Blooming - Bright spots in the point cloud

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Introduction

Blooming is an effect that occurs when extremely intense light from a point or region hits the imaging sensor and results in over-saturation. When the intensity gets too high the pixel gets “blinded” and the light from the pixel will spill into its neighbors with an intensity and radius that is proportional to the intensity of the incident light. This effect typically happens when a camera is pointed either directly, or indirectly by reflection, at a strong light source such as the sun or a strong lamp. Blooming can occur in the entire image or only in parts of an image. In the image below, the sun is reflecting in the ocean which causes a larger region of the image to become completely white. Distinguishing details within the affected regions becomes very difficult.

In this other image of a dandelion below, the starch of the dandelion appears to be bright orange in the region that is in front of the sun. The actual origin of this orange light comes from the pixels on the sides of the starch that are looking directly at the sun. The light from the sun is so intense that the light in those pixels spills into their darker, neighboring pixels that are seen in the starch.

Specular or reflective surfaces in the image scene which align such that the projector light is reflected straight back into the camera can cause blooming. Depth from over-saturated pixels cannot be accurately calculated and will, therefore, be filtered out. It is recommended to try to avoid these effects if possible.

Specular or reflective surfaces in the image scene which align such that the projector light is reflected straight back into the camera can cause blooming.

A typical scenario is when the camera is positioned to look straight down at a plane, such as a table. In this case, the projector may blind the camera and over-saturate it. The effect will appear as a completely white spot in the image which translates to a “hole” in the point cloud with no points, as can be seen in the image below.

How to deal with blooming

There are three ways to handle blooming:

  1. Position and angle the camera such that direct reflections from the projector or ambient light sources do not directly reflect back into the camera.

  2. Using 3D HDR at the expense of added acquisition time by adding a 3D frame that is exposed to cover the blooming highlights.

  3. In the event that the background is the source of blooming, change the background material to a more diffuse and absorptive material (ref. Optical properties of materials ).

Changing the angle of the camera is a cheap and efficient way of dealing with blooming. As mentioned above, It is preferable to offset the camera and tilt it so that the projector and other light sources do not directly reflect back into the camera as shown on the right side of the image below.

By simply tilting the camera, the data lost in the over-saturated region can be recovered, as seen in the right side of the image above. The left image below shows a point cloud taken when the camera is mounted perpendicular to the surface, while the right image shows the scene taken at a slight tilt.

 

 

A simple rule of thumb is to mount the camera so that the region of interest is in front of the camera as shown in the image below:

Quick tip: mount the camera so that the region of interest is in front of the camera.

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