2.2.2. Avalon-ST Video Video Packets
The Avalon-ST Video protocol further defines that any other symbol data transmitted in the first cycle (or beat) of the transmission is ignored. Uncompressed and rasterized, the pixel data is transmitted in the symbols that follow in subsequent cycles, starting with the top-left pixel.
Avalon-ST Video packets support RGB and YCbCr color spaces, with 4:4:4 or 4:2:2 chroma sub-sampling, with or without an optional alpha (transparency) channel. The 4:2:0 color space is only handled by the Clocked Video interfaces and the chroma resampler. For the other VIP IP cores, the 4:2:0 color space should be converted to or from 4:2:2 for processing.
Color channel data for RGB video packets is transmitted in the order of Blue, Green, Red. You can observe this order directly on the bus for symbols in sequence Avalon-ST configurations. For symbols (and pixels) in parallel configurations, the blue symbol occupies the least significant symbol position (D0) as shown the figure below, with the (x,y) raster position shown in brackets.
For 4:4:4 YCbCr data, chroma sample Cb is in the least significant position (or first symbol to be transmitted in a symbols in sequence configuration), Cr is the mid symbol, with the luminance (Y) occupying the most significant symbol position. The figure below shows an example with symbols in parallel.
For 4:2:2 YCbCr video, with sub-sampled chroma, the Cr and Cb symbols alternate, such that each Luma symbol is associated with either a Cr or Cb symbol as shown in the figure below.
For video with an alpha layer, the alpha channel occupies the first (least significant) symbol with the remaining color channels following as per the usual ordering with Blue or Cb occupying the next symbol after the alpha as shown in the figure below.
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