fix: preserve user-set uSampler uniform for custom shaders

src/core/p5.Renderer3D.js

@@ -1501,7 +1501,11 @@ export class Renderer3D extends Renderer {
     // the next time a shader is used. However, the texture() function
     // works differently and is global p5 state. If the p5 state has
     // been cleared, we also need to clear the value in uSampler to match.
-    fillShader.setUniform("uSampler", this.states._tex || empty);
+    this._settingFillUniforms = true;
+    if (this.states._tex || !fillShader._userSetSampler) {
+      fillShader.setUniform("uSampler", this.states._tex || empty);
+    }
+    this._settingFillUniforms = false;
     fillShader.setUniform(
       "uTint",
       this.states.tint?._getRGBA([255, 255, 255, 255]) ?? [255, 255, 255, 255]

src/webgl/p5.Shader.js

@@ -1103,6 +1103,10 @@ class Shader {
       return;
     }
 
+    if (uniformName === 'uSampler' && !this._renderer._settingFillUniforms) {
+      this._userSetSampler = true;
+    }
+
     // In p5.strands-related code, where some of the code may be in
     // p5.webgpu.js instead of the main p5.js build, we generally use
     // duck typing instead of instanceof to avoid accidentally importing

test/unit/visual/cases/webgl.js

@@ -1754,4 +1754,28 @@ visualTest('randomGaussian() in a fragment loop averages to the mean', (p5, scre
       screenshot();
     });
   });
+
+  visualTest(
+    'user-set uSampler on custom shader is not overridden',
+    function(p5, screenshot) {
+      p5.createCanvas(50, 50, p5.WEBGL);
+
+      const myShader = p5.createFilterShader(`precision highp float;
+uniform sampler2D uSampler;
+varying vec2 vTexCoord;
+void main() {
+  gl_FragColor = texture2D(uSampler, vTexCoord);
+}`);
+
+      const fbo = p5.createFramebuffer();
+      fbo.draw(() => p5.background(255, 0, 0));
+
+      p5.shader(myShader);
+      p5.noStroke();
+      myShader.setUniform('uSampler', fbo);
+      p5.plane(p5.width, p5.height);
+
+      screenshot();
+    }
+  );
 });

test/unit/visual/screenshots/WebGL/user-set uSampler on custom shader is not overridden/metadata.json

@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}

test/unit/webgl/p5.RendererGL.js

@@ -208,6 +208,29 @@ suite('p5.RendererGL', function() {
       expect(uSampler.texture.isFramebufferTexture).toBeFalsy();
       myp5.pop();
     });
+
+    test('user-set uSampler on custom shader is not overridden', function() {
+      myp5.createCanvas(10, 10, myp5.WEBGL);
+
+      const myShader = myp5.createFilterShader(`precision highp float;
+uniform sampler2D uSampler;
+varying vec2 vTexCoord;
+void main() {
+  gl_FragColor = texture2D(uSampler, vTexCoord);
+}`);
+
+      const fbo = myp5.createFramebuffer();
+      fbo.draw(() => myp5.background('red'));
+
+      myp5.shader(myShader);
+      myp5.noStroke();
+      myShader.setUniform('uSampler', fbo);
+
+      myp5.plane(myp5.width, myp5.height);
+
+      const pixel = myp5.get(5, 5);
+      assert.deepEqual(pixel, [255, 0, 0, 255]);
+    });
   });
 
   suite('default stroke shader', function() {