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L0
alpha1
alpha2
alpha3
L1
L2
L3
Blended Output
Internal Modules
78
SPRUHI7A – December 2012 – Revised June 2016
Copyright © 2012–2016, Texas Instruments Incorporated
High-Definition Video Processing Subsystem (HDVPSS)
1.2.3.4.3 Alpha Blender
The Alpha Blender uses the alpha values of the input layers and blends all of them as per the input order.
The block diagram of alpha blender is shown in
Figure 1-35. Alpha Blender Block Diagram
In
, L0 is the top layer and L3 is the bottom layer. alpha1, alpha2, alpha3 are the blending
levels of layers L1, L2, and L3 respectively.
Note: The total number of pixels per frame/field of any source driving the COMP module should be even.
1.2.3.5
Feedback Output
Blenders 0 and 1 (associated with DVO1 and DVO2) have a feedback output from blended image data.
The feedback image data can be selected from video alpha blending or final alpha blending, and sent to
external memory. Since feedback path stalls also stall the entire data pipeline, the amount of the stalls in
this path can not be such that they will cause an under-flow to the VENC. Hence, the feedback path needs
to be given proper high priority in the system for memory access when the corresponding VENC is also
functional. Both the paths (feedback path and Venc data path) are optional.
1.2.3.6
Background Color
There is a programmable background color in COMP module. Background color is configured in RGB
format with 10-bit in each R, G, and B. Any pixel with an alpha value of 0 will be replaced by this color. If
the video bottom layer is disabled, the video top layer will be blended with background color. If both video
layers are disabled, the background color is used as output of video alpha blending. For SD channel, only
one video stream comes in and it will be blended with the background color.
If the COMP is disabled, or the channel within the COMP driving a particular VENC is disabled, or all
inputs to a channel driving particular VENC are disabled, the underflow condition occurs at the VENC.
This underflow condition is due to data is not being driven out of the COMP fast enough to keep up with
the VENC read rate. Underflow can also happen due to lack of sufficient memory bandwidth or improper
VPDMA configuration for any of the input channel. When underflow condition happens, the background
color programmed in the COMP module is sent for the underflowed pixels.