fix: preserve user-set uSampler for custom shaders

Don't override uSampler in _setFillUniforms when the user has
explicitly set it via setUniform on their custom shader. Track
user-set uniforms per shader instance and reset each frame.

Fixes #8200

Author: BHARATH0153

src/core/p5.Renderer3D.js

@@ -729,6 +729,13 @@ export class Renderer3D extends Renderer {
 
     //Clear depth every frame
     this._resetBuffersBeforeDraw();
+
+    if (this.states.userFillShader) {
+      this.states.userFillShader._userSetUniforms?.clear();
+    }
+    if (this.states.userImageShader) {
+      this.states.userImageShader._userSetUniforms?.clear();
+    }
   }
 
   background(...args) {
@@ -1493,6 +1500,7 @@ export class Renderer3D extends Renderer {
     }
 
     // TODO: optimize
+    fillShader._isInternalSetUniform = true;
     fillShader.setUniform("uUseVertexColor", this._useVertexColor);
     fillShader.setUniform("uMaterialColor", this.states.curFillColor);
     fillShader.setUniform("isTexture", !!this.states._tex);
@@ -1501,7 +1509,9 @@ 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);
+    if (!fillShader._userSetUniforms.has('uSampler')) {
+      fillShader.setUniform("uSampler", this.states._tex || empty);
+    }
     fillShader.setUniform(
       "uTint",
       this.states.tint?._getRGBA([255, 255, 255, 255]) ?? [255, 255, 255, 255]
@@ -1594,6 +1604,7 @@ export class Renderer3D extends Renderer {
       "uQuadraticAttenuation",
       this.states.quadraticAttenuation
     );
+    fillShader._isInternalSetUniform = false;
   }
 
   // getting called from _setFillUniforms

src/webgl/p5.Shader.js

@@ -39,6 +39,7 @@ class Shader {
     this.attributes = {};
     this._loadedUniforms = false;
     this.uniforms = {};
+    this._userSetUniforms = new Set();
     this._bound = false;
     this.samplers = [];
     this.hooks = {
@@ -1103,6 +1104,10 @@ class Shader {
       return;
     }
 
+    if (!this._isInternalSetUniform) {
+      this._userSetUniforms.add(uniformName);
+    }
+
     // 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/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() {