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2026-06-03 09:00:11 +08:00
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+## ADDED Requirements
+
+### Requirement: Camera selector dropdown in viewport
+
+The viewport panel SHALL display a camera selector dropdown above the rendering canvas. The dropdown SHALL list "Free Camera" as the first entry, followed by all UsdCamera-typed prims found on the current USD stage.
+
+#### Scenario: Dropdown shows free camera and stage cameras
+- **WHEN** a USD stage is loaded and contains camera prims at paths `/cameras/main` and `/cameras/top`
+- **THEN** the camera selector dropdown lists: "Free Camera", "/cameras/main", "/cameras/top"
+
+#### Scenario: Dropdown shows only free camera with no stage cameras
+- **WHEN** a USD stage is loaded and contains no camera prims
+- **THEN** the camera selector dropdown lists only "Free Camera"
+
+#### Scenario: No stage loaded
+- **WHEN** no USD stage is loaded
+- **THEN** the camera selector dropdown is disabled or shows only "Free Camera"
+
+### Requirement: Switch to USD camera prim view
+
+When the user selects a USD camera prim from the dropdown, the viewport SHALL render the scene from that camera's viewpoint by reading its transform and projection attributes.
+
+#### Scenario: Select a USD camera from dropdown
+- **WHEN** the user selects "/cameras/main" from the camera dropdown
+- **THEN** the viewport renders the scene using the view matrix derived from that camera prim's world transform and the projection matrix derived from its focalLength, horizontalAperture, and clippingRange attributes
+
+#### Scenario: USD camera attribute mapping
+- **WHEN** the viewport is rendering through a UsdCamera prim
+- **THEN** the projection matrix SHALL be computed from the camera's `focalLength`, `horizontalAperture`, `verticalAperture`, `clippingRange`, and `projection` attributes
+- **AND** the view matrix SHALL be computed from the camera prim's world-space transform
+
+### Requirement: Free camera navigation disabled in USD camera mode
+
+When the viewport is in USD camera mode, orbit, pan, dolly, and zoom controls SHALL be disabled.
+
+#### Scenario: Attempt orbit while in USD camera mode
+- **WHEN** the viewport is viewing through a USD camera and the user performs Alt+LMB drag
+- **THEN** no orbit occurs and the viewport remains at the USD camera's viewpoint
+
+### Requirement: Free camera state preserved across mode switches
+
+The free camera's position, focal point, and orientation SHALL be preserved when switching to a USD camera and restored when switching back.
+
+#### Scenario: Switch to USD camera and back
+- **WHEN** the user positions the free camera, switches to a USD camera, then switches back to "Free Camera"
+- **THEN** the viewport returns to the exact free camera position and orientation before the switch
+
+### Requirement: Camera list refreshed on stage change
+
+The list of available USD camera prims SHALL be refreshed when the stage changes (file open, close, or stage reload).
+
+#### Scenario: Open a new stage with different cameras
+- **WHEN** the user opens a new USD file containing cameras at different paths
+- **THEN** the camera selector dropdown updates to reflect the new stage's camera prims
+
+### Requirement: Default camera mode is free camera
+
+The viewport SHALL start in free camera mode every time a stage is loaded.
+
+#### Scenario: Stage loaded defaults to free camera
+- **WHEN** a USD stage is loaded
+- **THEN** the camera selector is set to "Free Camera" and the viewport renders from the built-in free camera