One of the half dozen SpeedScreen technologies from Citrix is SpeedScreen Image Acceleration. Since I’ve been reviewing these technologies as part of my multimedia virtualization strategy work, I thought I’d briefly share some highlights of this valuable feature.

SpeedScreen Image Acceleration was introduced in Presentation Server 3.0 back in 2004 and is included in all product editions. It is one of the new features in last month’s Mac client release (along with SpeedScreen Progressive Display and various other enhancements), and it is an important capability of XenDesktop, too.

Image Acceleration improves the delivery of photographic bitmaps and high detail synthetic images. If a bitmap looks as if it is probably photographic or highly detailed, SpeedScreen Image Acceleration can add an extra level of lossy JPEG compression to speed up the transmission of the image to the client device. Lossy JPEG compression is quite CPU intensive, but since it is applied only when there is enough image detail that the probability of payoff is high, server CPU is not consumed needlessly.

With modern applications, even a background image can have considerable detail. For example, there may be subtle color changes from left to right or top-down. Even icons these days are often high detail images with gentle graduation in color. JPEG is very good at handling pictures with many different shades of color.

Under most circumstances, the compression scheme is able to remove redundant data with minimal loss of information, and the image still looks good to the user. So SpeedScreen Image Acceleration is enabled by default. But sometimes, applying a high level of lossy compression to a high detail, synthetic (non-photographic) image will produce noticeable artifacts or smudging. So there’s a trade-off between image quality and throughput. Since lossy compression isn’t always desired, the settings for Image Acceleration are fully controllable by policies. The system administrator can choose the preferred compression level (high, medium or low) and may restrict the use of lossy compression to network connections below a specified bandwidth threshold. Lossy compression is not appropriate for certain applications where image fidelity is critical, such as Picture Archival & Communications Systems (PACS) used in Healthcare to view X-rays and other scans, except to improve responsiveness while an image is being scrolled on the screen; but that’s a topic we can cover another time when we take a look at SpeedScreen Progressive Display.

Derek Thorslund
Product Strategist, Multimedia Virtualization