On Mon, 17 Dec 2001, Mike Frisch wrote: > > If you draw an interlaced frame to the screen in this manner, you have > > no way of knowing what order the fields will be shown. Not only that, > > if you don't know the clock the card is using to output the fields, if > > you don't draw the next frame in time it could loop between the two > > current fields (motion jumping back in time!). > > If I run Ogle or Xine on a 720x576 screen, why is this a concern to me? For one thing: You see a horizontal "tear" on the screen where the driver changes frame (field) whilst the video card is halfway through displaying the frame. Or - you are putting frames into video memory at the frame rate - say 25Hz. But the video card is showing fields "out of phase". So - you expected fields to be displayed as frame-1-odd frame-1-even frame-2-odd frame-2-even: which captures the correct time order. Instead you see frame-1-even frame-1-odd frame-2-even frame-2-odd. This is out of time order so you get horrible judder. This whole issue of field-orientation is a messy business, and this effect does depend on whether the original video is field-based. Often its frame-based, like film - in which case these problems aren't so obvious. I just put up with this at the moment - but more power to Billy with efforts to fix it properly. Though my approach would probably be to work on XVideo to provide a hook into the physical VSYNC. I don't see why its not enough to record by frames (both fields together), but make sure that each frame is shown exactly at VSYNC. Then the video card will automatically show each field in turn in the right time relationship. Steve
