Optical flow camera

[saihv]

About

Adds an optical flow camera that computes pixel velocity through a material/HLSL code. Inspired by carla-simulator/carla#4104, and the discussion at https://github.com/EpicGames/UnrealEngine/pull/6933 (thanks to @finger563 for the discussion and useful code!)

This PR does not make use of UE's native velocity buffer due to issues in its calculation of motion vectors (see https://github.com/EpicGames/UnrealEngine/pull/6933), and hence, can only compute optical flow based on camera motion and not dynamic objects. This is meant as an intermediate step while UE fixes the issues with velocity buffer (more discussion at https://github.com/florian-world/VelocityUE4).

The flow data is exposed in two ways, through two new 'cameras':

1. OpticalFlowCaptureComponent returns a 2-channel image where the channels correspond to vx and vy respectively. Corresponding material: HUDAssets/OpticalFlowMaterial.
2. OpticalFlowVisCaptureComponent is the other camera which converts the flow data to RGB for a more 'visual' output. Corresponding material: HUDAssets/OpticalFlowRGBMaterial.

How Has This Been Tested?

The two new cameras can be targeted through their ids 8 and 9; and can be visualized either through subwindows or through the normal Python image request API.

Pending accuracy tests of per-pixel flow.

Screenshots (if appropriate):

opticalflow.mp4

[jonyMarino]

@saihv What is the difference between OpticalFlow and OpticalFlowVis?

[saihv]

@jonyMarino

OpticalFlow is a 2-channel image where the channels correspond to vx and vy respectively (camera optical flow velocity). This is the data that is the actual raw optical flow. Corresponding material: HUDAssets/OpticalFlowMaterial.
OpticalFlowVis is a modified version that converts the flow data to RGB for a more 'visual' output that is easy to understand. Corresponding material: HUDAssets/OpticalFlowRGBMaterial.

[zimmy87]

Tested locally and it works for me; thanks for the contribution @saihv!

[alonfaraj]

Hi @saihv,
This is a great feature, thanks!
Maybe it's worth update the documentation , as well as the ros wrapper code.

[KaiXin-Chen]

Hi,
Thanks for the great functionality. However, there are something that I don't quite understand about it, mind giving me some clue on this?

1. If we store optical flow in the format of png ( using the argument PixelsAsFloat= false). it doesn't make sense for me of saving optical flow information in the format of uint8 (as optical flow can be negitive and should be a float)

2. If we store it using .pfm (PixelsAsFloat= true), the pfm file only have one channel, which also doesn't make sense to me.

Mind letting me know what I did wrong? Thanks a lot !

[artths]

Hi, Thanks for the great functionality. However, there are something that I don't quite understand about it, mind giving me some clue on this?

1. If we store optical flow in the format of png ( using the argument PixelsAsFloat= false). it doesn't make sense for me of saving optical flow information in the format of uint8 (as optical flow can be negitive and should be a float)
2. If we store it using .pfm (PixelsAsFloat= true), the pfm file only have one channel, which also doesn't make sense to me.

Mind letting me know what I did wrong? Thanks a lot !

It's not about storing. Response itself seem to have only 1 channel. This feature is broken.