Fix LÖVE 12 scissor seam

src/render/Renderer.lua

@@ -564,16 +564,24 @@ function Renderer:drawTiltedWorld(zoneList, sx, sy, wox, woy, target)
   return true
 end
 
+-- LÖVE 11 truncates scissor arguments to framebuffer pixels; the half-pixel
+-- bias keeps values divided back through a fractional DPI scale from landing
+-- one short.  LÖVE 12 passes fractional arguments through and rounds in the
+-- graphics backend instead, where that bias shifts each origin by one pixel
+-- and extends its far edge by two (#673).
+local SCISSOR_PIXEL_BIAS = 0.5
+if love and love.getVersion and select(1, love.getVersion()) >= 12 then
+  SCISSOR_PIXEL_BIAS = 0
+end
+
 -- Clamp a scissor rect to the viewport box, then round it outward to whole
--- framebuffer pixels.  love.graphics.setScissor truncates x, y, w and h to
+-- framebuffer pixels.  On LÖVE 11, x, y, w and h are truncated independently,
--- pixels independently, so a rect with fractional unit edges (Android's
+-- so a rect with fractional unit edges (Android's non-integer DPI puts
--- non-integer DPI puts fitScale/dpi in Sx/Sy) loses up to a pixel per side
+-- fitScale/dpi in Sx/Sy) loses up to a pixel per side and two adjacent SGB
--- and two adjacent SGB zones stop sharing an edge: the letterbox clear shows
+-- zones stop sharing an edge: the letterbox clear shows through as a seam at
--- through as a horizontal seam at every zone boundary (#373).  Rounding
+-- every zone boundary (#373).  Rounding outward makes neighbours overlap by
--- outward makes neighbours overlap by at most one row instead -- the overlap
+-- at most one row instead; SCISSOR_PIXEL_BIAS preserves that result across the
--- redraws the same canvas pixels one palette later, and past the canvas edge
+-- LÖVE 11 and 12 conversion rules.
--- there is nothing to draw.  The half pixel keeps LOVE's truncation on the
--- snapped edge rather than one short of it.
 local function scissorClamped(x, y, w, h, ox, oy, vpw, vph, dpiX, dpiY)
   local x2, y2 = math.min(x + w, ox + vpw), math.min(y + h, oy + vph)
   x, y = math.max(x, ox), math.max(y, oy)
@@ -581,9 +589,10 @@ local function scissorClamped(x, y, w, h, ox, oy, vpw, vph, dpiX, dpiY)
   dpiX, dpiY = dpiX or 1, dpiY or 1
   local px1, py1 = math.floor(x * dpiX), math.floor(y * dpiY)
   local px2, py2 = math.ceil(x2 * dpiX), math.ceil(y2 * dpiY)
-  love.graphics.setScissor((px1 + 0.5) / dpiX, (py1 + 0.5) / dpiY,
+  local b = SCISSOR_PIXEL_BIAS
-                           (px2 - px1 + 0.5) / dpiX,
+  love.graphics.setScissor((px1 + b) / dpiX, (py1 + b) / dpiY,
-                           (py2 - py1 + 0.5) / dpiY)
+                           (px2 - px1 + b) / dpiX,
+                           (py2 - py1 + b) / dpiY)
   return true
 end
 

tests/engine/title_zone_seams.lua

@@ -15,21 +15,41 @@ local check, eq, same = T.check, T.eq, T.same
 -- endFrame's blit closure, which needs canvases and a compiled shader.  The
 -- source is loaded directly so the rect arithmetic can be exercised with no
 -- GPU, the same way parity_picker_pointer_grab reads RomImporter (#254).
-local scissorClamped, captured
+local captured
-do
+local function loadScissor(loveMajor)
   local f = io.open("src/render/Renderer.lua", "rb")
   check(f ~= nil, "Renderer source is readable")
   local src = f and f:read("*a") or ""
   if f then f:close() end
+  local bias = src:match("\nlocal SCISSOR_PIXEL_BIAS = 0%.5.-\nend\n")
+  check(bias ~= nil, "scissor bias is still version-gated")
   local body = src:match("\nlocal function scissorClamped.-\nend\n")
   check(body ~= nil, "scissorClamped is still a single local function")
-  local fakeLove = { graphics = { setScissor = function(x, y, w, h)
+  local fakeLove = {
-    captured = { x = x, y = y, w = w, h = h }
+    getVersion = function() return loveMajor, 0, 0 end,
-  end } }
+    graphics = { setScissor = function(x, y, w, h)
-  local chunk = assert(loadstring("local love = ...\n" .. (body or "")
+      captured = { x = x, y = y, w = w, h = h }
+    end },
+  }
+  local chunk = assert(loadstring("local love = ...\n" .. (bias or "")
+    .. (body or "")
     .. "\nreturn scissorClamped"))
-  scissorClamped = chunk(fakeLove)
+  local scissor = chunk(fakeLove)
-  check(type(scissorClamped) == "function", "scissorClamped loads standalone")
+  check(type(scissor) == "function", "scissorClamped loads standalone")
+  return scissor
+end
+local scissorClamped = loadScissor(11)
+
+-- LÖVE 12 changed setScissor from truncating Lua integers to accepting floats
+-- and rounding in the backend.  Its arguments must therefore describe the
+-- already-snapped rectangle exactly, without LÖVE 11's half-pixel nudge.
+do
+  local scissor12 = loadScissor(12)
+  captured = nil
+  check(scissor12(50, 60, 10, 20, 0, 0, 100, 100, 2, 2),
+    "LÖVE 12 integer test rect draws")
+  same(captured, { x = 50, y = 60, w = 10, h = 20 },
+    "LÖVE 12 receives an unbiased snapped scissor")
 end
 
 -- The title's three SGB zones in canvas pixels (PaletteFX.zone turns the