Rolling Shutter vs. Global Shutter in Machine Vision

With the widespread adoption of machine vision in fields such as industrial automation and autonomous driving, the demand for higher image quality and accuracy in cameras has steadily increased. Applications in smart manufacturing, autonomous driving, and security surveillance require high levels of image precision and real-time processing, which drive the advancement of camera technology.

The camera sensor serves as the "eyes" of machines, enabling visual recognition, and its imaging performance is influenced by various parameters, with exposure time and shutter speed being particularly crucial. Shutter speed controls the exposure duration; the shorter the shutter time, the better it can capture fast-moving objects, while longer times are more suited to low-light environments.

Currently, there are two main shutter technologies: Rolling Shutter and Global Shutter. This article focuses on these two primary exposure technologies, discussing their working principles, advantages, and suitable applications, providing users with a reference for selecting the appropriate camera type.

What is a Rolling Shutter?

A rolling shutter captures an image by scanning each row from top to bottom, rather than capturing all pixels simultaneously. This results in a slight timing difference for the exposure of each row of pixels. When the camera or object is moving at high speeds, this row-by-row scanning may cause image distortion, leading to the "Jello Effect" or motion blur. The Jello Effect is a common type of image distortion where objects in the image appear warped or tilted, giving them a wavy, jelly-like appearance. Despite this, the high resolution and relatively low cost of rolling shutters make them highly suitable for applications where high resolution is needed but dynamic performance is less critical, such as static image capture or home monitoring.

Working Principle

The digital rolling shutter operates similarly to a mechanical rolling shutter in traditional film cameras. Just like the rotating metal disk in a film camera sequentially exposes each part to gradually allow light to enter, a digital rolling shutter’s image sensor exposes each row in succession, progressively capturing the image. Because there is a difference in exposure timing across rows of pixels, this row-by-row scanning can easily lead to distortion in high-speed dynamic scenes.

The illustration below demonstrates the step-by-step exposure process of a rolling shutter. Initially, the shutter gradually uncovers a portion of the film (or image sensor), allowing that part to be exposed to light. As the shutter progressively opens, each part of the frame completes its exposure sequentially until the shutter finally closes. This row-by-row exposure approach helps in understanding how a digital rolling shutter captures images step by step.

Figure: Rolling shutter exposure process

In a rolling shutter, each row of pixels starts exposure sequentially, resulting in different readout times from the top to the bottom of the frame. This “row time” depends on the sensor’s design parameters, including the sensor’s maximum frame rate (fps), image resolution, and configuration. For example, when using an 8 Megapixels USB Camera with Microphone for object recognition, row time affects image accuracy, especially when objects move quickly. Row time can be estimated using the following formula:

row_time = 1 / (maxFps * rows)

For this camera, with a resolution of 1920×1080 and a frame rate of 30 fps, the row time is approximately:

row_time=1/(30×1080)≈30.86μs

Tip: 1 μs (microsecond) equals one-millionth of a second (10^-6 seconds).

The larger the row time, the greater the capture time difference from the top to the bottom of the image, meaning each row of pixels has a larger exposure time difference, causing more noticeable distortion for fast-moving objects. The smaller the row time, the less image distortion occurs, resulting in an overall effect closer to the actual scene.

Factors Affecting the Jello Effect

The severity of the jello effect caused by a rolling shutter depends primarily on the following factors:

• Object’s Speed: The faster the movement, the more noticeable the distortion.
• Object’s Size: Larger objects show more pronounced warping in the image.
• Resolution: Higher resolution with the same frame rate results in shorter row times, which can relatively reduce distortion.

As shown in the image below, the typical manifestations of the jello effect are illustrated: due to the camera capturing each row sequentially, rotating fan blades and moving cars appear tilted and bent in the image. This distortion is particularly noticeable with fast-moving objects, stemming from the slight time differences in the exposure of each row of pixels.

Figure: Jello effect in rolling shutter

What is a Global Shutter?

A Global Shutter is an image sensor technology designed to capture information from all pixels in a frame simultaneously. Unlike the row-by-row scanning method of a rolling shutter, a global shutter exposes all pixels at the same time and then reads out the data together. This synchronized exposure effectively eliminates image distortion effects, such as the “rolling shutter effect” or "jello effect," in high-speed motion scenes. It can be likened to a snapshot where the entire scene is “captured” in a single instant, rather than scanned row by row, ensuring that every part of the image aligns with the actual scene.

Working Principle

A global shutter sensor integrates a shutter mechanism within the photosensitive device, allowing all pixels to open and close simultaneously. This technology is especially important when capturing fast-moving objects or complex dynamic scenes, as it prevents distortion or motion blur caused by row-by-row exposure, ensuring image integrity. With the synchronized exposure of a global shutter, objects maintain their original shape and position within the image.

As shown in the illustration, each frame captured by a global shutter records the entire scene at the same moment (during the exposure phase) before sequentially reading out the data. This method effectively reduces distortion in dynamic scenes, as all pixels are exposed at the same time rather than through row-by-row scanning.

Figure: Global Shutter Timing - Synchronized exposure and sequential data readout for each frame

Rolling Shutter vs. Global Shutter

After introducing the principles of global and rolling shutters, we will further explore the differences between these two technologies in terms of imaging quality, dynamic performance, and suitable applications. Understanding these distinctions can help engineers and users select the most appropriate camera type for specific applications, optimizing image quality and cost efficiency.

1. Comparison of Exposure Methods

A global shutter captures images by exposing all pixels simultaneously, ensuring that the sensor accurately records objects regardless of their speed, thus preserving image integrity. In contrast, a rolling shutter exposes each row of pixels in sequence, capturing information one row at a time. Although this method can reduce sensor costs, it is prone to "jello effect" or motion blur in high-speed scenarios, which can lead to image distortion.

Figure: Rolling shutter reads row-by-row; global shutter captures entire frame simultaneously

2. Image Distortion in Dynamic Scenes

In high-speed dynamic scenes, the advantages of a global shutter are particularly notable. Due to its synchronized exposure, moving objects do not appear distorted or blurred in the image, making it ideal for applications that require high-precision imaging, such as industrial machine vision, drone navigation, and autonomous driving. Rolling shutters perform well in static or low-speed scenarios, but when capturing fast-moving objects, the row-by-row exposure delay can cause objects to appear stretched or distorted in the frame. This can be a disadvantage in applications that require image consistency.

Figure: Motion distortion in fast action capture

3. Comparison of Key Features

We have further summarized the core characteristics, advantages, and disadvantages of each shutter type to help you more intuitively compare their suitability and selection criteria.

How to Choose the Appropriate Shutter Type

Global shutters, capable of exposing an entire scene simultaneously, are ideal for capturing high-speed motion. They are typically used in applications where maintaining consistent image quality is critical, such as sports events, traffic monitoring, and scientific research that requires precise capture of fast-moving actions. In these scenarios, image stability and accuracy are essential, as any form of motion blur or distortion can compromise the integrity of the captured scene. A global shutter ensures uniform exposure across the frame, providing a stable and reliable image even under dynamic conditions.

In industrial environments, particularly in machine vision systems, global shutters are often preferred. These systems are used to inspect high-speed objects on production lines, conduct automated inspections, and perform quality control tasks. Even minor image distortions can affect detection accuracy, making the consistency provided by a global shutter essential. The ability of a global shutter to capture undistorted images in high-speed settings is crucial for accurately representing the shape and position of objects within the frame. This level of precision is vital for the reliability and accuracy of industrial inspection systems, as it allows for consistent performance in detecting defects or irregularities. Thus, global shutters are favored in complex, dynamic environments where high-precision imaging is necessary, enabling these systems to maintain the high standards required in modern industry.

Rolling shutters, by contrast, are widely used in CMOS sensors due to their row-by-row scanning method, which allows for higher frame rates and lower power consumption. These characteristics make them well-suited for everyday imaging and static scenes, such as landscape or still-life photography, where object movement is minimal. In these applications, the relative lack of fast motion means that the rolling shutter effect does not significantly impact image quality. Moreover, the lower cost and reduced power requirements of rolling shutters make them an economical choice for consumer-grade imaging devices and applications where precise timing and motion accuracy are less critical.

Conclusion

Global shutter and rolling shutter are the two primary technologies in image sensors, each catering to specific imaging needs across various applications. In fields that demand precise, high-speed dynamic imaging—such as industrial automation, autonomous driving, and professional surveillance—global shutters are indispensable. Their synchronized exposure capability significantly reduces image distortion, making them the preferred choice for these high-stakes environments. Meanwhile, rolling shutters, with their lower cost and higher resolution, are widely used in static or low-dynamic scenarios, making them an ideal solution for consumer-grade and budget-sensitive projects.

As the demand for high-quality imaging and intelligent applications continues to grow, the use of global shutter technology will only increase, especially in industries requiring the capture of fast-moving objects. Looking ahead, as technology advances and production costs decrease, more applications are expected to transition to global shutter technology to improve system reliability and image quality. However, in scenarios where cost is a priority and dynamic requirements are low, rolling shutters will maintain their competitive edge.

In summary, understanding the key differences and suitable applications of global and rolling shutters helps users make more informed camera selection decisions in practical applications, achieving the best balance between performance and cost.