From 195a812ba216ba31718d2eec0ab3cca9b52f6433 Mon Sep 17 00:00:00 2001
From: Matthias <matthias@example.com>
Date: Sat, 4 Apr 2026 22:09:14 +0200
Subject: [PATCH] fix(image-pipeline): correct webgl source binding and context
 init

---
 .../backend/webgl/webgl-backend.ts            | 11 ++-
 .../image-pipeline/webgl-backend-poc.test.ts  | 78 +++++++++++++++++++
 2 files changed, 86 insertions(+), 3 deletions(-)

diff --git a/lib/image-pipeline/backend/webgl/webgl-backend.ts b/lib/image-pipeline/backend/webgl/webgl-backend.ts
index 23d4683..929503d 100644
--- a/lib/image-pipeline/backend/webgl/webgl-backend.ts
+++ b/lib/image-pipeline/backend/webgl/webgl-backend.ts
@@ -42,12 +42,14 @@ function assertSupportedStep(step: PipelineStep): void {
 function createGlContext(): WebGLRenderingContext {
   if (typeof document !== "undefined") {
     const canvas = document.createElement("canvas");
-    const context = canvas.getContext("webgl", {
+    const contextOptions: WebGLContextAttributes = {
       alpha: true,
       antialias: false,
       premultipliedAlpha: false,
       preserveDrawingBuffer: true,
-    });
+    };
+    const context =
+      canvas.getContext("webgl2", contextOptions) ?? canvas.getContext("webgl", contextOptions);
     if (context) {
       return context;
     }
@@ -55,7 +57,7 @@ function createGlContext(): WebGLRenderingContext {
 
   if (typeof OffscreenCanvas !== "undefined") {
     const canvas = new OffscreenCanvas(1, 1);
-    const context = canvas.getContext("webgl");
+    const context = canvas.getContext("webgl2") ?? canvas.getContext("webgl");
     if (context) {
       return context;
     }
@@ -229,6 +231,9 @@ function runStepOnGpu(context: WebglBackendContext, request: BackendStepRequest)
     throw new Error("WebGL framebuffer is incomplete.");
   }
 
+  gl.activeTexture(gl.TEXTURE0);
+  gl.bindTexture(gl.TEXTURE_2D, sourceTexture);
+
   const sourceLocation = gl.getUniformLocation(shaderProgram, "uSource");
   if (sourceLocation) {
     gl.uniform1i(sourceLocation, 0);
diff --git a/tests/image-pipeline/webgl-backend-poc.test.ts b/tests/image-pipeline/webgl-backend-poc.test.ts
index 0eef116..77b9689 100644
--- a/tests/image-pipeline/webgl-backend-poc.test.ts
+++ b/tests/image-pipeline/webgl-backend-poc.test.ts
@@ -339,6 +339,84 @@ describe("webgl backend poc", () => {
     expect(fakeGl.readPixels).toHaveBeenCalledTimes(1);
   });
 
+  it("keeps source texture bound on sampler unit when drawing", async () => {
+    const fakeGl = createFakeWebglContext({
+      readbackPixels: new Uint8Array([1, 2, 3, 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([9, 9, 9, 255]),
+      step: createCurvesStep(),
+      width: 1,
+      height: 1,
+    });
+
+    const sourceTexture = (fakeGl.createTexture as any).mock.results[0]?.value;
+    const outputTexture = (fakeGl.createTexture as any).mock.results[1]?.value;
+    expect(sourceTexture).toBeTruthy();
+    expect(outputTexture).toBeTruthy();
+
+    expect(fakeGl.framebufferTexture2D).toHaveBeenCalledWith(
+      fakeGl.FRAMEBUFFER,
+      fakeGl.COLOR_ATTACHMENT0,
+      fakeGl.TEXTURE_2D,
+      outputTexture,
+      0,
+    );
+
+    const bindTextureCalls = (fakeGl.bindTexture as any).mock.calls as Array<[number, unknown]>;
+    const bindTextureOrder = (fakeGl.bindTexture as any).mock.invocationCallOrder as number[];
+    const drawOrder = (fakeGl.drawArrays as any).mock.invocationCallOrder[0] as number;
+    const lastBindBeforeDrawIndex = bindTextureOrder
+      .map((callOrder, index) => ({ callOrder, index }))
+      .filter(({ callOrder, index }) => callOrder < drawOrder && bindTextureCalls[index]?.[0] === fakeGl.TEXTURE_2D)
+      .at(-1)?.index;
+
+    expect(lastBindBeforeDrawIndex).toBeTypeOf("number");
+    expect(bindTextureCalls[lastBindBeforeDrawIndex as number]?.[1]).toBe(sourceTexture);
+    expect(bindTextureCalls[lastBindBeforeDrawIndex as number]?.[1]).not.toBe(outputTexture);
+  });
+
+  it("initializes backend with webgl2-only context availability", async () => {
+    const fakeGl = createFakeWebglContext({
+      readbackPixels: new Uint8Array([11, 22, 33, 255]),
+    });
+    const getContextSpy = vi
+      .spyOn(HTMLCanvasElement.prototype, "getContext")
+      .mockImplementation((contextId) => {
+        if (contextId === "webgl2") {
+          return fakeGl;
+        }
+        if (contextId === "webgl") {
+          return null;
+        }
+        return null;
+      });
+
+    const { createWebglPreviewBackend } = await import("@/lib/image-pipeline/backend/webgl/webgl-backend");
+    const backend = createWebglPreviewBackend();
+    const pixels = new Uint8ClampedArray([200, 100, 50, 255]);
+
+    backend.runPreviewStep({
+      pixels,
+      step: createCurvesStep(),
+      width: 1,
+      height: 1,
+    });
+
+    expect(Array.from(pixels)).toEqual([11, 22, 33, 255]);
+    expect(fakeGl.drawArrays).toHaveBeenCalledTimes(1);
+    expect(getContextSpy).toHaveBeenCalledWith("webgl2", expect.any(Object));
+  });
+
   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");