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Feb 21, 2013, 05:01 AM
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Optical flow notes

So the next vehicle needs to use optical flow + sonar & the last board needs to be scrapped. The PX4FLOW module would be the best solution, but it's probably too heavy, necessitating a new board with smaller camera, motor control, IMU, & radio.

There are some optical flow algorithms on

They're algorithms from 1984 intended for movement of 1 pixel. The wisdom of the time said your best bet is to scan every pixel in the image for movement of 1 pixel so any ideas about logarithmic macroblock searches, blob detection, or edge detection probably won't work.

PX4FLOW says it scans a binned 188x120 image at 250hz. The algorithm in the PX4FLOW is described in

It's assembly language intensive. Their camera supports greyscale, allowing the fastest DMA capture.

It does basic, exhaustive absolute difference comparisons of 8x8 macroblocks in a search radius of 4 pixels. It scans 64 8x8 macroblocks, so only 1/5 of the entire image is scanned, but the scanned regions are at high enough resolutions to resolve textures.

Instead of averaging all 64 motion vectors, it takes the most frequently occurring vector. Then it does a subpixel absolute difference comparison using the closest macroblock to the final result to get subpixel motion.

The trick is using a gyro to cancel out picture movement from banking & turning, yielding only movement from horizontal motion.

The PX4Flow source code would be nice, especially for the assembly language routines. Using the smaller, easily obtained TCM8230, the nearest resolution drops to 160x120, but it needs software to output greyscale & it can't go higher than some 70fps.
Last edited by Jack Crossfire; Feb 21, 2013 at 05:12 AM.
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