SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
Figure 11-76 shows the alpha blending processing in detail.
The Z order of the Source Transparency Layer must have the value 3 (0x3: Z-order 3: layer above all the other layers).
1-alpha operator corresponds to the basic 1s-complement operation.
The alpha blending value is defined by:
Table 11-84 lists the percentage of alpha blending in the function of the alpha blending value on 8 bits.
Alpha Blending 1-Bit Value | Alpha Blending 4-Bit Value | Alpha Blending 8-Bit Value (Converted Value or Resulting Alpha) | Percent 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 (ARGB) where:
In that case, the processing is as follows:
The additional premultiplied alpha option is associated with the pipelines GFX, VID1, VID2, and VID3. The option is accessible through the [28] PREMULTIPLYALPHA bit for the respective pipeline register:
The following settings are available:
The prealphasel controls in Figure 11-76, correspond to the PREMULTIPLYALPHA of the pipelines mapped on the respective layers.
The logic marked in red in Figure 11-76 corresponds to the alpha value, computed when the write-back channel copies back to memory the premultiplied color component: A (destination) = A (source) + (1-A (source) × A (destination))
When the DISPC_WB_ATTRIBUTES[7] ALPHAENABLE bit is cleared, or when the overlay channel is not selected for WB, the computation of the A(destination) is disabled. The default value for DISPC_WB_ATTRIBUTES[7] ALPHAENABLE bit is 0x0. When the WB is configured to copy back one of the output channels (output of overlay), the following configurations are available:
The DISPC_WB_ATTRIBUTES[7] ALPHAENABLE bit is effective only when one of the output channels is written back, otherwise it is ignored.