ZRLE

ZRLE, or Zero-run Length Encoding, is a data compression scheme used by the Remote Framebuffer (RFB) protocol in VNC-based remote desktop systems. It is designed to efficiently encode rectangular regions of the screen to reduce bandwidth requirements during server-to-client transmission.

The technique works by dividing the target rectangle into fixed-size blocks, commonly 64x64 pixels. Each block is encoded independently using one of several sub-encodings. The most common approach is a palette-based encoding: a local color palette is constructed for the block, and the block’s pixels are represented as indices into that palette. Runs of identical palette indices are compressed with run-length encoding. If a block cannot be efficiently represented with a palette, a raw or alternative sub-encoding may be used.

For each block, the encoder selects the sub-encoding that yields the best compression, and the encoded block data is transmitted along with a small header indicating the chosen method and any necessary palette information. The client then decompresses the block accordingly. This modular approach allows ZRLE to adapt to different content types and network conditions.

ZRLE is particularly effective for desktop imagery with large flat areas or limited color variation, helping to minimize data transfer over constrained links. However, its performance can degrade on highly detailed or richly colored scenes, where palette quantization or multiple blocks may introduce overhead or reduce fidelity. It also incurs CPU overhead for both encoding and decoding.

ZRLE is supported by major VNC implementations, including TightVNC, TurboVNC, RealVNC, and TigerVNC, where it serves as one of the key encodings in the RFB protocol alongside other methods like RAW, RRE, and Hextile.