diff --git a/lib/image-pipeline/backend/webgl/shaders/detail-adjust.frag.glsl b/lib/image-pipeline/backend/webgl/shaders/detail-adjust.frag.glsl
index 9e032e7..b17e6dc 100644
--- a/lib/image-pipeline/backend/webgl/shaders/detail-adjust.frag.glsl
+++ b/lib/image-pipeline/backend/webgl/shaders/detail-adjust.frag.glsl
@@ -8,6 +8,7 @@ uniform float uDenoiseLuma;
 uniform float uDenoiseColor;
 uniform float uGrainAmount;
 uniform float uGrainScale;
+uniform float uImageWidth;
 
 float pseudoNoise(float seed) {
   float x = sin(seed * 12.9898) * 43758.5453;
@@ -36,7 +37,10 @@ void main() {
   }
 
   if (uGrainAmount > 0.0) {
-    float grainSeed = (gl_FragCoord.y * 4096.0 + gl_FragCoord.x) / max(0.5, uGrainScale);
+    float pixelX = floor(gl_FragCoord.x);
+    float pixelY = floor(gl_FragCoord.y);
+    float pixelIndex = ((pixelY * max(1.0, uImageWidth)) + pixelX) * 4.0;
+    float grainSeed = (pixelIndex + 1.0) / max(0.5, uGrainScale);
     float grain = (pseudoNoise(grainSeed) - 0.5) * uGrainAmount * 40.0;
     rgb += vec3(grain);
   }
diff --git a/lib/image-pipeline/backend/webgl/webgl-backend.ts b/lib/image-pipeline/backend/webgl/webgl-backend.ts
index 368d24d..02765b5 100644
--- a/lib/image-pipeline/backend/webgl/webgl-backend.ts
+++ b/lib/image-pipeline/backend/webgl/webgl-backend.ts
@@ -274,6 +274,11 @@ function applyStepUniforms(
     if (grainScaleLocation) {
       gl.uniform1f(grainScaleLocation, Math.max(0.5, detail.grain.size));
     }
+
+    const imageWidthLocation = gl.getUniformLocation(shaderProgram, "uImageWidth");
+    if (imageWidthLocation) {
+      gl.uniform1f(imageWidthLocation, request.width);
+    }
   }
 }
 
diff --git a/tests/image-pipeline/parity/fixtures.ts b/tests/image-pipeline/parity/fixtures.ts
index 6d17c03..e305a16 100644
--- a/tests/image-pipeline/parity/fixtures.ts
+++ b/tests/image-pipeline/parity/fixtures.ts
@@ -539,6 +539,7 @@ function runLightAdjustShader(
 
 function runDetailAdjustShader(
   input: Uint8Array,
+  width: number,
   uniforms: Map<string, number | [number, number, number]>,
 ): Uint8Array {
   const output = new Uint8Array(input.length);
@@ -549,6 +550,7 @@ function runDetailAdjustShader(
   const denoiseColor = Number(uniforms.get("uDenoiseColor") ?? 0);
   const grainAmount = Number(uniforms.get("uGrainAmount") ?? 0);
   const grainScale = Math.max(0.5, Number(uniforms.get("uGrainScale") ?? 1));
+  const imageWidth = Math.max(1, Number(uniforms.get("uImageWidth") ?? width));
 
   for (let index = 0; index < input.length; index += 4) {
     let red = input[index] ?? 0;
@@ -580,7 +582,11 @@ function runDetailAdjustShader(
     }
 
     if (grainAmount > 0) {
-      const grain = (pseudoNoise((index + 1) / grainScale) - 0.5) * grainAmount * 40;
+      const pixel = index / 4;
+      const x = pixel % imageWidth;
+      const y = Math.floor(pixel / imageWidth);
+      const pixelIndex = (y * imageWidth + x) * 4;
+      const grain = (pseudoNoise((pixelIndex + 1) / grainScale) - 0.5) * grainAmount * 40;
       red += grain;
       green += grain;
       blue += grain;
@@ -798,7 +804,11 @@ function createParityWebglContext(): WebGLRenderingContext {
       }
 
       if (currentProgram.kind === "detail-adjust") {
-        currentFramebuffer.attachment.data = runDetailAdjustShader(sourceTexture.data, currentProgram.uniforms);
+        currentFramebuffer.attachment.data = runDetailAdjustShader(
+          sourceTexture.data,
+          drawWidth,
+          currentProgram.uniforms,
+        );
         return;
       }
 
diff --git a/tests/image-pipeline/webgl-backend-poc.test.ts b/tests/image-pipeline/webgl-backend-poc.test.ts
index 3a2b8b7..c982f6f 100644
--- a/tests/image-pipeline/webgl-backend-poc.test.ts
+++ b/tests/image-pipeline/webgl-backend-poc.test.ts
@@ -65,6 +65,19 @@ function createUnsupportedStep(): PipelineStep {
   };
 }
 
+function createDetailAdjustStep(): PipelineStep {
+  return {
+    nodeId: "detail-1",
+    type: "detail-adjust",
+    params: {
+      sharpen: { amount: 20, radius: 1.4, detail: 10, masking: 0 },
+      clarity: 15,
+      denoise: { luminance: 12, color: 18, detail: 50 },
+      grain: { amount: 35, size: 2.5, roughness: 50 },
+    },
+  };
+}
+
 describe("webgl backend poc", () => {
   beforeEach(() => {
     vi.resetModules();
@@ -425,6 +438,29 @@ describe("webgl backend poc", () => {
     expect(getContextSpy).toHaveBeenCalledWith("webgl2", expect.any(Object));
   });
 
+  it("passes image width uniform for detail-adjust grain parity", async () => {
+    const fakeGl = createFakeWebglContext({
+      readbackPixels: new Uint8Array([11, 22, 33, 255]),
+    });
+    vi.spyOn(HTMLCanvasElement.prototype, "getContext").mockImplementation((contextId) => {
+      if (contextId === "webgl") {
+        return fakeGl;
+      }
+      return null;
+    });
+
+    const { createWebglPreviewBackend } = await import("@/lib/image-pipeline/backend/webgl/webgl-backend");
+    const backend = createWebglPreviewBackend();
+    backend.runPreviewStep({
+      pixels: new Uint8ClampedArray(7 * 3 * 4),
+      step: createDetailAdjustStep(),
+      width: 7,
+      height: 3,
+    });
+
+    expect(fakeGl.uniform1f).toHaveBeenCalledWith(expect.anything(), 7);
+  });
+
   it("downgrades compile/link failures to cpu with runtime_error reason", async () => {
     const { createBackendRouter } = await import("@/lib/image-pipeline/backend/backend-router");
     const { createWebglPreviewBackend } = await import("@/lib/image-pipeline/backend/webgl/webgl-backend");