Sunday, 28 February 2016

i.MX6SX - Video processing via VADC & PXP (UDOO Neo)

Unlike the rest of the i.mx6 family, the i.mx6sx has limited hardware video decoding support for example there is no h.264 or mpeg support. The i.mx6sx hosts a simpler video analogue to digital converter (VADC) which allows the conversion of a NTSC or PAL signal to a YUV444 format. The YUV444 is slightly unusual in that its encoded as a 32 bit value which equates to YUV32 in V4L2 terms. Given the unusual YUV32 output decoding becomes more interesting because it heavily relies on the PXP (Pixel Pipeline) engine for colour space conversion and onward rendering to a display. The PXP can be considered a smaller brother of the IPU (Image Processing Unit) as found on the rest of i.mx6 family. In addition to colour space conversion the PXP has the capability to flip (horizontal/vertically), scale and blend/overlay images from multiple sources. The one caveat is that PXP on the i.mx6sx has limited input and output formats that it can accept.



Engine check sensor image
To give you an idea of the what the PXP can do, the video above shows the output of a NTSC camera blended with a graphics overlay. The camera is actually a parking (reversing) camera (courtesy of the UDOO team), this is highlighted by the red/yellow/green lines and the blinking 'STOP' text which form part of the camera output. Given the automotive theme the graphics overlay represents a engine check sensor dash. The output video (720x480) fits nicely on the UDOO 7" LVDS display. As the UDOO Neo is targeted towards sensor technology (IOT),  we use the tilt angle from the gyroscope to determine the amount of alpha blending to apply (the graphics fade away or become fully visible).  Furthermore depending on axis of the tilt the combined video output is flipped horizontally or vertically (in real time) noticeable by the 'STOP' text and graphics appearing reversed or upside down.

Given the above, I have put together a simple example that demonstrates to how read the camera output and pass it to the PXP for onward processing. The example is partially based on existing freescale code snippets but modified to use the PXP and then simply renders to a frame buffer. The example will only run against a frame buffer and not on X11. Unlike the above video, the example does not use GPU hardware acceleration therefore the CPU usage will be high. In order to build the example you require PXP header/library to be built and installed which are part of the imx-lib package.

To build:

make clean
make

To run:

./imx6sx_vdac_pxp

Optional parameters

-t Time in seconds to run
-h flip image horizontally
-v flip image vertically
-i invert colours


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