Altera Video and Image Processing Suite User Manual
Page 20
IP Core
Stall Behavior
Error Recovery
Alpha Blending
Mixer/
Mixer II
All modes stall for a few cycles after each
output frame and between output lines.
Between frames, the IP core processes non-
image data packets from its input layers in
sequential order. The core may exert
backpressure during the process until the
image data header has been received for all
its input.
During the mixing of a frame, the IP core:
• Reads from the background input for
each non-stalled cycle.
• Reads from the input ports associated
with layers that currently cover the
background image.
Because of pipelining, the foreground pixel
of layer N is read approximately N active
cycles after the corresponding background
pixel has been read.
• If the output is applying backpressure or
if one input is stalling, the pipeline stalls
and the backpressure propagates to all
active inputs.
• When alpha blending is enabled, one
data sample is read from each alpha port
once each time that a whole pixel of data
is read from the corresponding input
port.
There is no internal buffering in the IP core,
so the delay from input to output is just a
few clock cycles and increases linearly with
the number of inputs.
The Alpha Blending Mixer IP core
processes video packets from the
background layer until the end of packet
is received.
• Receiving an
endofpacket
signal too
early for the background layer—the
IP core enters error mode and
continues writing data until it has
reached the end of the current line.
The
endofpacket
signal is then set
with the last pixel sent.
• Receiving an
endofpacket
signal
early for one of the foreground layers
or for one of the alpha layers—the IP
core stops pulling data out of the
corresponding input and pads the
incomplete frame with undefined
samples.
• Receiving an
endofpacket
signal late
for the background layer, one or
more foreground layers, or one or
more alpha layers—the IP core enters
error mode.
This error recovery process maintains
the synchronization between all the
inputs and is started once the output
frame is completed. A large number of
samples may have to be discarded
during the operation and backpressure
can be applied for a long time on most
input layers. Consequently, this error
recovery mechanism could trigger an
overflow at the input of the system.
1-14
Stall Behavior and Error Recovery
UG-VIPSUITE
2015.05.04
Altera Corporation
Video and Image Processing Suite Overview