From 28c0789b19cecf32c9578d97421c92bda201d213 Mon Sep 17 00:00:00 2001
From: root <root@stein2>
Date: Sun, 13 Sep 2026 18:46:20 +0200
Subject: [PATCH] Shader graph: Abs node, the distance from 0 per channel

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 README.md                  |  2 +-
 src/core/shader/nodes.ts   |  7 +++++++
 tests/core/shaders.test.ts | 21 +++++++++++++++++++++
 tools/e2e/28-shader.mjs    | 30 ++++++++++++++++++++++++++++++
 4 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 58ca89f0..65284d40 100644
--- a/README.md
+++ b/README.md
@@ -37,7 +37,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 762 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 763 checks (GM_STEPS=20 for one step)
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs
 node tools/seed-actor.mjs && node tools/smoke-actor.mjs
diff --git a/src/core/shader/nodes.ts b/src/core/shader/nodes.ts
index 377b5ec6..1b2d3d82 100644
--- a/src/core/shader/nodes.ts
+++ b/src/core/shader/nodes.ts
@@ -341,6 +341,12 @@ export const ShaderFract = unary(
   'fract',
   'The part after the decimal point: 2.75 becomes 0.75. A rising value becomes a sawtooth from 0 to 1.',
 );
+export const ShaderAbs = unary(
+  'ShaderAbs',
+  'Abs',
+  'abs',
+  'The distance from 0, sign dropped: -0.3 becomes 0.3. A value crossing 0 becomes a V. Per channel for colours.',
+);
 
 export const ShaderStep: ShaderNodeDef = {
   type: 'ShaderStep',
@@ -392,6 +398,7 @@ export const SHADER_NODES: readonly ShaderNodeDef[] = [
   ShaderSin,
   ShaderFloor,
   ShaderFract,
+  ShaderAbs,
   ShaderStep,
   ShaderPow,
 ];
diff --git a/tests/core/shaders.test.ts b/tests/core/shaders.test.ts
index 9b2c327a..f3214687 100644
--- a/tests/core/shaders.test.ts
+++ b/tests/core/shaders.test.ts
@@ -470,6 +470,27 @@ describe('compileShader', () => {
     expect(fragment).toMatch(/float n\d+_result = pow\(max\(n\d+_u, 0\.0\), 2\.0\);/);
     expect(fragment).toMatch(/vec3 n\d+_result = pow\(max\(n\d+_color, 0\.0\), vec3\(2\.2\)\);/);
   });
+
+  it('writes Abs as the GLSL call, per channel for a colour', () => {
+    const f = makeProject();
+    const s = shaderEditor(f, 'Fold');
+    const uv = s.add(SHADER_UV);
+    const centred = s.add('ShaderSubtract', { b: 0.5 });
+    const folded = s.add('ShaderAbs');
+    const tint = s.add('ShaderColor', { value: 0xff8000 });
+    const magnitude = s.add('ShaderAbs');
+    const output = s.add(SHADER_OUTPUT);
+    s.link(uv, 'u', centred, 'a');
+    s.link(centred, 'result', folded, 'value');
+    s.link(folded, 'result', output, 'alpha');
+    s.link(tint, 'color', magnitude, 'value');
+    s.link(magnitude, 'result', output, 'color');
+    const build = compileShader(s.asset().graph);
+    expect(build.problems).toEqual({});
+    const { fragment } = build.compiled!;
+    expect(fragment).toMatch(/float n\d+_result = abs\(n\d+_result\);/);
+    expect(fragment).toMatch(/vec3 n\d+_result = abs\(n\d+_color\);/);
+  });
 });
 
 describe('a sprite wearing a shader', () => {
diff --git a/tools/e2e/28-shader.mjs b/tools/e2e/28-shader.mjs
index 1c52edbf..6145f73c 100644
--- a/tools/e2e/28-shader.mjs
+++ b/tools/e2e/28-shader.mjs
@@ -127,6 +127,9 @@ export async function shader(t) {
   // 28c. A Property node: its name is the row a sprite shows; a bad name is a problem on the node.
   await click(canvas.x + 80, canvas.y + 320, 'right');
   await sleep(300);
+  // Property is the twelfth entry, below the palette's scroll mask: search it into view first.
+  await typeText('Property');
+  await sleep(150);
   await clickLast('Property');
   await sleep(300);
   const propertyNode = (await madeNodes()).find((n) => n.type === 'ShaderProperty');
@@ -503,6 +506,33 @@ export async function shader(t) {
     { rootNear, rootFar },
   );
 
+  // 28k-iii. Abs folds u around 0.5 into a V: |u - 0.5| is below 0.25 only in the middle half.
+  await redOrBlue(
+    [
+      node('uv', 'ShaderUV'),
+      node('centred', 'ShaderSubtract', { b: 0.5 }, 260, 0),
+      node('fold', 'ShaderAbs', {}, 260, 120),
+      node('cut', 'ShaderStep', { edge: 0.25 }, 260, 240),
+    ],
+    [
+      edge('uv', 'u', 'centred', 'a'),
+      edge('centred', 'result', 'fold', 'value'),
+      edge('fold', 'result', 'cut', 'value'),
+      edge('cut', 'result', 'mix', 't'),
+    ],
+  );
+  await sleep(400);
+  const foldLeft = await pixel(-0.4, -0.3);
+  const foldMiddle = await pixel(0, -0.3);
+  const foldRight = await pixel(0.4, -0.3);
+  check(
+    'Abs folds the cut into a V: red in the middle, blue at both edges',
+    colourDistance(foldMiddle, '#ff0000') < 6 &&
+      colourDistance(foldLeft, '#0000ff') < 6 &&
+      colourDistance(foldRight, '#0000ff') < 6,
+    { foldLeft, foldMiddle, foldRight },
+  );
+
   // 28l. Sample texture reads the picture where the graph says. On a pasted two-colour picture:
   // flipped u swaps the halves; u + 0.5 past the border shows the Output's Edge – Repeat continues
   // with the far half, Mirror with the near one.