FFmpeg filter composition: combine crop, scale and subtitles in one -vf chain; repeated options keep only the last filter

# Compose one FFmpeg video filter chain When separate wrapper stages each append `-vf` for the same output stream, the options do not compose: the last matching option wins. Build one ordered chain instead. FFmpeg's `-vf` is an alias for `-filter:v`; comma-separated filters form a connected chain. Multiple inputs, branching and several outputs need a deliberate complex graph. For a measured 1920×1080 source, the intended order is: ```text crop=1080:1080:(iw-ow)/2:(ih-oh)/2,scale=720:720,subtitles=filename=captions.ass ``` Cropping happens before resizing; captions are laid out against the resized output. Inspect the ASS script's resolution, fonts and styling before rendering. The [standard-library command builder](https://github.com/the-media-vampire/veyrul-integrations/blob/0dc5ead8bb5ae92611f402b2343da54e88be3f59/examples/ffmpeg-filter-chain/compose_filters.py) emits a JSON argument list and **does not execute FFmpeg**: ```sh python3 compose_filters.py input.mov output-new.mp4 --source-width 1920 --source-height 1080 --crop-width 1080 --crop-height 1080 --width 720 --height 720 python3 -B test_compose.py ``` The example selects the first video and optional first audio stream, encodes H.264/AAC, and intentionally omits source metadata and chapters. Choose and verify a different mapping or metadata policy when the task requires it. Do not use this default for spatial audio, all-track preservation, HDR mastering, interlaced content or non-square-pixel inputs. No codec, subtitle renderer or font availability is promised. Pass the resulting list to a subprocess without a shell only after inspection and authorization. Shell avoidance does not eliminate FFmpeg filtergraph escaping. This small example therefore accepts only a simple `.ass` basename; stage an authorized copy under that name in the command's working directory. Arbitrary subtitle paths and filter expressions are refused. Input and output paths are resolved separately; `-n` refuses existing outputs. Read the actual source dimensions first. ## Evidence and limits Three deterministic test groups exercise exact order, changed dimensions, path-as-one-argument behavior, overwrite refusal, impossible crops and filter-syntax rejection. The builder was run; no production video or visual comparison was rendered. The first check exposed a test expectation about macOS `/var` path normalization; its corrected expectation uses the resolved path. Input preservation and error cases pass. Primary source inspection is pinned to FFmpeg 8.1: - [Option matching and `-vf` alias](https://github.com/FFmpeg/FFmpeg/blob/9047fa1b084f76b1b4d065af2d743df1b40dfb56/fftools/ffmpeg_opt.c) - [Simple filtergraph and stream selection documentation](https://github.com/FFmpeg/FFmpeg/blob/9047fa1b084f76b1b4d065af2d743df1b40dfb56/doc/ffmpeg.texi) - [Filterchain syntax, escaping, crop, scale and subtitles](https://github.com/FFmpeg/FFmpeg/blob/9047fa1b084f76b1b4d065af2d743df1b40dfb56/doc/filters.texi) Use the [media pilots](https://github.com/the-media-vampire/veyrul-integrations/tree/0dc5ead8bb5ae92611f402b2343da54e88be3f59/examples/media-pilots) for caption timing, beat grids and audio handoff. Inspect saved sources, report only permitted observed results, and reread corrections on the next related task. This is founder-operated engineering evidence, not independent user adoption. Reusable code: https://github.com/the-media-vampire/veyrul-integrations/blob/0dc5ead8bb5ae92611f402b2343da54e88be3f59/examples/ffmpeg-filter-chain/compose_filters.py Checks: https://github.com/the-media-vampire/veyrul-integrations/blob/0dc5ead8bb5ae92611f402b2343da54e88be3f59/examples/ffmpeg-filter-chain/test_compose.py Founder-operated guide. Numeric command construction was checked locally; FFmpeg rendering and visual quality were not tested by this guide.

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      "note": "Original tested command-only Python builder; inspect before execution"
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      "note": "Three deterministic test groups; no video render"
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