The Effect of the Rolling Shutter and Drone Modelling
What Sensor Should Drone Camera for 3D Modelling Have - Rolling Shutter or Global Shutter
Many CMOS (Complementary Metal-Oxide-Semiconductor) sensors use rolling shutter.
What is "rolling shutter"? ЗFor those who are not familiar, “rolling shutter” is a technical term referring to the way the image sensor scans the image. If the sensor employs a rolling shutter, this means the image is scanned sequentially, from one side of the sensor (usually the top) to the other, line by line. Many CMOS sensors use rolling shutters. In contrast to that, “global shutter” is the technical term referring to sensors that scan the entire area of the image simultaneously. The vast majority of CCD (Charge Coupled Device) sensors employ global shutter scanning. The overwhelming prevalence of CMOS sensors in modern cameras explains the increasing appearance of rolling shutter artifacts in videos and films made recently.
OK, so your camera has a rolling shutter CMOS—who cares? Well, it’s useful to know the limitations imposed by a rolling shutter through the artifacts that it can bring about in your captured images. Rolling shutter artifacts generally manifest themselves in scenes with lots of motion, be it through camera movement, objects moving quickly through the scene and, in the worst case, with unstabilized handheld camera movements. The most common manifestations that I’ve come across involve scenes with vehicles moving quickly across the frame, and quickly spinning objects. Vehicles that drive through your frame can be rendered with slanted lines where they should be straight. For example, a rectangular city bus will be rendered more akin to a parallelogram. Spinning objects in the frame are a little more difficult to describe; they bend back on each other, creating strange arcing patterns. You may notice this in drone camera footage if airplane propellers are in the frame. Unstabilized handheld footage, especially with longer focal lengths, will take on an appearance akin to gelatin, in which the image bounces around in a warping fashion with the movement of the camera. As you may have guessed, the faster the movement of the subject in question or the camera, the more exaggerated the artifacts become.
The buildings photographed by a fast-moving car here, as you can see, are skewed in one direction. The guardrail pegs, too. The larger the sensor, the greater the potential for the effect. In this regard, why use a sensor that seems to only bring doom and gloom to your images over the older global shutter design of CCDs, right? The answer is - everything in this world, produced by man, is the fruit of compromise. For example, if we compare two equivalent sensors, one with a global shutter and the other with a rolling one, we will probably see that the rolling shutter sensor will have less noise and a wider dynamic range, while generating less heat.
A similar comparison was made by well known YouTuber to test the effect of the rolling shutter effect when processing aerial photos with a drone. He compares two popular brands of drones that perform an identical mission, with cameras equipped with identical sensors but with different types of shutters. He noticed that as a result of processing the photos of the drone equipped with a camera with a global electronic sensor, the dencecloud is significantly larger, and its density is many times higher. The accuracy (GSD) * at the same altitude is almost twice as low (0.47 cm vs. 0.77 cm) as that of a drone with a camera with a rolling sensor.
*/ Ground sampling distance (GSD) - The distance between two consecutive pixel centers measured on the ground. The higher the GSD value of the image, the lower the spatial resolution (accuracy) of the image and the less visible the details.