|
The DirectDraw modes are entirely replaced by the Direct3D mode. We strongly recommend to use the Direct3D mode instead of the DirectDraw modes. |
In this mode, a live image with an output speed of the primary surface mode and at the same time display of the overlay can be achieved.
The video image is digitized to a non-visible area of the VGA board. This area always has to be on the VGA board. By defining a key colour and drawing with that colour to the output window, the video image will be displayed only in those parts where the key colour is used. When the window is filled with the key colour, the video image is displayed completely. Graphics and text elements which use a different colour will not be destroyed (non-destructive overlay). The fading is done by the VGA chip and requires hardly any CPU cycles. This mode is not supported by all VGA chips and is only in certain modes possible.
The best text and graphics overlay is achieved with the following video mode:
Mode = IS_SET_DM_DIRECTDRAW | IS_SET_DM_ALLOW_OVERLAY
If the video image should be scaled to the size of a window, the following can be used:
Mode = IS_SET_DM_DIRECTDRAW | IS_SET_DM_ALLOW_OVERLAY | IS_SET_DM_ALLOW_SCALING
So as to prevent the interlace effect with moving images, and extra mode can be activated when scaling. This is achieved with:
Mode = IS_SET_DM_DIRECTDRAW | IS_SET_DM_ALLOW_OVERLAY | IS_SET_DM_ALLOW_SCALING | IS_SET_DM_ALLOW_FIELDSKIP
Fig. 39: DirectDraw overlay surface mode
