Video Mixing IP Cores

6

2015.01.23

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The Video Mixing IP cores mix together up to 12 image layers .

This run-time control is partly provided by an Avalon-MM slave port with registers for the location, and on or off status of each foreground layer. The dimensions of each layer are then specified by Avalon-ST Video control packets.

Note: It is expected that each foreground layer fits in the boundaries of the background layer.

IP Cores		Feature
Alpha Blending Mixer		• Supports picture-in-picture and image blending with per
		pixel alpha support.
		• Supports dynamic changing of location and size of each
		layer during run time.
		• Allows the individual layers to be switched on and off.
		• Supports 1 pixel per transmission.
Mixer II		• Supports picture-in-picture mixing.
		• Supports dynamic changing of location and size of each
		layer during run time.
		• Allows the individual layers to be switched on and off.
		• Supports up to 4 pixels in parallel.
		• Includes built in test pattern generator as background
		layer

The Video Mixing IP cores read the control data in two steps at the start of each frame. The buffering happens inside the IP core so that the control data can be updated during the frame processing without unexpected side effects.

The first step occurs after the IP core processes and transmits all the non-image data packets of the background layer, and it has received the header of an image data packet of type 0 for the background. At this stage, the on/off status of each layer is read. A layer can be disabled (0), active and displayed (1) or consumed but not displayed (2). The maximum number of image layers mixed cannot be changed dynamically and must be set in the parameter editor.

The IP core processes the non-image data packets of each active foreground layer, displayed or consumed, in a sequential order, layer 1 first. The IP core integrally transmits the non-image data packets from the

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	6-2	Alpha Blending	UG-VIPSUITE
			2015.01.23

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

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Video Mixing Parameter Settings

6-3

at the system-level if erroneous pixels have to be discarded. The IP core ignores all non-image data packets (including control packets) and discards them just before the processing of a frame starts.

The valid range of alpha coefficients is 0 to 1, where 1 represents full translucence, and 0 represents fully opaque.

For n-bit alpha values (RGBAn) coefficients range from 0 to 2n–1. The model interprets (2n–1) as 1, and all other values as (Alpha value)/2n. For example, 8-bit alpha value 255 > 1, 254 > 254/256, 253 > 253/256, and so on.

The value of an output pixel ON, where N is the maximum number of layers, is deduced from the following recursive formula:

ON = (1 – aN)pN + aNON–1

O0 = p0

where pN is the input pixel for layer N and aN is the alpha pixel for layer N. The Alpha Blending Mixer IP core skips consumed and disabled layers. The IP core does not use alpha values for the background layer (a0); you must tie the alpha0 port off to 0 when you instantiate the IP core in the parameter editor.

Note: All input data samples must be in unsigned format. If the number of bits per pixel per color plane is N, then each sample consists of N bits of data which are interpreted as an unsigned binary number in the range [0, 2N–1]. All output data samples produced by the Alpha Blending Mixer IP core are also in the same unsigned format.

Video Mixing Parameter Settings

Table 6-1: Alpha Blending Mixer Parameter Settings

Parameter		Value		Description
Maximum layer width		32-2600, Default =		Specify the maximum image width for the
		1024		layer background in pixels. No layer width
				can be greater than the background layer
				width. The maximum image width is the
				default width for all layers at start-up.
Maximum layer height		32-2600, Default = 768		Specify the maximum image height for the
				layer background in pixels. No layer height
				can be greater than the background layer
				height. The maximum image height is the
				default height for all layers at start-up.
Bits per pixel per color plane		4-20, Default = 8		Select the number of bits per pixel (per color
				plane).
Number of color planes in		1, 2, 3		Select the number of color planes that are
sequence				sent in sequence over one data connection.
				For example, a value of 3 for R'G'B' R'G'B'
				R'G'B'.

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6-4		Video Mixing Parameter Settings					UG-VIPSUITE
						2015.01.23
		Parameter			Value		Description
		Number of color planes in		1, 2, 3			Select the number of color planes in parallel.
		parallel
		Number of layers being mixed		2–12			Select the number of image layers to overlay.
							The higher number layers are mixed on top
							of the lower number layers. The background
							layer is always layer 0.
		Alpha blending		On or Off			• When you turn on this parameter, the IP
							core generates alpha data sink ports for
							each layer (including an unused port
							alpha_in_0 for the background layer).
							This requires a stream of alpha values; one
							value for each pixel.
							• When you turn off this parameter, the IP
							core does not generate any alpha data sink
							ports, and the image layers are fully
							opaque.
		Alpha bits per pixel		2, 4, 8			Select the number of bits used to represent
							the alpha coefficient.
	Table 6-2: Mixer II Parameter Settings
		Parameter			Value		Description
		Output format (used for		4:4:4, 4:2:2			Select the sampling rate format for the
		background layer)					background layer.
		Color space (used for		•	RGB		Select the color space you want to use for the
		background layer)		•	YCbCr		background layer.
		Maximum frame width		32-2600, Default =			Specify the maximum image width for the
				1920			layer background in pixels.
		Maximum frame height		32-2600, Default =			Specify the maximum image height for the
				1080			layer background in pixels.
		Bits per pixel per color plane		4-20, Default = 8			Select the number of bits per pixel (per color
							plane).
		Number of color planes		2, 3			Select the number of color planes to be
							transmitted.
		Number of pixels in parallel		1, 2, 4			Select the number of pixels transmitted or
							received in parallel.
		Color planes transmitted in		1			The Mixer II IP core supports only color
		parallel					planes in parallel.

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