The server and client communicate via channels. Each channel is dedicated to a specific type of data. The available channels are following.

Multiple Channels

  1. Main - control and configuration

  2. Display - graphics commands images and video streams

  3. Inputs - keyboard and mouse inputs

  4. Cursor - pointer device position and cursor shape

  5. Playback - audio received from the server to be played by the client

  6. Record - audio captured on the client side

  7. Smartcard - passthrough of smartcard data from the client machine to the guest OS

  8. USB - redirection of USB devices plugged into the client to the guest OS

Image Compression

Spice offers several image compression algorithms, which can be chosen on server initiation and dynamically at run-time. Quic is a Spice proprietary image compression technology based on the SFALIC algorithm. The Lempel-Ziv (LZ) algorithm is another option. Both Quic and LZ are local algorithms encoding each image separately. Global LZ (GLZ) is another proprietary Spice technology that uses LZ with history-based global dictionary. GLZ takes advantage of repeating patterns among images to shrink the traffic and save bandwidth, which is critical in a WAN environment. Spice also offers an automatic mode for compression selection per image, where the choice between LZ/GLZ and Quic is heuristically based on image properties. Conceptually, synthetic images are better compressed with LZ/GLZ and real images are better with Quic.

Video Compression

Spice uses loss-less compression for images sent to the client. However, video streams are handled differently. Spice server heuristically identifies video areas and sends them as a video stream coded using M-JPEG. This handling saves a lot of traffic, improving Spice performance, especially in a WAN environment. However, in some circumstances the heuristic behavior might cause low quality images (e.g. identifying updated text area as a video stream). Video streaming can be chosen on server initiation and dynamically at run-time.

Mouse modes

Spice supports two mouse modes: server and client. The mode can be changed dynamically and is negotiated between the client and the server.

  1. Server mouse - When a user clicks inside the Spice client window, the client mouse is captured and set invisible. In this mode, the server controls the mouse position on display. However, it might be problematic on WAN or on a loaded server, where mouse cursor might have some latency or non-responsiveness.

  2. Client mouse - Not captured and is used as the effective pointing device. To enable client mouse, the VDI host application must register an absolute pointing device (e.g. USB tablet in QEMU). This mode is appropriate for WAN or or for a loaded server, since cursor has smooth motion and responsiveness. However, the cursor might lose synchronization (position and shape) for a while.

Other Features

  1. Multiple Monitors - any number of monitors is supported

  2. Arbitrary Resolution - when using the QXL driver, the resolution of the guest OS will be automatically adjusted to the size of the client window.

  3. USB Redirection - Spice can be used to redirect USB devices that are plugged in the client to the guest OS. This redirection can either be automatic (all newly plugged devices are redirected), or manual (the user selects which devices (s)he wants to redirect).

  4. Smartcard Redirection - data from smartcard that are inserted into the client machine can be passed through to the guest OS. The smartcard can be used by both the client OS and the guest OS.

  5. Bidirectional Audio - Spice supports audio playback and recording. Playback is compressed using the CELT algorithm

  6. Lip-sync - between video and audio. Available only when video streaming is enabled.

  7. Migration - switching channel connectivity for supporting server migration

  8. Pixmap and Palette caching