The IOverlayNotify interface provides an upstream filter, such as a decoder, with notifications of changes to the rendering window. This includes notifications of changes to the palette, color key, and window position, and visible region (clipping) changes.
Most software video decoders let the video renderer draw the decompressed images they produce by passing the media samples to the IMemInputPin interface on the renderer's input pin.
However, some video decoding filters (typically hardware decompression boards) handle the drawing of the images themselves, perhaps by using a VGA connector. These filters do not need to use IMemInputPin, but can instead use the IOverlay interface provided by the renderer input pin. Through this interface, the decoder can be notified when the window position or size changes, or when the current system palette changes in order to install and change color keys and palettes.
When to Implement
To get notifications of when the window size or position changes, when the current system palette changes, or when a different color key is used, decoders that do their own drawing should implement an IOverlayNotify interface. A pointer to this interface can then be passed to the IOverlay interface on the renderer's input pin, through the IOverlay::Advise method, to set up an advise link (essentially a callback mechanism). After the advise link is established, the renderer calls the decoder's IOverlayNotify methods when the appropriate events occur. An advise link can be canceled by calling IOverlay::Unadvise.
When to Use
The video renderer is the only filter that calls the methods on this interface. This is done automatically by the default video renderer. If you are writing a replacement video renderer, you will need to use the methods on this interface if your filter supports IOverlay and this interface is passed to your filter in an IOverlay::Advise call.
Methods in Vtable Order
| IUnknown methods | Description |
| QueryInterface | Returns pointers to supported interfaces. |
| AddRef | Increments the reference count. |
| Release | Decrements the reference count. |
| IOverlayNotify methods | Description |
| OnPaletteChange | Provides notification that the palette of the window has changed. |
| OnClipChange | Provides notification that the visible region of the window has changed. |
| OnColorKeyChange | Provides notification that the chroma key has changed. |
| OnPositionChange | Provides notification that the position has changed. |
Provides notification that the window's visible region has changed. It is essential that any overlay hardware be updated to reflect the change to the visible region before returning from this method.
HRESULT OnClipChange(
RECT *pSourceRect,
RECT *pDestinationRect,
RGNDATA *pRgnData
);
Returns an HRESULT value.
The calls to OnClipChange happen in synchronization with the window. It is called with an empty clip list to freeze the video before the window moves, and then called again when the window has stabilized with the new clip list.
If the window rectangle is all zeros, the window is invisible. The filter must take a copy of the information if it needs to store it.
Provides notification that the window's color key has changed.
HRESULT OnColorKeyChange(
COLORKEY * pColorKey
);
Returns an HRESULT value.
Provides notification that the palette of the window has changed.
HRESULT OnPaletteChange(
DWORD dwColors,
const PALETTEENTRY * pPalette
);
Returns an HRESULT value.
Before returning, the filter should copy the array of RGBQUAD values, if necessary.
Provides notification that the position has changed.
HRESULT OnPositionChange(
const RECT *pSourceRect,
const RECT *pDestinationRect
);
Returns an HRESULT value.
This method is a callback intended for use by hardware overlay cards that do not want the expense of synchronous clipping updates, and just want to know when the source or destination video positions change.
Unlike the IOverlayNotify::OnClipChange method, this method is not called in synchronization with the window changing but, rather, at some point after the window has changed (basically in time with WM_SIZE messages received). This is therefore suitable for overlay cards that do not inlay their data to the frame buffer.
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