Fix doubled stereo image on UE 5.5/5.6 (zero-based eye index misclassified as full pass)

[adambenovic]

Problem

On UE 5.5/5.6 games, everything rendered near the camera (hands, HUD, world-space UI) appears heavily doubled with a cross-eyed effect, while distant geometry looks mostly fine. Related: #329 (UE 5.5.3 games not entering VR correctly), #368 (UE 5.6 compatibility).

Root cause

FFakeStereoRenderingHook::calculate_stereo_view_offset classifies a call as a mono "full pass" via:

const auto is_full_pass = view_index == 0 && !index_was_ever_two && !index_was_ever_negative;

UE <= 5.4 called CalculateStereoViewOffset with view index -1 (INDEX_NONE) or 2 early on, flipping those flags so index 0 was subsequently treated as a real eye. UE 5.5/5.6 only ever sends indices 0/1, so view 0 stays permanently misclassified as a full pass. That skips the per-eye mod callbacks (on_pre/post_calculate_stereo_view_offset) and update_hmd_state() for one eye only, while the other eye receives the full treatment.

Measured on Satisfactory (CSS custom UE 5.6.1, 1.2 experimental branch) with first-N-call instrumentation: the two eye view locations ended up ~33.6 cm apart (including a 15.7 cm vertical offset) instead of IPD (~6.5 cm), and the offset varied with head position. Near-field content at such disparity is unfusable - hence the double image.

Fix

On UE5 (already detected via m_has_double_precision / LWC double-precision math), index 0 in a stereo pass is always a real eye - a genuine mono pass arrives as INDEX_NONE (-1), which the function already filters earlier. One-line change gating the full-pass classification to non-UE5 binaries.

Testing

• Satisfactory 1.2 experimental (CSS UE 5.6.1, D3D12, OpenXR via Virtual Desktop): double image fully resolved, eye separation verified at IPD via instrumentation, extended play session stable with the UEVR Enhancements gameplay mod.
• The remaining UE 5.6 hook warnings (Slate.DrawToVRRenderTarget cvar removed upstream, etc.) are unrelated to this fix and the slate path still works via the engine''s RDG deprecation bridge, which routes to the legacy RenderTexture_RenderThread vtable entry that UEVR already hooks.

🤖 Generated with Claude Code

[joeyhodge]

Approval from me, Good work!

[joeyhodge]

I have been testing on many different uevr/ue versions and have noticed no regressions myself with this included