Contrast Distortion

An artifact called contrast distortion occurs due to lens imperfections and optical phenomena like diffraction and chromatic aberration. The optical imperfections cause a slight blur between neighboring pixels even at a “perfect” focus distance. The pixel blurring increases depending on the size of the camera aperture and the corresponding depth of field and distance from the correct focus distance.

Note

Contrast distortion is caused by blurring in the camera lens.

Contrast distortion appears when we have an abrupt change from a highly absorptive to a reflective surface. This could be the black to white transition on a checkerboard or regions with specular reflections, like a shiny metal cylinder. The errors are visible as surface distortion/deformation artifacts in the 3D point cloud. This artifact is also called the Laplacian effect because the distortion curve in the point cloud looks like a laplacian curve.

The image below depicts a 2D image where a Laplacian filter is applied on the bottom half to demonstrate the intensity transition. Notice how the filter highlights the edges, i.e. transitions in the color image wherever the intensity changes.

Contrast distortion occurs along the image x-axis (camera’s baseline) but not along the y-axis. The strength of the contrast distortion depends on the gradient of the change in intensity within a small region of pixels. A sharp or significant gradient produces a more pronounced distortion in the point cloud, as seen in the image below.

Warning

Contrast distortion only occurs along the camera’s baseline.

Let us look at two similar cylinders, A and B. Cylinder B has a greater difference in brightness between dark and bright areas along its body than cylinder A. Additionally, the intensity transition area of cylinder B is narrower, and therefore, the brightness gradient is steeper. These two characteristics enhance the contrast distortion’s undesirable effect on the point cloud of cylinder B; the artifact gets taller and steeper.

The different reflection directions create a strong contrast distortion effect, perceived as a black stripe in the second image. In the resulting point cloud to the right, the over-exposure along the surface of the cylinder creates a “ridge” that distorts and deforms the cylinder from its actual, cylindrical shape.

To correct and/or remove this artifact, check out how to Deal with the contrast distortion.