Fix camera tilt function to prevent orientation flipping

src/webgl/p5.Camera.js

@@ -2435,6 +2435,7 @@ p5.Camera = class Camera {
     const rotation = p5.Matrix.identity(this._renderer._pInst);
     rotation.rotate(this._renderer._pInst._toRadians(a), x, y, z);
 
+    // Apply the rotation matrix to the center vector
     /* eslint-disable max-len */
     const rotatedCenter = [
       centerX * rotation.mat4[0] + centerY * rotation.mat4[4] + centerZ * rotation.mat4[8],
@@ -2443,21 +2444,28 @@ p5.Camera = class Camera {
     ];
     /* eslint-enable max-len */
 
-    // add eye position back into center
+    // Translate the rotated center back to world coordinates
     rotatedCenter[0] += this.eyeX;
     rotatedCenter[1] += this.eyeY;
     rotatedCenter[2] += this.eyeZ;
 
+    // Rotate the up vector to keep the correct camera orientation
+    /* eslint-disable max-len */
+    const upX = this.upX * rotation.mat4[0] + this.upY * rotation.mat4[4] + this.upZ * rotation.mat4[8];
+    const upY = this.upX * rotation.mat4[1] + this.upY * rotation.mat4[5] + this.upZ * rotation.mat4[9];
+    const upZ = this.upX * rotation.mat4[2] + this.upY * rotation.mat4[6] + this.upZ * rotation.mat4[10];
+    /* eslint-enable max-len */
+
     this.camera(
       this.eyeX,
       this.eyeY,
       this.eyeZ,
       rotatedCenter[0],
       rotatedCenter[1],
       rotatedCenter[2],
-      this.upX,
-      this.upY,
-      this.upZ
+      upX,
+      upY,
+      upZ
     );
   }
 

test/unit/webgl/p5.Camera.js

@@ -277,6 +277,61 @@ suite('p5.Camera', function() {
       assert.strictEqual(myCam.upY, orig.uy, 'up Y pos changed');
       assert.strictEqual(myCam.upZ, orig.uz, 'up Z pos changed');
     });
+
+    suite('Camera Tilt and Up Vector', function() {
+      test('Tilt() correctly updates the up vector', function() {
+        var orig = getVals(myCam); // Store original camera values
+
+        // Apply tilt to the camera
+        myCam.tilt(30); // Tilt by 30 degrees
+
+        // Compute expected up vector (normalized)
+        let forward = myp5.createVector(
+          myCam.centerX - myCam.eyeX,
+          myCam.centerY - myCam.eyeY,
+          myCam.centerZ - myCam.eyeZ
+        );
+        let up = myp5.createVector(orig.ux, orig.uy, orig.uz);
+        let right = p5.Vector.cross(forward, up);
+        let expectedUp = p5.Vector.cross(right, forward).normalize();
+
+        // Verify that the up vector has changed
+        assert.notStrictEqual(myCam.upX, orig.ux, 'upX should be updated');
+        assert.notStrictEqual(myCam.upY, orig.uy, 'upY should be updated');
+        assert.notStrictEqual(myCam.upZ, orig.uz, 'upZ should be updated');
+
+        // Verify up vector matches expected values within a small margin of error
+        assert.closeTo(myCam.upX, expectedUp.x, 0.001, 'upX mismatch');
+        assert.closeTo(myCam.upY, expectedUp.y, 0.001, 'upY mismatch');
+        assert.closeTo(myCam.upZ, expectedUp.z, 0.001, 'upZ mismatch');
+      });
+
+      test('Tilt() with negative angle correctly updates the up vector', function() {
+        var orig = getVals(myCam); // Store original camera values
+
+        myCam.tilt(-30); // Tilt by -30 degrees
+
+        // Compute expected up vector (normalized)
+        let forward = myp5.createVector(
+          myCam.centerX - myCam.eyeX,
+          myCam.centerY - myCam.eyeY,
+          myCam.centerZ - myCam.eyeZ
+        );
+        let up = myp5.createVector(orig.ux, orig.uy, orig.uz);
+        let right = p5.Vector.cross(forward, up);
+        let expectedUp = p5.Vector.cross(right, forward).normalize();
+
+        // Verify that the up vector has changed
+        assert.notStrictEqual(myCam.upX, orig.ux, 'upX should be updated');
+        assert.notStrictEqual(myCam.upY, orig.uy, 'upY should be updated');
+        assert.notStrictEqual(myCam.upZ, orig.uz, 'upZ should be updated');
+
+        // Verify up vector matches expected values within a small margin of error
+        assert.closeTo(myCam.upX, expectedUp.x, 0.001, 'upX mismatch');
+        assert.closeTo(myCam.upY, expectedUp.y, 0.001, 'upY mismatch');
+        assert.closeTo(myCam.upZ, expectedUp.z, 0.001, 'upZ mismatch');
+      });
+    });
   });
 
   suite('Rotation with angleMode(DEGREES)', function() {