The alpha blending value is defined
by:
- The component value A when
using an ARGB or RGBA pixel format (alpha in the source pixel data is
converted to 8-bit alpha value in the input pipeline logic):
- For ARGB-1555, the
alpha blending is defined using a 1-bit value. It is converted into
an 8-bit value by duplicating the 1-bit value (see Table 12-479).
- For ARGB-4444, the
alpha blending is defined using a 4-bit value. It is converted into
an 8-bit value by duplicating the 4-bit value (see Table 12-479).
- If the pixel format
contains no alpha blending value, the pixel alpha value is
considered to be 0xFF, and if alpha is equal to 0xFF, there is no
multiplication.
- For BITMAP or YUV
formats, there is no alpha blending factor associated with each
pixel value. Only the global alpha blending factor associated with
the window displaying the BITMAP or YUV format is used.
- The global alpha blending
value can be set in the DSS_VID_GLOBAL_ALPHA
register of the input pipeline, and is updated in synch with the selected
output channel.
- The modulated alpha blending
value (that is, a total alpha blending value, when a combination of the
pixel alpha blending value A and a global alpha blending are present) is
determined as: Alpha = (Pixel Alpha × Global Alpha) / 256.
Table 12-479 shows the remapping and the percentage of alpha blending achieved.
Table 12-479 DISPC
Overlay Alpha Blending – ARGB
Alpha Blending 1-Bit Value
(ARGB16-1555) |
Alpha Blending 4-Bit Value
(ARGB16-4444) |
Alpha Blending 8-Bit Value
(Converted Value) |
% Blending |
| 0x0 |
0x0 |
0x00 |
100% (transparent) |
| N/A |
0x1 |
0x11 |
93.33% |
| N/A |
0x2 |
0x22 |
86.6% |
| N/A |
... |
... |
... |
| N/A |
0xE |
0xEE |
6.6% |
| 0x1 |
0xF |
0xFF |
0% (opaque) |
Premultiplied
Alpha
The image ARGB may
have its RGB component already premultiplied with the alpha (AR'G'B') where:
- R' = A*R
- G' = A*G
- B' = A*B
In that case, the
processing is as follows:
- Color components of
premultiplied layers are multiplied with the global alpha, if global alpha
is not equal to 0.
- Color components of the
composed underlying layer are multiplied with (1 - A*global alpha).
The additional
premultiplied alpha option is accessible through the DSS_VID_ATTRIBUTES [28]
PREMULTIPLYALPHA register bit of the input pipeline. The following settings are
available:
- PREMULTIPLYALPHA bit = 0:
Source is not premultiplied with alpha. Full blending is done in the overlay
manager.
- PREMULTIPLYALPHA bit = 1:
Source is premultiplied with alpha. Partial blending is done.
Note: There is no alpha blending
between the background and the next layer (layer-0) above it.
Layer-0 cannot be pre-multiplied,
if the overlay output is driving the display, since that layer (or the
background color) defines the bottom most layer. In this case, layer-0 alpha is
1 always.