = floor(sc / cell) * cell;               // top of this cell
    float row = cc0.x * axisX + cc0.y * axisY;        // along the axis
    if (row > edge) { discard; }                      // below the horizon
    tn = clamp((row - top) / max(edge - top, 1.0), 0.0, 1.0);
    parity = mod(floor(sc.x / cell) + floor(sc.y / cell), 2.0);
    if (glowAmt > 0.0) {
      vec2 cc = (floor(sc / cell) + 0.5) * cell;
      glowD = length(cc - glowPos) * glowInvR;
    }
  }
  float pos = tn * count;
  float base = min(floor(pos), count - 1.0);
  vec3 c = bandAt(base);
  if (base < count - 1.0 && (pos - base) > start) {
    if (parity < 0.5) { c = bandAt(base + 1.0); }
  }
  // The sunset's warmth, radiating from the disc: posterised to a few rungs
  // and checker-dithered between them -- the same 8-bit move as the bands,
  // so the glow reads as painted light rather than as a smooth airbrush --
  // measured cell-to-cell on the flat frame and angle-to-angle on the
  // skybox, so its rings ride whichever grid the checker itself is on.
  if (glowAmt > 0.0) {
    float g = glowAmt * pow(clamp(1.0 - glowD, 0.0, 1.0), 2.0);
    float lvl = floor(g * 4.0);
    if (g * 4.0 - lvl > 0.5 && parity < 0.5) { lvl += 1.0; }
    c = mix(c, glowColor, min(lvl / 3.0, 1.0) * 0.65);
  }
  return vec4(c, alpha);
}
]]

-- ------- the ramp
--
-- The bands as a one-texel-per-band TEXTURE rather than as a uniform array,
-- which is what they used to be: `uniform vec3 bands[8]`, filled from Lua and
-- read through a loop counter. On desktop GL that is as portable as it looks.
-- On Android it was not. The sky's lower bands came back BLACK -- a hard-edged
-- strip running from partway down the gradient to the horizon point, with the
-- moon still drawn correctly over it, and with the haze BELOW the sky (the
-- palest band again, but delivered by love.graphics.clear instead of by the
-- array) landing in exactly the right colour. Same colour, two routes, one of
-- them black: the fault was the array, not the palette.
--
-- Which of the ES failure modes it was hardly matters -- a driver that
-- truncates a partially-filled array, a fragment uniform budget the guaranteed
-- floor of which is sixteen vectors (eight bands plus the glow plus LOVE's own
-- built-ins is over it), a reflection that finds bands[0] and nothing after --
-- because they all have the same shape: slots past the first few read as zero,
-- and zero is black.
--
-- A sampler has none of them. One texture unit replaces eight uniform vectors,
-- there is no array to index and no budget to overrun, and a texel that does
-- not exist cannot read as black because the image is built at exactly the
-- width the shader divides by. Nearest and clamped, so a sample lands on one
-- band's own colour and an out-of-range one lands on the end band rather than
-- on nothing.
--
-- Rebuilt only when the bands move, which is when the clock or the display
-- mode does; Sky.bands drops it as it rebuilds the list it is made from.
local function rampFor(bands)
  if cache.ramp and cache.rampFor == bands then return cache.ramp end
  if not (love.image and love.image.newImageData
          and love.graphics and love.graphics.newImage) then return nil end
  local n = #bands
  if n < 1 then return nil end
  local ok, data = pcall(love.image.newImageData, n, 1)
  if not (ok and data) then return nil end
  for i = 1, n do
    local c = bands[i]
    pcall(data.setPixel, data, i - 1, 0, c[1], c[2], c[3], 1)
  end
  local built, img = pcall(love.graphics.newImage, data)
  if not (built and img) then return nil end
  -- nearest: a band is a flat colour, not something to interpolate between.
  -- clamp: the shader clamps its index too, so this is the second of two
  -- guards against ever sampling off the end -- and it returns the edge band.
  pcall(img.setFilter, img, "nearest", "nearest")
  pcall(img.setWrap, img, "clamp", "clamp")
  cache.ramp, cache.rampFor = img, bands
  return img
end

Sky._rampFor = rampFor            -- named for the suite

-- The band ramp for the CURRENT bands, plus how many texels wide it is --
-- for a pass that wants to read the same sky this one paints. The water's
-- reflection is the one caller: it looks the reflected direction up on this
-- very ramp, so the sky on the lake and the sky over it are one palette,
-- through one display-mode transform, off one clock.
--
-- nil where the ramp could not be built, which is exactly when Sky.paint
-- falls back to flat bands -- so a driver that loses the gradient loses the
-- reflected gradient with it rather than showing two different skies.
function Sky.ramp()
  local bands = Sky.bands()
  if not (bands and bands[1]) then return nil end
  local img = rampFor(bands)
  if not img then return nil end
  return img, #bands, bands
end

-- How far the twilight glow reaches around the disc, in canvas pixels, for
-- a `w`-wide frame. The same number Sky.paint sends as `glowInvR`.
Sky.GLOW_REACH = 0.55

local shader = nil            -- nil = untried, false = unavailable

local function getShader()
  if shader == nil then
    shader = false
    if love.graphics and love.graphics.newShader then
      local ok, sh = pcall(love.graphics.newShader, SHADER_SRC)
      if ok and sh then
        shader = sh
      elseif V and V.mod and V.mod.log then
        -- once, and only where it can be read: the fallback below is a sky
        -- without its dither, which is easy to look at and impossible to
        -- diagnose without this line
        V.mod.log:warn("sky shader did not compile: %s -- the bands draw flat, "
                       .. "with no dither between them", tostring(sh))
      end
    end
  end
  return shader or nil
end

Sky._getShader = getShader        -- named for the suite

-- The flat fallback: the same bands as solid rectangles, no checker, on the same
-- quantised edges. For a driver that could not compile the shader -- which is
-- also every headless run.
local function paintFlat(w, h, bands, edge, alpha, cell, top)
  local g = love.graphics
  local n = #bands
  local span = edge - (top or 0)
  local prev = 0
  for i = 1, n do
    local cut = (i == n) and math.min(h, math.ceil(edge))
                or math.floor(((top or 0) + i / n * span) / cell + 0.5) * cell
    cut = math.max(prev, math.min(cut, math.min(h, math.ceil(edge))))
    if cut > prev then
      local c = bands[i]
      g.setColor(c[1], c[2], c[3], alpha)
      g.rectangle("fill", 0, prev, w, cut - prev)
    end
    prev = cut
  end
end

-- ------- the discs
--
-- The sun and moon, as cell art: a circle of whole diorama cells with a
-- lighter core, a dithered rim, and -- for the moon -- a few fixed crater
-- cells. Drawn as plain rectangles on the same grid as the sky's own dither,
-- through the same display-mode transform as every palette here, and
-- SCISSORED to the sky's region: the horizon point is where a setting body
-- disappears, so it can never hang under the map at a high pitch.
--
-- SIZED BY THE FRAME, not by the world: a celestial body's apparent size is
-- an angle, so zooming the ground in and out must not swell and shrink the
-- sun with it. The radius is a fraction of the frame height, converted to
-- whole cells so the disc still sits on the diorama's grid -- chunky cells
-- up close, fine ones at survey zoom, the same size body either way.
Sky.DISC_FRAC = 0.030     -- disc radius, as a fraction of the frame height
Sky.DISC_MIN = 3          -- but never fewer cells than this across a radius

-- crater centres as fractions of the radius, so they ride any disc size.
-- Public because the water's reflection draws the same moon (see Water):
-- one list, so the disc on the lake cannot drift from the one in the sky.
Sky.MOON_CRATERS = { { -0.4, -0.2 }, { 0.2, 0.45 }, { 0.5, -0.4 },
                     { -0.15, 0.7 }, { 0.05, 0.05 } }

-- a crater's radius, as a fraction of the disc's -- the r/5 paintDisc uses
Sky.CRATER_FRAC = 0.2

local MOON_CRATERS = Sky.MOON_CRATERS

-- The disc's four shades as the display mode has them, lightest first.
-- Shared with the reflection pass, so the sun on the water is the same sun
-- that is in the sky, in the same mode's palette.
function Sky.discShades(moon)
  local src = moon and DayNight.MOON_COLORS or DayNight.SUN_COLORS
  return PaletteFX.effectiveColors(src) or src
end

-- Whether this body is the LOOMING low sun -- the sunset exaggeration.
local function looming(body)
  return (body.glowAmt or 0) > 0.25 and not body.moon
end

-- The disc's radius for a `h`-tall frame on a `cell`-pixel grid: in CANVAS
-- PIXELS, and in whole cells. Sized by the FRAME rather than by the world
-- (see DISC_FRAC), so a zoom does not swell the sun.
--
-- Read by paintDisc below and by the reflection, which needs the same
-- number in radians -- a disc drawn one size and mirrored another would
-- read as two different suns.
function Sky.discRadius(h, cell, body)
  cell = math.max(1, cell or 1)
  local r = math.max(Sky.DISC_MIN,
                     math.floor(h * Sky.DISC_FRAC / cell + 0.5))
  if body and looming(body) then r = r + math.max(1, math.floor(r * 0.4)) end
  return r * cell, r
end

-- One disc's worth of cell art -- shared verbatim by the screen-space
-- painter below (the flat screen) and by the BAKE the VR eyes texture
-- their world-anchored quad with (Sky.discImage). `plot(dx, dy, c)` gets
-- every kept cell in disc-local cell coordinates and its 0..255 colour.
local function discCells(r, moon, shades, twilight, plot)
  local core = shades[1]
  local main = shades[twilight and 3 or 2]
  local craterR = math.max(1, math.floor(r / 5))
  for dy = -r, r do
    for dx = -r, r do
      local d = math.sqrt(dx * dx + dy * dy)
      if d <= r + 0.1 then
        local c = d <= r * 0.5 and core or main
        -- dithered rim: the outer ring keeps only one parity of its cells
        local keep = d <= r - 0.9 or (dx + dy) % 2 == 0
        if moon then
          for _, cr in ipairs(MOON_CRATERS) do
            local cdx = dx - math.floor(cr[1] * r + 0.5)
            local cdy = dy - math.floor(cr[2] * r + 0.5)
            if cdx * cdx + cdy * cdy <= craterR * craterR then
              c = shades[3]
            end
          end
        end
        if keep then plot(dx, dy, c) end
      end
    end
  end
end

local function paintDisc(body, edge, cell, w, h)
  local g = love.graphics
  if not (body and body.y and g.setScissor) then return end
  local shades = Sky.discShades(body.moon)
  local twilight = looming(body)
  local _, r = Sky.discRadius(h, cell, body)
  -- snap the centre to the cell grid, like everything else in this sky
  local bx = math.floor(body.x / cell) * cell + cell / 2
  local by = math.floor(body.y / cell) * cell + cell / 2
  if by - r * cell > edge then return end     -- wholly below the horizon point
  local sx, sy, sw, sh = g.getScissor()
  g.setScissor(0, 0, math.ceil(w), math.floor(edge))
  discCells(r, body.moon, shades, twilight, function(dx, dy, c)
    g.setColor(c[1] / 255, c[2] / 255, c[3] / 255, 1)
    g.rectangle("fill", bx + dx * cell - cell / 2,
                by + dy * cell - cell / 2, cell, cell)
  end)
  if sx then g.setScissor(sx, sy, sw, sh) else g.setScissor() end
  g.setColor(1, 1, 1, 1)
end

-- ------- the disc as a TEXTURE, for the VR eyes
--
-- A VR eye must not paint the disc in screen space at all: a canvas-grid
-- painting re-snaps to different cells every head movement (jitter) and
-- holds its pattern square to the CANVAS (a rolled or pitched head
-- watches the sun's face turn). So the same cell art is baked once into
-- a texture, and Voxel3D hangs it on a quad ANCHORED IN THE WORLD --
-- projected through the eye's own matrix like any geometry, stable under
-- every head motion. Rebaked only when the palette or the twilight state
-- moves the colours.
local discBake = { key = nil, img = nil }

Sky.DISC_BAKE_R = 9          -- bake radius, in cells
Sky.DISC_BAKE_PX = 8         -- texture pixels per cell

function Sky.discImage(moon, twilight)
  if not (love.graphics and love.graphics.newCanvas) then return nil end
  local shades = Sky.discShades(moon)
  local key = (moon and "m" or "s") .. (twilight and "t" or "-")
  for i = 1, math.min(3, #shades) do
    local c = shades[i]
    key = key .. ":" .. c[1] .. "," .. c[2] .. "," .. c[3]
  end
  if discBake.key == key and discBake.img then return discBake.img end
  local r, px = Sky.DISC_BAKE_R, Sky.DISC_BAKE_PX
  local size = (2 * r + 1) * px
  local ok, canvas = pcall(love.graphics.newCanvas, size, size)
  if not (ok and canvas) then return nil end
  pcall(canvas.setFilter, canvas, "nearest", "nearest")
  local g = love.graphics
  local done = pcall(function()
    g.push("all")
    g.origin()
    g.setCanvas(canvas)
    g.clear(0, 0, 0, 0)
    g.setBlendMode("alpha")
    discCells(r, moon, shades, twilight, function(dx, dy, c)
      g.setColor(c[1] / 255, c[2] / 255, c[3] / 255, 1)
      g.rectangle("fill", (dx + r) * px, (dy + r) * px, px, px)
    end)
    g.pop()
  end)
  if not done then return nil end
  discBake.key, discBake.img = key, canvas
  return canvas
end

-- Whether this body is the looming low sun, for callers sizing the baked
-- disc (the same exaggeration paintDisc applies through discRadius).
function Sky.discLooming(glowAmt, moon)
  return (glowAmt or 0) > 0.25 and not moon
end

-- Paint the sky into the bound canvas, filling it from the top edge down to
-- `horizonY` (or to SPAN of the frame when the horizon is out of it).
--
-- `cell` is the diorama's pixel size in canvas pixels -- the pass's own
-- pixels-per-world-pixel, handed in every frame so a zoom lands immediately.
--
-- `body` is the sun or moon to hang, already projected to canvas pixels by
-- the caller's own camera (Voxel3D.skyBody), with the twilight glow riding
-- along; nil hangs nothing and warms nothing.
--
-- `top` anchors the gradient in space rather than to the frame: the canvas
-- row band 1 starts on (often negative -- above the frame), from a caller
-- that mapped a fixed elevation span to its own camera (see ELEV_SPAN).
-- nil or 0 is the flat screen's behaviour: zenith band at the top edge.
--
-- `axis` tips the whole painting to a rolled camera's true horizon: a unit
-- {ax, ay} pointing "toward the ground" on the canvas (Voxel3D.horizonLine),
-- with `horizonY` and `top` then read as distances ALONG it rather than as
-- rows. nil is the level default. Only the shader path can tilt; the flat
-- fallback paints level, which only a headless run ever sees. Under an
-- axis the DISC is not painted here at all -- the VR caller hangs the
-- baked disc (Sky.discImage) in the world instead; `body` still carries
-- the twilight glow into the bands.
--
-- `ray` makes the gradient a SKYBOX: the eye's own ray fan (the camera
-- record's skyRay, from VRRig.eyeCamera), letting every pixel take its
-- band from its TRUE elevation -- so no motion of the head, on any axis,
-- moves a band; only the clock does. nil keeps the linear frame gradient
-- the flat screen has always painted.
--
-- Returns false when there is nothing to paint, in which case the caller's flat
-- fill is the whole sky. That fill is the palest band, so a frame that declines
-- this looks like a hazy day rather than like a bug.
function Sky.paint(w, h, sky, horizonY, cell, body, top, axis, ray)
  local bands = sky and sky.bands
  if not (bands and bands[1]) then return false end
  if not (w and h and w > 0 and h > 0) then return false end
  local g = love.graphics
  if not (g and g.rectangle) then return false end
  -- with a ray fan the shader's own per-pixel elevation test is the only
  -- boundary and the whole frame goes through it; along an axis the
  -- caller's edge is already the signed distance and has no row to be
  -- clamped to; level callers keep the SPAN fallback
  local edge
  if ray then
    edge = h
  elseif axis then
    edge = horizonY
  else
    edge = Sky.region(h, horizonY)
  end
  if not edge then return false end
  local alpha = sky[4] or 1
  cell = math.max(1, math.floor((cell or 1) + 0.5))

  -- State to put aside. The scene's shader is one, and the blend mode another --
  -- a pass that left "replace" behind would make the fade-in strength meaningless
  -- -- but the DEPTH MODE is the one that would break the frame: a rectangle
  -- drawn under the pass's own ("lequal", true) stamps itself across the depth
  -- buffer at the near plane and hides the entire world behind the sky.
  local prevShader = g.getShader and g.getShader() or nil
  local cmp, write
  if g.getDepthMode then cmp, write = g.getDepthMode() end
  if g.setDepthMode then g.setDepthMode("always", false) end
  local blend, blendAlpha
  if g.getBlendMode then blend, blendAlpha = g.getBlendMode() end
  if g.setBlendMode then g.setBlendMode("alpha") end

  local glowAmt = body and not body.moon and (body.glowAmt or 0) or 0
  -- the skybox glow needs the sun's world DIRECTION (skyBody carries it);
  -- a body without one has nothing to measure angles against, so no glow
  if ray and glowAmt > 0 and not (body and body.dx) then glowAmt = 0 end
  -- the world direction a canvas fraction (u, v) looks along, normalised
  -- -- for sizing the angular checker and the glow's angular reach below
  local function rayDirAt(u, v)
    local b, du, dv = ray.base, ray.du, ray.dv
    local x = b[1] + du[1] * u + dv[1] * v
    local y = b[2] + du[2] * u + dv[2] * v
    local z = b[3] + du[3] * u + dv[3] * v
    local l = math.sqrt(x * x + y * y + z * z)
    if l < 1e-9 then return 0, 0, -1 end
    return x / l, y / l, z / l
  end
  local function rayAngle(u0, v0, u1, v1)
    local ax, ay, az = rayDirAt(u0, v0)
    local bx, by, bz = rayDirAt(u1, v1)
    local d = ax * bx + ay * by + az * bz
    return math.acos(math.max(-1, math.min(1, d)))
  end
  local sh = getShader()
  local ramp = sh and rampFor(bands)
  if not ramp then sh = nil end       -- no ramp, no gradient: paint it flat
  if sh then
    local sent = pcall(function()
      -- the bands arrive as a texture, one texel each, and `count` is that
      -- texture's width -- see rampFor for why they are not a uniform array
      sh:send("ramp", ramp)
      sh:send("count", #bands)
      sh:send("edge", edge)
      sh:send("top", math.min(top or 0, edge - 1))
      sh:send("axisX", axis and axis[1] or 0)
      sh:send("axisY", axis and axis[2] or 1)
      sh:send("useRay", ray and 1 or 0)
      if ray then
        sh:send("rayBase", ray.base)
        sh:send("rayDu", ray.du)
        sh:send("rayDv", ray.dv)
        sh:send("raySpan", Sky.ELEV_SPAN)
        sh:send("invSize", { 1 / w, 1 / h })
        -- the angular checker's cell: the angle one dither cell spans at
        -- the frame's centre, so the sky-glued grid comes out the same
        -- size on screen as the diorama's own pixel grid
        sh:send("cellAng",
                math.max(1e-4, rayAngle(0.5, 0, 0.5, 1) * cell / h))
      end
      sh:send("cell", cell)
      sh:send("start", Sky.DITHER and Sky.DITHER_START or 2)
      sh:send("alpha", alpha)
      sh:send("glowAmt", glowAmt)
      if glowAmt > 0 then
        local gc = body.glowColor or { 248, 224, 168 }
        if ray then
          -- the glow in ANGLES: its direction is the sun's own, and its
          -- reach is the same fraction of the view the pixel reach was
          -- of the frame, so the two paths agree on how wide it looks
          local dx, dy, dz = body.dx, body.dy, body.dz
          local l = math.sqrt(dx * dx + dy * dy + dz * dz)
          sh:send("glowDir", { dx / l, dy / l, dz / l })
          sh:send("glowInvA", 1 / math.max(
            1e-3, rayAngle(0, 0.5, 1, 0.5) * Sky.GLOW_REACH))
        else
          sh:send("glowPos", { body.x, body.y })
          sh:send("glowInvR", 1 / math.max(1, w * Sky.GLOW_REACH))
        end
        sh:send("glowColor", { gc[1] / 255, gc[2] / 255, gc[3] / 255 })
      end
    end)
    if sent then
      g.setShader(sh)
      g.setColor(1, 1, 1, 1)
      -- tilted or rayed, the sky's reach is not a row: the full frame
      -- goes through the shader and the discard is the boundary
      local rectH = (axis or ray) and h or math.min(h, math.ceil(edge))
      g.rectangle("fill", 0, 0, w, rectH)
      g.setShader()
    else
      sh = nil
    end
  end
  if not sh then
    paintFlat(w, h, bands, (axis or ray) and math.min(h, edge) or edge,
              alpha, cell, math.min(top or 0, edge - 1))
  end
  -- the disc goes over the glow, under nothing: plain rectangles, so it is
  -- there whether or not the shader built. NOT under an axis or a ray fan:
  -- those cameras hang the baked disc in the world instead (drawWorldDisc,
  -- with Sky.discImage)
  if not (axis or ray) then
    paintDisc(body, math.min(h, edge), cell, w, h)
  end
  g.setColor(1, 1, 1, 1)

  if g.setBlendMode and blend then g.setBlendMode(blend, blendAlpha) end
  if g.setDepthMode then g.setDepthMode(cmp or "always", write or false) end
  if prevShader and g.setShader then g.setShader(prevShader) end
  return true
end

-- Drop the compiled shader (window resize, hot reload), so a re-created graphics
-- context builds a new one instead of drawing with a handle from the old. The
-- ramp is a GPU object on the same context and goes with it.
function Sky.invalidate()
  shader = nil
  if cache.ramp and cache.ramp.release then pcall(cache.ramp.release, cache.ramp) end
  cache.ramp, cache.rampFor = nil, nil
  if discBake.img and discBake.img.release then
    pcall(discBake.img.release, discBake.img)
  end
  discBake.key, discBake.img = nil, nil
end

return Sky