Zynq Ult VCU TRD User Guide
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UG1250 (v2019.1) May 29, 2019
Chapter 4:
System Considerations
frame with GUI control elements while other parts of the frame are transparent. A
mechanism called
pixel alpha
is used to control the opacity of each pixel in the graphics
plane.
The pixel format used for the graphics plane is called ARGB8888 or AR24. It is a packed
format that uses 32 bits to store the data value of one pixel (32 bits per pixel or BPP), 8 bits
per component (BPC) —also called color depth or bit depth. The individual components are:
alpha value (A), red color (R), green color (G), blue color (B). The alpha component describes
the opacity of the pixel: An alpha value of 0% means the pixel is fully transparent (invisible);
an alpha value of 100% means the pixel is fully opaque.
The pixel formats used for the video plane are NV12, NV16, XV15 and XV20. These are
two-plane versions of the YUV 4:2:0 and YUV 4:2:2 format, respectively. The three
components are separated into two sub-images or planes.
In NV12 and XV15 formats, chroma planes are sub-sampled in both the horizontal and
vertical dimensions by a factor of 2. That is to say, for a 2x2 square of pixels, there are 4 Y
samples but only 1 U sample and 1 V sample. Bit-depth for each sample is 8-bit for NV12
and 10-bit for XV15. The Y plane is first in memory. A combined CbCr plane immediately
follows the Y plane in memory.
In NV16 and XV20 formats, chroma planes are sub-sampled only in the horizontal
dimension by a factor of 2. Thus, there is the same amount of lines in chroma planes as in
the luma plane. For a 2x2 group of pixels, there are 4 Y samples and 2 U and 2 V samples
each. Bit-depth for each sample is 8-bit for NV16 and 10-bit for XV20. The Y plane is first in
memory. A combined CbCr plane immediately follows the Y plane in memory. The CbCr
plane is the same width and height, in bytes, as the Y plane.
Aside from the pixel format, a video buffer is further described by a number of other
parameters (see
). For this design, the relevant parameters are width, height, and
stride as the PS display pipeline does not allow for setting an x or y offset.
