Understanding Video Compression: Factors That Influence Bitrate and Encoding Strategies

The Bilibili video cloud team is responsible for video storage, encoding, and transmission. This article introduces the fundamentals of video compression and explains why bitrate is a critical metric for streaming.

Raw video data is extremely large (e.g., a 1080p 30 fps RGB stream requires about 1.4 Gb per second). Without compression, such streams cannot be stored or transmitted efficiently, leading to buffering and poor user experience.

Compression reduces redundancy in both spatial and temporal dimensions and removes information that the human eye cannot perceive. Modern encoders therefore apply color‑space conversion, block partitioning, intra‑prediction, motion search, motion compensation, DCT, quantization, and entropy coding to lower bitrate while preserving subjective quality.

Factors affecting video bitrate

1. Content characteristics – Simple scenes contain less information and can be compressed more aggressively; complex scenes retain more detail and require higher bitrate. Sub‑factors include:

Resolution and frame rate – Higher resolution and frame rate increase raw data volume, but the relationship with bitrate is not linear because higher resolution often brings stronger spatial correlation.

Spatial correlation – Uniform areas (e.g., large color blocks) allow intra‑prediction to reuse similar pixels, reducing residual data. Highly textured or random areas provide little redundancy.

Temporal correlation – Consecutive frames that are similar enable motion‑compensation to reference previous frames, lowering residual size.

Upload source bitrate – The bitrate of the original uploaded file does not directly determine the final streaming bitrate; transcoding normalizes quality and removes unnecessary redundancy.

2. Encoding algorithms and standards – Successive codec generations (AVC/H.264, HEVC/H.265, AV1) improve compression efficiency, allowing the same visual quality at lower bitrate. For example, HEVC can reduce bitrate by ~40 % compared with AVC for HDR or 8K content.

Bilibili currently supports three standards (AVC, HEVC, AV1) to serve a wide range of devices and to enable advanced formats such as HDR and 8K. Users can manually select a stream, while the platform also auto‑selects the optimal codec based on device capabilities.

3. Bitrate control strategies – Encoders adjust quantization parameters to meet bitrate or quality targets.

ABR (Average Bitrate) maintains a stable bitrate, suitable for live streaming, but may sacrifice quality consistency.

CRF (Constant Rate Factor) allocates more bits to perceptually important regions (e.g., sky vs. grass, frequently referenced blocks) and less to less important areas, achieving better quality‑bitrate trade‑offs.

In summary, video bitrate is influenced by content complexity, codec sophistication, and bitrate‑control algorithms. Advanced codecs and well‑tuned control modes can significantly lower bitrate while preserving visual quality, leading to smoother playback experiences.