diff --git a/app/public/app.js b/app/public/app.js
index 3452bd3..2bfa765 100644
--- a/app/public/app.js
+++ b/app/public/app.js
@@ -1481,6 +1481,12 @@ async function viewReader(mangaId, chapterId, startPage) {
     scheduleLeafTextures()
   }
   const imgReady = im => im.complete && im.naturalWidth > 0
+  // Lado de la hendidura del lomo: l o r (borde de la página) o m (página doble, en el centro).
+  const gutterOf = (i, n) => {
+    if (wide.has(i)) return 'm'
+    const rtl = settings.dir === 'rtl'
+    return shown.length === 2 ? (rtl === (n === 0) ? 'l' : 'r') : rtl ? 'r' : 'l'
+  }
   // Hacia una página que aún no ha llegado, la actual se queda (con un indicador tras un momento) hasta que
   // llegue o pasen 4 s, en vez de pasar la hoja y dejar negro debajo. Si se pide otra página mientras,
   // manda la última.
@@ -1494,7 +1500,7 @@ async function viewReader(mangaId, chapterId, startPage) {
     const stage = document.createElement('div')
     stage.className = 'stage'
     stage.classList.toggle('two', shown.length === 2)
-    stage.innerHTML = shown.map(i => `<img data-i="${i}" src="${pages[i]}" alt="" draggable="false">`).join('')
+    stage.innerHTML = shown.map((i, n) => `<img data-i="${i}" data-gutter="${gutterOf(i, n)}" src="${pages[i]}" alt="" draggable="false">`).join('')
     const imgs = [...stage.querySelectorAll('img')]
     if ($('.stage', box) && dir && !wantDrag && !imgs.every(imgReady)) {
       waitTimer = setTimeout(() => box.classList.add('waiting'), 200)
@@ -1534,6 +1540,7 @@ async function viewReader(mangaId, chapterId, startPage) {
     else { box.replaceChildren(stage) }
     stage.querySelectorAll('img').forEach(img => img.addEventListener('load', () => {
       const i = Number(img.dataset.i)
+      if (isWide(img)) img.dataset.gutter = 'm'
       if (isWide(img) && !wide.has(i)) {
         wide.add(i)
         if (shown.length === 2 && shown.includes(i)) goTo(shown[0], false, true) // reagrupar
@@ -1645,19 +1652,28 @@ async function viewReader(mangaId, chapterId, startPage) {
       gl.attachShader(prog, shader(gl.FRAGMENT_SHADER, `#version 300 es
         precision highp float;
         uniform sampler2D uFront, uBack;
-        uniform bool uPaper;
+        uniform bool uPaper, uSpineLeft, uSpread;
         in vec2 vUV;
         in float vC;
         out vec4 color;
+        // La hendidura del lomo: los mismos puntos que data-gutter en style.css, para que no salte al empezar el giro.
+        float gutter(float d) {
+          if (d < 0.012) return mix(0.4, 0.6, d / 0.012);
+          if (d < 0.03) return mix(0.6, 0.8, (d - 0.012) / 0.018);
+          if (d < 0.06) return mix(0.8, 0.93, (d - 0.03) / 0.03);
+          if (d < 0.1) return mix(0.93, 1.0, (d - 0.06) / 0.04);
+          return 1.0;
+        }
         void main() {
           vec4 t;
           float b;
+          float fromSpine = uSpineLeft ? vUV.x : 1.0 - vUV.x;
           if (gl_FrontFacing) {
             t = texture(uFront, vUV, -0.5);
-            b = 1.0 - (1.0 - vC) * 0.3;
+            b = (1.0 - (1.0 - vC) * 0.3) * gutter(uSpread ? abs(vUV.x - 0.5) * 2.0 : fromSpine);
           } else {
             t = uPaper ? vec4(0.914, 0.898, 0.863, 1.0) : texture(uBack, vec2(1.0 - vUV.x, vUV.y), -0.5);
-            b = 1.0 - (1.0 + vC) * 0.3;
+            b = (1.0 - (1.0 + vC) * 0.3) * gutter(fromSpine);
           }
           color = vec4(t.rgb * b, t.a);
         }`))
@@ -1743,7 +1759,7 @@ async function viewReader(mangaId, chapterId, startPage) {
     canvas.addEventListener('webglcontextlost', () => { if (leafGL?.gl === gl) { leafGL = null; leafTextures.clear() } })
     leafGL = { canvas, gl, prog, posBuf, uvBuf, aniso, shadowProg, shadowVao, shadowBuf, shadowCount: m, shadowData: new Float32Array(shadowSegs * 16 * 5),
       uSize: gl.getUniformLocation(shadowProg, 'uSize'), positions: new Float32Array(count * 4), uvs: new Float32Array(count * 2), indexCount: n,
-      uView: gl.getUniformLocation(prog, 'uView'), uPaper: gl.getUniformLocation(prog, 'uPaper'), maxSize: gl.getParameter(gl.MAX_TEXTURE_SIZE) }
+      uView: gl.getUniformLocation(prog, 'uView'), uPaper: gl.getUniformLocation(prog, 'uPaper'), uSpineLeft: gl.getUniformLocation(prog, 'uSpineLeft'), uSpread: gl.getUniformLocation(prog, 'uSpread'), maxSize: gl.getParameter(gl.MAX_TEXTURE_SIZE) }
     return leafGL
   }
   function leafTexture(im) {
@@ -1817,6 +1833,8 @@ async function viewReader(mangaId, chapterId, startPage) {
     const scale = pw / W
     gl.scissor(Math.round(paper.l * scale), Math.round((H - paper.t - paper.h) * scale), Math.round(paper.w * scale), Math.round(paper.h * scale))
     gl.uniform1i(L.uPaper, back ? 0 : 1)
+    gl.uniform1i(L.uSpineLeft, spineLeft ? 1 : 0)
+    gl.uniform1i(L.uSpread, frontIm.dataset.gutter === 'm' ? 1 : 0)
     gl.activeTexture(gl.TEXTURE0)
     gl.bindTexture(gl.TEXTURE_2D, front)
     gl.activeTexture(gl.TEXTURE1)
@@ -2033,10 +2051,15 @@ async function viewReader(mangaId, chapterId, startPage) {
         spineLeft = sign > 0
         const keep = document.createElement('img')
         keep.className = 'leafkeep'
+        keep.dataset.gutter = stay.dataset.gutter
         keep.src = stay.src
         place(keep, rel(stay))
-        box.append(keep)
-        extra.push(keep)
+        // Su hendidura es transparente: sin negro detrás, se vería la página nueva y la hendidura se aclararía.
+        const keepBack = document.createElement('i')
+        keepBack.className = 'leafkeepback'
+        place(keepBack, rel(stay))
+        box.append(keepBack, keep)
+        extra.push(keepBack, keep)
         old.style.visibility = 'hidden' // la hoja y la copia ocupan su lugar
       } else {
         // Una página: el lomo está a la izquierda en izq→der y a la derecha en der→izq. Hacia delante, la
@@ -2375,7 +2398,13 @@ async function viewReader(mangaId, chapterId, startPage) {
     render()
   })
   $('#fit').addEventListener('change', e => { settings.fit = e.target.value; savePrefs(); render() })
-  $('#dir').addEventListener('change', e => { settings.dir = e.target.value; box.dataset.dir = settings.dir; syncDirUi(); savePrefs() })
+  $('#dir').addEventListener('change', e => {
+    settings.dir = e.target.value
+    box.dataset.dir = settings.dir
+    box.querySelectorAll('.stage img').forEach((im, n) => { im.dataset.gutter = gutterOf(Number(im.dataset.i), n) })
+    syncDirUi()
+    savePrefs()
+  })
   $('#filter').addEventListener('change', e => { settings.filter = e.target.value; saveGlobal(); applyFilter() })
   $('#anim').value = settings.anim
   $('#anim').addEventListener('change', e => { settings.anim = e.target.value; saveGlobal() })
diff --git a/app/public/style.css b/app/public/style.css
index 32566ca..d25e6f1 100644
--- a/app/public/style.css
+++ b/app/public/style.css
@@ -397,6 +397,19 @@ body.reading #netbanner { display: none; }
 .paged .curldrop { position: absolute; z-index: 2; pointer-events: none; opacity: 0; }
 .paged .curldrop.l { background: linear-gradient(90deg, rgba(0,0,0,.55), rgba(0,0,0,.2) 35%, rgba(0,0,0,0) 75%); }
 .paged .curldrop.r { background: linear-gradient(270deg, rgba(0,0,0,.55), rgba(0,0,0,.2) 35%, rgba(0,0,0,0) 75%); }
+/* Hendidura del lomo: la página se funde con el negro de detrás. Los puntos coinciden con gutter() en app.js. */
+.paged:not(.fit-width) img[data-gutter] {
+  --gutter-stops: rgba(0,0,0,.4) 0, rgba(0,0,0,.6) 1.2%, rgba(0,0,0,.8) 3%, rgba(0,0,0,.93) 6%, #000 10%;
+  --gutter-mask: linear-gradient(var(--gutter-side), var(--gutter-stops));
+  -webkit-mask-image: var(--gutter-mask);
+  mask-image: var(--gutter-mask);
+}
+.paged:not(.fit-width) img[data-gutter="l"] { --gutter-side: to right; }
+.paged:not(.fit-width) img[data-gutter="r"] { --gutter-side: to left; }
+.paged:not(.fit-width) img[data-gutter="m"] {
+  --gutter-mask: linear-gradient(to right, #000 45%, rgba(0,0,0,.93) 47%, rgba(0,0,0,.8) 48.5%, rgba(0,0,0,.6) 49.4%, rgba(0,0,0,.4) 50%, rgba(0,0,0,.6) 50.6%, rgba(0,0,0,.8) 51.5%, rgba(0,0,0,.93) 53%, #000 55%);
+}
+.paged .leafkeepback { position: absolute; z-index: 2; pointer-events: none; background: #000; }
 .paged .leafkeep { position: absolute; z-index: 2; pointer-events: none; max-width: none; max-height: none; object-fit: fill; }
 
 /* Info de la obra */