Altera Video and Image Processing Suite - Manual (Page 113)

background layer. The IP core treats the non-image data packets from the foreground layers differently
depending on their type.
• Control packets (type 15)— processed to extract the width and height of each layer and are discarded
on the fly.
• Other/user packets (types 1–14)—propagated unchanged.
The second step corresponds to the usual behavior of other Video and Image Processing IP cores that
have an Avalon-MM slave interface. After the IP core has processed and/or propagated the non-image
data packets from the background layer and the foreground layers, it waits for the
Go
bit to be set to 1
before reading the top left position of each layer.
Consequently, the behavior of the Alpha Blending Mixer and Mixer II IP cores differ slightly from the
other Video and Image Processing IP cores. The behavior of the Video Mixing IP cores is illustrated by
the following pseudo-code:
go = 0;
while (true)
{
status = 0;
read_non_image_data_packet_from background_layer();
read_control_first_pass(); // Check layer status
(disable/displayed/consumed)
for_each_layer layer_id
{
// process non-image data packets for displayed or consumed
layers
if (layer_id is not disabled)
{
handle_non_image_packet_from_foreground_layer(layer_id);
}
}
while (go != 1)
wait;
status = 1;
read_control_second_pass(); // Copies top-left coordinates to
internal registers
send_image_data_header();
process_frame();
}
Alpha Blending
The alpha frames contain a single color plane and are transmitted in video data packets.
When you turn on Alpha blending in the Alpha Blending Mixer parameter editor, the Avalon-ST input
ports for the alpha channels expect a video stream compliant with the Avalon-ST Video protocol. The
first value in each packet, transmitted while the
startofpacket
signal is high, contains the packet type
identifier 0. This condition holds true even when the width of the alpha channels data ports is less than 4
bits wide. The last alpha value for the bottom-right pixel is transmitted while the
endofpacket
signal is
high.
It is not necessary to send control packets to the ports of the alpha channels. The width and height of each
alpha layer are assumed to match with the dimensions of the corresponding foreground layer. The Alpha
Blending Mixer IP core recovers cleanly if there is a mismatch, although there may be throughput issues
6-2 Alpha Blending
UG-VIPSUITE
2015.05.04
Altera Corporation Video Mixing IP Cores
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