From 38e931bc797cfb591171d259805c3e5f8c2eb824 Mon Sep 17 00:00:00 2001
From: Tanner Collin <git@tannercollin.com>
Date: Tue, 17 Feb 2026 11:36:44 -0700
Subject: [PATCH] feat: Implement mouse wheel zoom to cursor position

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>
---
 src/Camera.cpp         | 25 +++++++++++++++++++++++--
 src/Camera.h           |  2 +-
 src/ViewportWidget.cpp |  3 ++-
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/src/Camera.cpp b/src/Camera.cpp
index da25aff..cd733e9 100644
--- a/src/Camera.cpp
+++ b/src/Camera.cpp
@@ -40,11 +40,32 @@ void Camera::mouseMoveEvent(QMouseEvent* event)
     m_lastPos = event->pos();
 }
 
-void Camera::wheelEvent(QWheelEvent* event)
+void Camera::wheelEvent(QWheelEvent* event, const QVector3D& worldPos)
 {
     QPoint numDegrees = event->angleDelta() / 8;
     if (!numDegrees.isNull()) {
-        setZoom(m_zoom + numDegrees.y() / 5.0f);
+        float zoomAmount = numDegrees.y() / 5.0f;
+        float oldZoom = m_zoom;
+        float newZoom = oldZoom + zoomAmount;
+
+        QMatrix4x4 rotation;
+        rotation.rotate(m_xRot / 16.0f, 1, 0, 0);
+        rotation.rotate(m_yRot / 16.0f, 0, 1, 0);
+        QVector3D p_camera = rotation * worldPos;
+
+        if (std::abs(p_camera.z() + oldZoom) < 1e-6) {
+            setZoom(newZoom);
+            return;
+        }
+
+        float ratio = (p_camera.z() + newZoom) / (p_camera.z() + oldZoom);
+
+        float newPanX = (p_camera.x() + m_panX) * ratio - p_camera.x();
+        float newPanY = (p_camera.y() + m_panY) * ratio - p_camera.y();
+
+        setZoom(newZoom);
+        setPanX(newPanX);
+        setPanY(newPanY);
     }
 }
 
diff --git a/src/Camera.h b/src/Camera.h
index 1b35c59..baa2d34 100644
--- a/src/Camera.h
+++ b/src/Camera.h
@@ -21,7 +21,7 @@ public:
 
     void mousePressEvent(QMouseEvent* event);
     void mouseMoveEvent(QMouseEvent* event);
-    void wheelEvent(QWheelEvent* event);
+    void wheelEvent(QWheelEvent* event, const QVector3D& worldPos);
 
     QMatrix4x4 modelViewMatrix() const;
 
diff --git a/src/ViewportWidget.cpp b/src/ViewportWidget.cpp
index 7eb3f90..40d2633 100644
--- a/src/ViewportWidget.cpp
+++ b/src/ViewportWidget.cpp
@@ -239,7 +239,8 @@ void ViewportWidget::mouseMoveEvent(QMouseEvent *event)
 
 void ViewportWidget::wheelEvent(QWheelEvent *event)
 {
-    m_camera->wheelEvent(event);
+    QVector3D worldPos = unproject(event->pos(), m_currentPlane);
+    m_camera->wheelEvent(event, worldPos);
 }
 
 void ViewportWidget::keyPressEvent(QKeyEvent *event)