MB86290A Datasheet, PDF (46/184 Pages) Fujitsu Component Limited. – Graphics Controller Hardware Specifications

5.2.2 Overlay
Simple priority mode
The top layer has the higher priority. Each pixel color is determined
according to the following rules:
1. If the C layer is not transparent, the C-layer color is displayed.
2. If the C layer is transparent and W-layer image is at that position,
the W-layer color is displayed.
3. If the C layer is transparent and there is no W layer image at that
position, and if the M-layer color is not transparent, the M-layer
color is displayed.
4. If the C and M layers are transparent and there is no W-layer image
at that position, the B-layer color is displayed.
Transparent color is set by putting a specific transparent color code in the
register.
Blend mode
The W, M and B layers are managed in the same way as the simple priority
mode described above. The result of the W/M/B layer priority color is
blended with the C-layer color according to the blending ratio specified in the
register. This mode is applied when the alpha bit of that pixel in the C layer
is 1. If this alpha bit is set to 0, the result is the same as the simple priority
mode.
When the C-layer display priority is cursor display, the cursor color and C
layer color are alpha blended at the pixel position with alpha bit = 1. The
alpha blend ratio is calculated as follows:
♦ When BRS bit of BRATIO register = 0
Display color = ((C layer color x blend coefficient) +
(Mixed color of W/M/B layers x (1-blend coefficient))
♦ When BRS bit of BRATIO register = 1
Display color = (C layer color x (1-blend coefficient)) +
(Mixed color of W/M/B layers x blend coefficient)
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