[x265] [ANN] New Release v4.2

[Kirithika Kalirathnam]

Hello everyone,
We are pleased to announce the release of x265 v4.2. This version includes a new feature, optimizations, and critical bug fixes.
Please note that, starting with this release, we will be adopting a pure Trunk-Based Development (TBD) branching model. All development and release activities will be carried out on the master branch, which will serve as the single source of truth.

Version 4.2
===========

Release date - 19th April, 2026.

New feature
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1. Threaded Motion Estimation (Experimental feature)-It uses a dedicated thread pool to precompute Motion Estimation in parallel. Improves encoding speed up to 1.5x for 1080p & lower resolution on multi core machines with low frequency setting. On high frequency systems or on machines with low number of cores, the overhead of additional Motion estimation work may outweigh parallelism

Enhancements to existing features
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1.Add new Levels 6.3 to 7.2 specified in ITU-T H.265 (V9) (09/2023) and above
2.Improve Slices feature with check zeroMv
3.Enable frame parallelism with MCSTF feature
4.Updated support to signal AOM FGM params
5.Improve quality with SBRC feature
6.Updated DolbyVision P5 VUI defaults

API changes
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1. API Support to enable Threaded Motion Estimation(--threaded-me)

Optimizations
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1. RISC V optimizations including SAD, SATD, DCT, IDCT, block copy, pixel utilities, SAO, loopfilter, transpose kernels resulting in 2x encoding speed.
2. ARM SIMD optimizations including the use of NEON and SVE instruction set extensions. The following algorithms now have optimized SIMD implementations: DST, IDCT, SSE, SSD ,intra_pred_planar, pelFilterLumaStrong, interpolation, planecopy, dequant_normal, blockcopy, pixel variance resulting in 8% faster encoding speed compared to v4.1

Bug fixes
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1. Fix memory leaks (no command line option, SEI buffer, analysis save/load)
2. Fix chroma qp offset for non yuv444 inputs
3. Fix max supported input resolution
4. Fix bugs with ARM SIMD optimizations
5. Fix Alpha and Multiview feature flag support in x265_config
6. Fix test harness issues, CMake errors
7. Fix inconsistent output with aq-motion
8. Fix crash with hist-scenecut on high bit-depth builds
9. Fix lookahead concurrency bug
10. Fix shared link issue (R_X86_64_PC32), yuv recon output issue, rd-refine and dynamic-refine issue, inputs for Windows named pipe, weighted prediction delta_chroma_offset, crf and vbv issue in abr-ladder, psnr and ssim reported with MCSTF feature, internally overflowed VBV variables

Known issues
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1. Output mismatch between analysis save & load with cutree with reuse level < 10
2. Inconsistent hash mismatch with abr-ladder feature
3. Performance regression observed with threaded-me feature on high frequency systems and for higher resolutions (4k)

Thanks,
Kirithika
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