Intel 82854 GMCH - Manual (Page 15)

Intel® Digital Set Top Box Display Driver 15
User’s Guide for Microsoft* Windows* CE 5.0
AlphaBlendMode<DWORD>
This key is used to enable ARGB32 pixel format on the framebuffer. When in this mode, the alpha channel in the
frame buffer is used by the HW internally to blend the graphics plane over the video overlay plane. The value for
this registry key can be either 0: disabled, or 1: enabled.
Notes:
• This feature is only supported with the “Depth” registry key set to 0x20 (32). It is ignored otherwise.
• With this mode enabled, any framebuffer pixel with a zero alpha value is transparent – which unfortunately
means that the CE desktop and CE utilities will be transparent. Direct Draw must be used to render non-
transparent pixels to the framebuffer. If GDI is used, only black will be seen.
• This feature is enabled automatically by the display driver. The driver does this by enabling destination color
keying and by configuring the framebuffer to use ARGB32 pixel format. The one side effect this configuration
is that any pixel that has an alpha value of 0xFF that also matches the color key selected will be transparent
instead of opaque. For this reason, it is recommended to use alpha values from 0x00-0xFE.
• When in this mode, the HW assumes that the framebuffer is pre-multiplied. When using Direct Draw to render
to the framebuffer, it is necessary to use the correct pre-multiplied/non-premultiplied flags. If this is done
incorrectly, the final rendered image may not look correct. When using the CPU to draw on the framebuffer (ex:
SW emulated BLT, memcpy, and so on), you must take into account the fact that the framebuffer is pre-
multiplied.
VRWorkaroundMode<DWORD>
This key is used to enable the Video Renderer Workaround that prevents the stock CE 5.0 video renderer from
drawing the color key on the frame buffer. This key is ignored if “AlphaBlendMode” is disabled. This key should be
enabled whenever “AlphaBlendMode” is enabled and the stock CE 5.0 video renderer is used.