Image Quality Factors

Distortion

Image Quality Factors
  1. Introduction
  2. How does distortion occur?
  3. How to measure distortion
    1. TV distortion method (ISO 9039)
    2. Line geometric distortion method (ISO 17850)
    3. Local geometric distortion method (ISO 17850)
  4. Conclusion
  5. References

Introduction

Image distortion occurs when the straight lines of an image appear to be deformed or curved unnaturally. There are three types of lens distortion called barrel, pincushion, and waveform (also known as moustache) distortion. It is important to note that distortion occurs differently depending on the lens system and whether the lens can or cannot be removed from the camera.

In general, the function of rotationally symmetric optical systems is to form an image that is geometrically similar to the object. There are a few exceptions for some particular systems, such as fish-eye lenses, where geometric conditions are deliberately not maintained. Ideally, this function is accomplished according to the geometry of perspective projection. Deviations from the ideal image geometry are called distortion.

Figure 1: An example of barrel and pincushion distortion

How does distortion occur?

Distortion normally occurs from aberrations near the edges of an image. Each type of distortion usually develops through different variables. Barrel distortion, for example, is often the result of a lens in at full zoom while pincushion distortion occurs most often from telephoto lenses. Waveform distortion is the result of a large angle camera that is in zoom mode where it combines both barrel and pincushion distortion.

How to measure distortion

ISO 903911 is the standard that defines methods to measure a lens that is separated from a camera. Sometimes, however, the lens cannot be removed from a camera (such as in a mobile phone) and thus the time-consuming methods described in ISO 9039 will not be adequate. As a result, ISO 178502 was developed to define methods to measure distortion using a camera lens combination.

TV distortion method (ISO 9039)

The oldest method that uses the camera-lens combination is the TV distortion method, which was first specified for the analysis of TV camera systems. This method requires a test chart with a regular grid of geometric structures such as the ones shown below.

TE260 dot chart
TE251 grid chart
Figure 2: The TE260 (left) dot chart and the TE251 grid chart are two popular charts for measuring distortion

The TV distortion method is typically based on a system that shows a steadily increasing distortion from the image center to the corners. The bending of a straight line in the original image is quantified at the top edge of the image (Figures 3 and 4). The ratio of the bending over the height of the image multiplied by 100 is the percentage of picture height distortion. This method is compliant with the method described in EBU Tech 32493.

$$D=\frac{\Delta H}{H}\cdot 100$$

Picture height of barrel distortion
Figure 3: Picture height distortion with barrel shape (negative)
Picture height of pincushion distortion
Figure 4: Picture height distortion with pincushion shape (positive)

Line geometric distortion method (ISO 17850)

Certain lens systems (especially small ones in mobile devices) are corrected for distortion at the maximum image height. These systems show the highest level of distortion at lower distances from the optical center. The resulting type of distortion is oftentimes a mixture of barrel and pincushion distortion (Figure 5) and described with the term wave distortion.

Line geometric distortion
Figure 5: Line geometric distortion creating the pincushion effect

For systems with a wave distortion, the measured picture height distortion can be zero or close to zero even if a strong distortion is visible in the image. The line geometric distortion method can be broken down into three separate methods to measure the distortion of these systems as specified in ISO 17850.

1) Horizontal line distortion

This method is applicable when the vertical line Ai is located closer to the vertical line at the center of the image than to Bi, use formula 2: $$Dhi=\frac{\left(Bi-Ai\right)}{2V}\times100%$$

Otherwise, use formula 3: $$Dhi=\frac{\left(Ai-Bi\right)}{2V}\times100%%$$

Where:
i   =   a suffix representing each picture height;
Ai, Bi, and V shall be represented by the number of pixels from the output image.

2) Vertical line distortion

Use this method when the horizontal line αi is located closer to the horizontal line through the center of the image than to βi, use formula 4: $$Dvi=\frac{\left(\beta i-\alpha i\right)}{2V}\times100%$$

Otherwise, use formula 5: $$Dvi=\frac{\left(\alpha i-\beta i\right)}{2V}\times100%$$

Where:
i is a suffix representing each picture width;
αi, βi, and V shall be represented by the number of pixels in the output image.

3) Total line distortion

The total line distortion is calculated as: $$\left|D_{\mathrm{line}}i\right|=\sqrt{\mathrm{Dh}\mathrm{i}^2+\mathrm{Dv}\ \mathrm{i}^2}%$$

The sign for Dlinei represents the higher absolute value of either the horizontal or the vertical line distortion.

Local geometric distortion method (ISO 17850)

The local geometric distortion method is utilized when a single number for the distortion is not sufficient. In other words, a function for the distortion is needed to correct the distortion in the image processing. Keep in mind that when a specific image height needs to be addressed, the local geometric distortion is more reliable.

To measure the local geometric distortion, it is assumed that the distortion close to the optical center is zero and a regular grid can be calculated based on the geometric positions of the 9 structures (3x3) in the center of the image. This grid is expanded to the whole image and defines the nominal positions for each of the structures.

Local geometric distortion H-values
Figure 6: H-values for determination of the local geometric distortion

The distortion is then measured using the following formula:

Where:

H* is the distance of the dot from the image center

H is the nominal distance of the dot from the image center based on an expanded regular grid.

If a single number is derived from the local geometric distortion, the maximum distortion measured for any of the geometric structures in the image is the one that is reported.

This method is described in detail in ISO 17850.

Conclusion

Image distortion is when the straight lines of an image appear to be deformed or curved unnaturally creating different types of distortion including barrel, pincushion, and waveform. Distortion is often the result of the geometrics of the lens and can greatly disrupt the quality of the image.

It is important to test and analyze lens distortion to ensure high image quality. We recommend following ISO 17850 and using proper test charts to capture various images using different lens functions e.g., zoom and then analyze the results using an evaluation software (e.g., iQ-Analyzer) to see where improvement can be made to the camera system.