UsdLayerManager/openspec/changes/camera-wireframe-viewport/proposal.md

Why

Camera prims exist in the USD scene but are invisible in the viewport — there is no visual indication of a camera's position, orientation, or field of view. Users cannot tell where cameras are in the scene, cannot click to select them, and cannot use the transform manipulator to reposition them. This is a critical gap for any scene with multiple cameras (rendering, animation, layout).

What Changes

• Draw a 3D frustum wireframe for every UsdGeomCamera prim in the viewport, rendered into the FBO using the existing GL overlay infrastructure (BindDrawTarget/UnbindDrawTarget).
• Highlight the active camera (the one currently driving the viewport via the camera toolbar) with a distinct colour; all other cameras use the standard camera wire colour.
• Highlight the selected camera with the selection accent colour.
• Support viewport click-selection of camera prims by screen-space hit-testing mouse clicks against projected frustum segments.
• Camera prims, once selected, can be translated/rotated/scaled through the existing TransformManipulator (they are Xformable; no manipulator changes are required).

Capabilities

New Capabilities

• camera-frustum-overlay: Per-frame GL wireframe rendering of all UsdGeomCamera prims' frustums into the viewport FBO, with colour-coding for active vs. selected vs. inactive cameras.
• camera-viewport-picking: Screen-space hit-testing that maps a viewport mouse click to a camera prim when the click is within a threshold distance of any projected frustum edge; integrates with the existing prim-selection callbacks.

Modified Capabilities

(none — no existing spec-level requirements are changing)

Impact

• Modified: src/core/UsdSceneRenderer.h/.cpp — add DrawCameraWireframes(stage, selectedPaths, activeCameraPath, viewProjMatrix) and PickCameraAtPoint(stage, mouseScreenPos, viewProjMatrix, imagePos, viewW, viewH, outPath).
• Modified: src/ui/ViewportPanel.cpp — call DrawCameraWireframes each frame after DrawBoundingBoxes; call PickCameraAtPoint inside the single-click pick path before the existing PickObject geometry pick so camera hits take priority.
• No new external dependencies. Uses existing GLSL shader program already compiled for DrawAxis/DrawBoundingBoxes.