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Optical Flow

A project log for VR Camera: FPGA Stereoscopic 3D 360 Camera

Building a camera for 360-degree, stereoscopic 3D photos.

Colin PateColin Pate 11/15/2017 at 18:120 Comments

It's been a while since my last update, not because I've stopped working on this project, but because I've been sinking almost every minute of my free time into rewriting the stitching software. The output is finally decent enough that I figured it's time to take a break from coding, install the other four cameras, and write a new post about what I've been doing. A few days after my last log entry, I happened across Facebook's Surround 360 project, an amazing open-source 360 camera. The hardware was pretty cool but it cost $30,000, which was a little outside my budget. What really caught my eye was their description of the stitching software, which uses an image processing technique called "optical flow" to seamlessly stitch their 360 images.

Optical flow is the movement of a point or object between two images. It can be time-based and used to detect how fast and where things are moving. In this case, the two images are taken at the same time from different cameras that are located next to each other, so it tells us how close the point is. If the object is very far away from the cameras, there will be no flow between the images, and it will be at the same location in both. If the object is close, it will appear shifted to the left in the image taken from the right camera, and shifted to the right in the image taken from the left camera. This is also known as parallax, and it's part of how your eyes perceive depth.

With optical flow, we can create a "true" VR image. That means that every column of pixels ideally becomes the column of pixels that would be seen if you physically placed a camera at that location. This allows a seamless 3D image to be viewed at any point in the 360 sphere. With my previous stitching algorithm,  only the columns at the center of the left and right eye images for each camera were accurate representations of the perspective from that angle. Between these points, the perspective became more and more distorted as it approached the midpoint between the two cameras. If we use optical flow, we can guess where each point in the image would be located as seen by a camera at column in the image.

This is easier said than done, however. I decided to roll my own feature-matching and optical-flow algorithm from the ground up, which in hindsight, may have been a little over-ambitious. I started off developing my algorithm using perfect stereo images from a set of example stereo images, and it took two weeks to get an algorithm working with decent results. Using it with the images from the 360 camera opened a whole other can of worms. It took redoing the lens calibration, adding color correction, and several rewrites of the feature-matching algorithm to get half-decent results.

Here's a re-stitched image using the new algorithm:

As you can see, there's still plenty of work to be done, but it's a step in the right direction!  I'll upload my code so you can check out my last few weeks of work.

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