Google is extending the InVideo advertising format it has pioneered on YouTube, the video sharing site it now owns, and plans to extend it across video partner sites in its content network. This will enable advertisers to run a single contextually-targeted video overlay campaign across a large network of sites.

Brightcove, which provides a broadband video platform for publishers, is among the companies that has announced support for the trial service. The text and graphical overlay adverts are intended to be less intrusive than pre-roll videos, appearing semi-transparent over the bottom 20% of the video window.

It will give media publishers the additional ability, beyond selling their own advertising, to target tailored in-stream overlay messages from a network of advertisers using Google. Publishers and content providers can control which videos get which advertisements and when the advertisements play in each video.

“Video and rich media continue to account for an increasingly large segment of online content — Brightcove customers alone reach 130 million unique users a month across thousands websites,” said Chris Johnston of Brightcove.

The Google AdSense for video beta is currently available through Brightcove in a limited trial release with select customers. The company said they plan to make the feature generally availability to publishers in 2008.

The current InVideo advertisements use the Flash format, at a nominal size of 480×70 pixels. Clicking on the Flash overlay can send users directly to an external web page or open an optional extended video advertising unit. Unfortunately, Google still does not seem to support widescreen video properly, and assumes a frame rate of 30 frames per second.