When processing digital video in software like FFmpeg or web studios like SliceFlow, you will frequently encounter two fundamental operational modes: Stream Copy (often represented as -c copy) and Re-encoding (such as -c:v libx264).
While both methods produce video segments, they work on fundamentally different principles. Understanding their trade-offs helps you choose the exact right mode for your workflow.
Comparison Summary Matrix
| Feature | Stream Copy Mode | Re-encode Mode |
|---|---|---|
| Visual Quality | 100% Lossless (0% loss) | Lossy (Generational compression) |
| Processing Speed | Instantaneous (1–3s) | Moderate (10–60s+ per gigabyte) |
| CPU / GPU Load | Virtually Zero (I/O only) | 100% Core Saturation |
| Frame Accuracy | Keyframe-aligned (I-frames) | Exact millisecond frame accuracy |
How Stream Copying Operates
When Stream Copy is invoked, FFmpeg demuxes the input container (e.g. extracting video and audio elementary streams from an MP4 envelope). Instead of passing these packets to a decoder and filtering pipeline, FFmpeg directly feeds the raw packet payload into the output muxer.
Because no mathematical DCT (Discrete Cosine Transform) or chroma sub-sampling takes place, the operation is limited only by storage write speed.
When Should You Use Re-encoding?
Although Stream Copy is superior for speed and quality, Re-encoding is required when:
- You require frame-accurate cutting precisely at a non-keyframe (P-frame or B-frame).
- You need to transcode to a different container or codec (e.g., converting an MKV with HEVC video to an MP4 with H.264 for legacy device compatibility).
- You need to downsample 4K footage to 1080p or reduce file size for email distribution.
SliceFlow's Hybrid Flexibility
SliceFlow provides toggle buttons allowing you to select between Stream Copy (Lossless) and Re-encode (Accurate) directly in the Cut Settings panel. You get the best of both worlds according to your specific project needs.