Handling Dynamic Format Changes (Windows Embedded CE 6.0)
1/6/2010
Video renderers in DirectShow accept only video formats that can be drawn efficiently. For example, the window-based run-time renderer will accept only the RGB format that matches the current display device mode (for example, RGB565 when the display is set to 65,536 colors).
As a last resort, it also accepts 8-bit palletized formats, because most display cards can draw this format efficiently.
When the renderer has DirectDraw loaded, it might later ask the source filter to switch to something that can be written onto a DirectDraw surface and drawn directly through display hardware.
In some cases, the renderer's upstream filter might try to change the video format while the video is playing. This often occurs when a video stream has a palette change. It is most often the video decompressor that initiates a dynamic format change.
An upstream filter attempting to change formats dynamically should always call the IPin::QueryAccept method on the renderer input pin (for filters based on CTransformFilter, this is implemented in CTransformFilter::CheckInputType).
The formats a renderer will allow an upstream filter to change dynamically are undefined. However, at a very minimum, it should allow the upstream filter to change palettes.
When an upstream filter changes media types, it will attach the format to the first sample delivered in that new type.
If the renderer holds many samples in a queue waiting to be rendered, it should delay changing the format until the sample with the type change is actually about to be rendered.
When a video renderer detects a palette change in the stream it should send the EC_PALETTE_CHANGED notification to the filter graph manager. The DirectShow video renderers detect whether a palette has really changed in dynamic format or not.
The video renderers do this not only to filter out the number of EC_PALETTE_CHANGED notifications sent but also to reduce the amount of palette creation, installation, and deletion required.
Finally, the video renderer might also detect that the size of the video has changed, in which case, it should send the EC_VIDEO_SIZE_CHANGED notification. An application might use this notification to negotiate space in a compound document. The actual video dimensions are available through the IBasicVideo control interface.
The DirectShow renderers detect whether the video has actually changed size prior to sending these events.