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Display Subsystem Functional Description
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PREMULTIPLYALPHA bit = '0' : Source is not pre-multiplied with alpha. Full blending is done in the
DISPC.
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PREMULTIPLYALPHA bit = '1' : Source is pre-multiplied with alpha. Partial blending is done.
NOTE:
The pre-multiplied alpha option is only valid when bit fields
[4:1] GFXFORMAT and
[4:1] VIDFORMAT, respectively, are set to ARGB or RGBA
formats. Otherwise, the PREMULTIPLYALPHA bit fields are ignored by the hardware.
The alpha blending value is defined by the pixel value (ARGB or RGBA formats). A global alpha blending
value can be defined and used in combination with the pixel alpha blending value. If the pixel format
contains no alpha blending value, the pixel alpha value is considered to be 0xFF.
In case of ARGB-444, the alpha blending is defined using a 4-bit value. It is converted into an 8-bit value
by duplicating the 4-bit value.
details the alpha blending 4-bit values and the corresponding
blending percentage.
Table 7-28. Alpha Blending 4-Bit Values
Alpha Blending 4-Bit Value (ARGB-444)
Alpha Blending 8-Bit Value
% Blending
(Converted Value)
0x0
0x00
100% (transparent)
0x1
0x11
93.33%
0x2
0x22
86.6%
...
...
...
0xE
0xEE
6.6%
0xF
0xFF
0% (opaque)
7.4.2.4.2 Transparency Color Keys
7.4.2.4.2.1 Normal Mode
This section describes the features available in normal mode.
The two transparency color keys are the video source transparency color key and the graphics destination
transparency color key. The encoded pixel color value is compared to the transparency color key. For
CLUT bitmaps, the palette index is compared to the transparency color key and not to the palette value
pointed out by the palette index.
NOTE:
The video source transparency color key and graphics destination transparency color key
cannot be active at the same time.
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Video source transparency color key value:
The video source transparency color key value defines the encoded pixel data considered as the
transparent pixel. The encoded pixel values with the source color key value are pixels not visible on
the screen, and the underlayer encoded pixel values or solid background color are visible.
The video source transparency color key can be used only if the color space conversion and the
up/down-scaling modules are disabled. The format of the data is RGB 16. (This feature handles the
hardware cursor displayed by one of the video layers.)
To enable the video source transparency color key, set to 0x1 the DSS.
TCKLCDSELECTION bit for LCD output or the DSS.
[13] TCKDIGSELECTION bit for
digital output. Program the DSS.
[10] TCKLCDENABLE bit (LCD output) or the
DSS.
[12] TCKDIGENABLE bit (digital output) to enable or disable the transparency
color key.
An example is shown in
: The video source transparency is applied on video1 (VID1) and
video2 (VID2) layers. The pixels with the transparency color key are not displayed; instead, underlying
layers are shown.
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Display Subsystem
SWPU177N – December 2009 – Revised November 2010
Copyright © 2009–2010, Texas Instruments Incorporated