diff --git a/src/webgl/GeometryBuilder.js b/src/webgl/GeometryBuilder.js
index b3b2d20216..7f34911bc7 100644
--- a/src/webgl/GeometryBuilder.js
+++ b/src/webgl/GeometryBuilder.js
@@ -1,6 +1,7 @@
 import * as constants from '../core/constants';
 import { Matrix } from '../math/p5.Matrix';
 import { Geometry } from './p5.Geometry';
+import { GeometryPart, createPartState } from './p5.GeometryPart';
 
 /**
  * @private
@@ -22,6 +23,13 @@ class GeometryBuilder {
     this.geometry.gid = `_p5_GeometryBuilder_${GeometryBuilder.nextGeometryId}`;
     GeometryBuilder.nextGeometryId++;
     this.hasTransform = false;
+
+    // material parts. when the material state (texture, specular, ambient,
+    // shininess) changes between draws inside the callback, a new part is
+    // opened, so model() renders the result per part like a multi-material obj.
+    // fill stays baked into vertexColors, so a plain fill change never splits.
+    this.parts = [];
+    this.currentPart = null;
   }
 
   /**
@@ -64,11 +72,13 @@ class GeometryBuilder {
       );
     }
 
+    const transformedVertices = this.transformVertices(input.vertices);
+    const transformedNormals = this.transformNormals(input.vertexNormals);
     let startIdx = this.geometry.vertices.length;
-    for (const v of this.transformVertices(input.vertices)) {
+    for (const v of transformedVertices) {
       this.geometry.vertices.push(v);
     }
-    for (const vn of this.transformNormals(input.vertexNormals)) {
+    for (const vn of transformedNormals) {
       this.geometry.vertexNormals.push(vn);
     }
     for (const val of input.uvs) {
@@ -121,6 +131,83 @@ class GeometryBuilder {
     for (const c of vertexColors) {
       this.geometry.vertexColors.push(c);
     }
+
+    this._addToCurrentPart(
+      input,
+      transformedVertices,
+      transformedNormals,
+      vertexColors
+    );
+  }
+
+  /**
+   * @private
+   * snapshot the material state that can't live per vertex (texture, specular,
+   * ambient, shininess), in p5's own state names. fill stays in vertexColors, so
+   * a plain fill() change never opens a new part.
+   */
+  _snapshotPartState() {
+    const s = this.renderer.states;
+    const state = createPartState();
+    if (s._tex) state.texture = s._tex;
+    if (s._useSpecularMaterial) state.specularColor = s.curSpecularColor;
+    if (s._hasSetAmbient) state.ambientColor = s.curAmbientColor;
+    if (s._useShininess !== 1) state.shininess = s._useShininess;
+    return state;
+  }
+
+  _sameColor(a, b) {
+    if (a === b) return true;
+    if (!a || !b) return false;
+    return a.length === b.length && a.every((v, i) => v === b[i]);
+  }
+
+  _sameMaterial(a, b) {
+    return a.texture === b.texture &&
+      a.shininess === b.shininess &&
+      this._sameColor(a.specularColor, b.specularColor) &&
+      this._sameColor(a.ambientColor, b.ambientColor);
+  }
+
+  /**
+   * @private
+   * append one draw to the current part, opening a new part when the material
+   * changed. copied element by element like the combined geometry above, so the
+   * part stays aligned whatever shape the uvs are.
+   */
+  _addToCurrentPart(input, vertices, normals, vertexColors) {
+    // parts are made of fills; a stroke-only draw contributes no faces
+    if (!this.renderer.states.fillColor) return;
+
+    const state = this._snapshotPartState();
+    if (
+      !this.currentPart ||
+      !this._sameMaterial(state, this.currentPart.partState)
+    ) {
+      this.currentPart = new GeometryPart(
+        `${this.geometry.gid}|part${this.parts.length}`,
+        state
+      );
+      this.parts.push(this.currentPart);
+    }
+
+    const part = this.currentPart;
+    const startIdx = part.vertices.length;
+    for (const v of vertices) {
+      part.vertices.push(v);
+    }
+    for (const vn of normals) {
+      part.vertexNormals.push(vn);
+    }
+    for (const val of input.uvs) {
+      part.uvs.push(val);
+    }
+    for (const c of vertexColors) {
+      part.vertexColors.push(c);
+    }
+    for (const f of input.faces) {
+      part.faces.push(f.map(idx => idx + startIdx));
+    }
   }
 
   /**
@@ -179,6 +266,15 @@ class GeometryBuilder {
    */
   finish() {
     this.renderer._pInst.pop();
+    // expose the material parts only when there really are multiple materials,
+    // and not while custom per-vertex attributes are in play (those aren't
+    // split per part yet). single-material builds keep the geometry as its own
+    // part, so nothing changes for them (zero regression).
+    const hasUserProps =
+      Object.keys(this.geometry.userVertexProperties).length > 0;
+    if (this.parts.length >= 2 && !hasUserProps) {
+      this.geometry.parts = this.parts;
+    }
     return this.geometry;
   }
 }
diff --git a/test/unit/visual/cases/webgl.js b/test/unit/visual/cases/webgl.js
index ed2af5fdc6..a2daf5eeea 100644
--- a/test/unit/visual/cases/webgl.js
+++ b/test/unit/visual/cases/webgl.js
@@ -932,6 +932,59 @@ visualSuite('WebGL', function () {
       p5.model(geom);
       screenshot();
     });
+
+    visualTest(
+      'a texture change splits into parts that each keep their texture',
+      async function (p5, screenshot) {
+        p5.createCanvas(50, 50, p5.WEBGL);
+        const texA = await p5.loadImage('test/unit/assets/cat.jpg');
+        const texB = await p5.loadImage('test/unit/assets/spheremap.jpg');
+        const geom = p5.buildGeometry(() => {
+          p5.push();
+          p5.translate(-12, 0, 0);
+          p5.texture(texA);
+          p5.box(12);
+          p5.pop();
+          p5.push();
+          p5.translate(12, 0, 0);
+          p5.texture(texB);
+          p5.box(12);
+          p5.pop();
+        });
+        p5.background(255);
+        p5.rotateX(0.4);
+        p5.rotateY(0.4);
+        p5.noStroke();
+        p5.model(geom);
+        screenshot();
+      }
+    );
+
+    visualTest(
+      'a textured part and an untextured part both render',
+      async function (p5, screenshot) {
+        p5.createCanvas(50, 50, p5.WEBGL);
+        const tex = await p5.loadImage('test/unit/assets/cat.jpg');
+        const geom = p5.buildGeometry(() => {
+          p5.push();
+          p5.translate(-12, 0, 0);
+          p5.texture(tex);
+          p5.box(12);
+          p5.pop();
+          p5.push();
+          p5.translate(12, 0, 0);
+          p5.fill('red');
+          p5.box(12);
+          p5.pop();
+        });
+        p5.background(255);
+        p5.rotateX(0.4);
+        p5.rotateY(0.4);
+        p5.noStroke();
+        p5.model(geom);
+        screenshot();
+      }
+    );
   });
 
   visualSuite('font data', () => {
diff --git a/test/unit/visual/screenshots/WebGL/buildGeometry()/a texture change splits into parts that each keep their texture/000.png b/test/unit/visual/screenshots/WebGL/buildGeometry()/a texture change splits into parts that each keep their texture/000.png
new file mode 100644
index 0000000000..4d7b4e4575
Binary files /dev/null and b/test/unit/visual/screenshots/WebGL/buildGeometry()/a texture change splits into parts that each keep their texture/000.png differ
diff --git a/test/unit/visual/screenshots/WebGL/buildGeometry()/a texture change splits into parts that each keep their texture/metadata.json b/test/unit/visual/screenshots/WebGL/buildGeometry()/a texture change splits into parts that each keep their texture/metadata.json
new file mode 100644
index 0000000000..2d4bfe30da
--- /dev/null
+++ b/test/unit/visual/screenshots/WebGL/buildGeometry()/a texture change splits into parts that each keep their texture/metadata.json	
@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}
\ No newline at end of file
diff --git a/test/unit/visual/screenshots/WebGL/buildGeometry()/a textured part and an untextured part both render/000.png b/test/unit/visual/screenshots/WebGL/buildGeometry()/a textured part and an untextured part both render/000.png
new file mode 100644
index 0000000000..d6dc63e496
Binary files /dev/null and b/test/unit/visual/screenshots/WebGL/buildGeometry()/a textured part and an untextured part both render/000.png differ
diff --git a/test/unit/visual/screenshots/WebGL/buildGeometry()/a textured part and an untextured part both render/metadata.json b/test/unit/visual/screenshots/WebGL/buildGeometry()/a textured part and an untextured part both render/metadata.json
new file mode 100644
index 0000000000..2d4bfe30da
--- /dev/null
+++ b/test/unit/visual/screenshots/WebGL/buildGeometry()/a textured part and an untextured part both render/metadata.json	
@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}
\ No newline at end of file
diff --git a/test/unit/webgl/p5.Geometry.js b/test/unit/webgl/p5.Geometry.js
index 44b492b859..85631d8f91 100644
--- a/test/unit/webgl/p5.Geometry.js
+++ b/test/unit/webgl/p5.Geometry.js
@@ -336,5 +336,82 @@ suite('p5.Geometry', function () {
       expect(geom.vertices.length).toBeGreaterThan(0);
       expect(geom.faces.length).toEqual(0);
     });
+
+    test('a texture change splits the build into parts', function() {
+      myp5.createCanvas(50, 50, myp5.WEBGL);
+      const texA = myp5.createGraphics(10, 10);
+      const texB = myp5.createGraphics(10, 10);
+      const geom = myp5.buildGeometry(() => {
+        myp5.texture(texA);
+        myp5.box(8);
+        myp5.texture(texB);
+        myp5.sphere(8);
+      });
+      // one part per material, in draw order
+      expect(geom.parts.length).toEqual(2);
+      expect(geom.parts[0].partState.texture).not.toBeNull();
+      expect(geom.parts[1].partState.texture).not.toBeNull();
+      expect(geom.parts[0].partState.texture)
+        .not.toEqual(geom.parts[1].partState.texture);
+    });
+
+    test('a fill change alone does not split the build', function() {
+      myp5.createCanvas(50, 50, myp5.WEBGL);
+      const geom = myp5.buildGeometry(() => {
+        myp5.fill('red');
+        myp5.box(8);
+        myp5.fill('blue');
+        myp5.sphere(8);
+      });
+      // fill bakes into vertexColors, so the geometry stays a single part
+      expect(geom.parts.length).toEqual(1);
+      expect(geom.vertexColors.length).toBeGreaterThan(0);
+    });
+
+    test('per-part materials render the same as drawing them directly',
+      function() {
+        assertGeometryRendersMatch(function() {
+          myp5.push();
+          myp5.translate(-10, 0);
+          myp5.specularMaterial(255, 0, 0);
+          myp5.shininess(50);
+          myp5.box(8);
+          myp5.pop();
+          myp5.push();
+          myp5.translate(10, 0);
+          myp5.specularMaterial(0, 0, 255);
+          myp5.shininess(200);
+          myp5.box(8);
+          myp5.pop();
+        }, [checkLights]);
+      }
+    );
+
+    test('instancing draws every material part with the instance count',
+      function() {
+        const renderer = myp5.createCanvas(50, 50, myp5.WEBGL);
+        const texA = myp5.createGraphics(10, 10);
+        const texB = myp5.createGraphics(10, 10);
+        const geom = myp5.buildGeometry(() => {
+          myp5.texture(texA);
+          myp5.box(8);
+          myp5.texture(texB);
+          myp5.sphere(8);
+        });
+        expect(geom.parts.length).toEqual(2);
+
+        const fillSpy = vi.spyOn(renderer, '_drawFills');
+        myp5.background(255);
+        myp5.fill(255);
+        myp5.model(geom, 4);
+
+        // one instanced draw per part, each carrying the same instance count
+        expect(fillSpy).toHaveBeenCalledTimes(geom.parts.length);
+        for (const call of fillSpy.mock.calls) {
+          expect(call[1].count).toEqual(4);
+        }
+        fillSpy.mockRestore();
+      }
+    );
   });
 });