Contrast Detection Probability (CDP)
Contrast Transfer Accuracy (CTA)
Image Quality FactorsIntroduction
Contrast detection probability (contrast transfer accuracy as it is now known) refers to the camera system's ability to correctly identify an object's contrast in its field of view. For example, advanced driver assistance systems (ADAS) must have the capability to identify different objects in their field of view. Without proper identification, the camera system may make an incorrect adjustment and create a dangerous situation. With that in mind, it is imperative to test the CDP (CTA) of a camera system to ensure high performance and safety.
The background of CDP/CTA
Until recently, the automotive industry lacked international standards for testing and evaluating these camera systems' image quality. As a result, the IEEE-P2020 working group* was formed to establish parameters that will ultimately lead to the publication of an international standard on automotive image quality.1
At this time, the group has defined specific key performance indicators (KPIs), including CDP/CTA, and published a white paper that outlines the group's work towards an international standard for automotive image quality testing and specifications.2
CDP/CTA is the most important key performance indicator (KPI) that the group is currently working on. First presented by Dr. Marc Geese in 2018, CDP/CTA is designed to be independent of the components tested so a test team could use it to perform the same performance tests on different subsystems (e.g., lenses, sensors, windshields, etc.).
*Disclaimer: The opinions contained within the IEEE-P2020 working group solely represent the views of this working group and do not necessarily represent the position of either the IEEE or the IEEE Standards Association.
Why CDP/CTA over SNR?
Traditionally, SNR is used to describe a camera's performance when reproducing the contrast of a scene. However, for a setting with extremely high dynamic range, such as those often encountered by ADAS (e.g., a scene with flickering light), the SNR value is not the best indicator of full system performance in these situations.
SNR is a well-established metric that measures both the intended signal and the background noise that obscures the signal. Unfortunately, it is hard to link the SNR values directly to an application.
So, while most engineers are familiar with the required SNR for their industry, the values themselves still cannot be directly linked to an imaging system's task to detect a specific contrast under a particular condition. CDP/CTA, however, can provide this missing information and thus, vastly improve the design requirements that are now based around facts as opposed to a "best guess.3
SNR drops are another issue that often appears in ADAS cameras. These drops are a phenomenon of a multi-exposure HDR algorithm that results in a non-linear SNR curve. This issue is especially prevalent in luminance regions where multiple images are combined, often leading to an SNR drop from an acceptable high value to a low critical value.
Additional issues with SNR also arise when predicting how an ADAS camera system will perform using an SNR value. The SNR drops, for instance, do not account for other elements in the scene, such as fog or dust between the camera and the object(s).
The development of CDP/CTA is a result of SNR being an inadequate performance measurement for the automotive industry. Unlike SNR, CDP/CTA directly links the system under test and can describe its performance regardless of the natural conditions. CDP can also provide an overview of multiple image quality components using the same metric.
Numerous tests are currently underway to support CDP/CTA as an internationally recognized test procedure for automotive camera systems.
How to test for CDP/CTA
As CDP/CTA is a relatively new concept, there are limited tools on the market that can measure and provide accurate results. For example, a typical image quality test setup using a test chart can only offer a limited number of data points in a synthetic scene. However, dynamic scenes, such as those experienced by automotive cameras, are better analyzed using numerous data points from the scene. To recreate dynamic scenes in a test lab, our engineers have developed the DTS.
Dynamic Test Stand (DTS)
Essentially, the DTS can test an extensive dynamic scene as opposed to a synthetic scene in a test lab environment. In a test lab, the DTS can precisely recreate driving conditions that would be much harder to duplicate in an unpredictable outdoor setting or with a single test chart.
The DTS tests for CDP/CTA using six separate illumination boxes, each with 36 gray patches for 216 different intensities with a dynamic range of 120 dB. Each box can show different intensity levels simultaneously while creating a more dynamic testing situation similar to what an automotive camera will face in a natural environment. One image set can provide numerous data points for CDP/CTA, giving you much more insight into the system's complete performance under test.
The DTS has the only integrated analysis software specifically for evaluating CDP/CTA on the market. The software creates a heat map based on the data points collected (see images below) from the device under test. These heat maps show each data point plotted depending on the system's performance at a wide range of luminance and contrast. Yellow shows where the system performs highly, while blue indicates lower performance. It is important to note that each system under test has different requirements. So even a map that appears to have lower performance may, in fact, still be acceptable for the specific camera under test.
Benefits of the DTS and its CDP/CTA evaluation
The DTS and its evaluation software give automotive companies a reliable way to characterize their systems and use the results to evaluate their designs' performance and safety in a test lab setting.
Other benefits include:
- Numerous data points for each measurement
- Eliminating the majority of road testing
- Validating the specifications of finished products
- It easily allows manufacturers to make specification requirements via heat maps
- Output .xml file with CDP/CTA, OECF, and PDF (probability density function) results
The DTS can also generate flickering light sequences to evaluate the camera system's response to changing light intensities. For more information, please see our flicker article.
Conclusion
Contrast detection probability or contrast transfer accuracy refers to an automotive camera system's ability to correctly identify an object's contrast in its field of view. Without proper identification, the system may make an incorrect adjustment and create a dangerous situation.
CDP/CTA is one of the current KPIs under development by the IEEE-P2020 working group. The group was formed to enact these testing parameters to lead to an internationally recognized automotive image quality standard eventually.
The Dynamic Test Stand (DTS), one of the first CDP/CTA testing solutions on the market, has been developed in accordance with the proceedings of the IEEE-P2020 group. The DTS can generate a dynamic testing scene using 216 intensities at 120 dB to test a camera system's CDP/CTA performance. It is also equipped with CDP evaluation software that uses heat maps to show exactly how the system performs using different contrast and luminance.
The end goal of these automotive ADAS camera systems has always been to improve our daily transportation efficiency and safety. But before we can implement these systems into everyday vehicles, they must first be rigorously tested to ensure the avoidance of dangerous situations.