Screen Content Coding (SCC) and HEVC‑SCC Tools: Principles and Performance Gains

1. What is SCC (Screen Content Coding)

With the rapid development of cloud computing and mobile Internet, applications such as screen sharing, remote teaching, video conferencing, and wireless display have become increasingly popular. The video content in these scenarios often consists of screen content rather than natural camera‑captured scenes. Screen content is characterized by large flat regions, repeated patterns, high saturation or a limited set of colors, and blocks of identical digits or letters, without sensor noise. These characteristics make screen‑content coding a challenging problem. Traditional video codecs (e.g., H.264/AVC, early H.265/HEVC) were designed for camera‑captured video and do not exploit these specific features. Adding SCC‑specific tools to HEVC can significantly improve compression performance.

2. Main tools of HEVC‑SCC and typical BD‑Rate gains

The ITU, ISO, and IEC jointly defined the HEVC‑SCC extension in 2016. To improve coding efficiency for screen content, four tools were introduced:

Intra Block Copy (IBC)

Palette Mode (PLT)

Adaptive Color Transform (ACT)

Adaptive Motion Vector Resolution (AMVR)

These tools provide notable BD‑Rate reductions (typical gains): IBC –19.1%, PLT –11.1%, ACT –0.7%, AMVR –1.5% on the "YUV + Text and Graphics with Motion" test set. However, they also increase encoder complexity, so selective use is required.

3. Principles of IBC and PLT

IBC (Intra Block Copy) works like inter‑prediction but uses reference blocks from the same picture. It exploits the high spatial redundancy of screen content (repeated patterns, text, UI elements). Early versions were limited to 2Nx2N blocks with 1‑D motion vectors; later extensions introduced 2‑D vectors and larger reference areas, greatly improving efficiency. IBC is treated as a special inter‑prediction mode in HEVC‑SCC, reusing most existing HEVC motion‑compensation infrastructure.

PLT (Palette Mode) encodes blocks that contain a small number of distinct colors by transmitting a palette table and palette indices for each pixel. An escape index is used for colors not present in the palette. The palette size varies per CU, and a palette predictor is built from previously encoded neighboring CUs to reduce overhead. After palette indices are generated, they are scanned (horizontal or vertical) and encoded using COPY_LEFT_MODE or COPY_ABOVE_MODE.

4. Current status of SCC

SCC is an optional extension of the HEVC standard; cloud‑side implementations and optimizations are mature and widely deployed.

Mobile‑side HEVC‑SCC is limited by performance and power consumption (e.g., Snapdragon 8 Gen2, Dimensity 9000).

The next‑generation codec AV1, led by Google, has adopted SCC as a standard. While mobile decoding of SCC is now feasible, generating SCC‑encoded content still faces challenges such as tool hardening and power‑efficiency optimization.