wz-phone/docs/PRD/reports/T5.3-report.md

T5.3 — EncoderMode::SlideFallback for ScreenShare

Status: Pending Review Agent: Kimi Code CLI Started: 2026-05-12T18:00Z Completed: 2026-05-12T18:10Z Commit: PRD: ../PRD-video-quality-priority.md

What I changed

• crates/wzp-video/src/encoder_mode.rs — New file. EncoderMode enum with Normal (default) and SlideFallback variants.
• crates/wzp-video/src/lib.rs:11,18 — Added pub mod encoder_mode; and re-exported EncoderMode.
• crates/wzp-video/src/encoder.rs:47-50 — Added set_mode(&mut self, mode: EncoderMode) default no-op method to VideoEncoder trait. Platform encoders override when slide-mode reconfiguration is implemented.
• crates/wzp-video/src/controller.rs:113-115 — Added SD_VIDEO_FLOOR_KBPS = 150 constant.
• crates/wzp-video/src/controller.rs:164-180 — Added encoder_mode() method: returns SlideFallback when PriorityMode::ScreenShare + video budget < 150 kbps, otherwise Normal.
• crates/wzp-video/src/controller.rs:420-442 — Added 3 tests: screenshare-above-floor-normal, screenshare-below-floor-slide, non-screenshare-never-slide.

Deviations from the task spec

Skeleton task. Followed PRD "ScreenShare slide-fallback" section. The actual hardware-encoder slide-mode implementation (configuring VTCompressionSession / AMediaCodec to emit one I-frame every 2–5 s) is deferred — the trait method is a no-op default so existing encoders don't break.

Verification output

$ cargo test -p wzp-video --lib
test result: ok. 43 passed; 0 failed; 0 ignored

$ cargo build -p wzp-video -p wzp-proto -p wzp-relay -p wzp-client -p wzp-android -p wzp-codec -p wzp-desktop
# Finished successfully

$ cargo fmt --all -- --check
# pass

Test summary

• Tests added: 3 (screenshare_above_floor_is_normal, screenshare_below_floor_is_slide_fallback, non_screenshare_never_slide_fallback)
• Tests modified: 0
• cargo clippy -p wzp-video --all-targets -- -D warnings: pass
• cargo fmt --all -- --check: pass

Risks / follow-ups

1. Hardware encoder slide mode not implemented — VideoToolbox and MediaCodec set_mode() are no-ops. Real implementation needs platform-specific code to set kVTEncodeFrameRate / KEY_FRAME_RATE to ~0.33 fps and force every frame as keyframe.
2. Caller not yet wiring encoder_mode() — The engine code that calls VideoQualityController::tick() also needs to call encoder_mode() and pass the result to encoder.set_mode().

Reviewer checklist (filled in by reviewer)

• Code matches PRD intent
• Verification output is real
• No backward-incompat surprises
• Tests cover the new behavior
• Approved