[카메라] rolling shutter artifact

http://www.redhax.net/wiki/Understanding_Rolling_Shutter_Artifacts

 Understanding Rolling Shutter Artifacts

 

The image is scanned acrossed each row of pixels in order from top to bottom.On digital cinema cameras that have a rolling electronic shutter, strobes and other similar phenomenon can cause the image to be split across a horizontal line. This happens during two brief periods where the camera is either begining an exposure or reading an image from the sensor chip. The split is called a rolling shutter artifact. There is no actual shutter involved, the shutter refers to a process occuring on the sensor chip that simulates the behavior of a mechanical shutter while accomplishing the neccessary task of transmitting the image data from the sensor chip to the rest of the camera.
Rolling shutter artifacts also exist in cameras that have a mechanical rotary shutter. However because the edge of the mechanical shutter is between the back element of the lense and the film plane it is very out of focus and creates a gradual line. Digital shutters create a sharp line that is only slightly softened by how gradually the flash comes up to power.

How a Digital Rolling Shutter Works

A digital camera sensor is composed of a grid of individual light sensor sites called photosites. Each photosite site corresponds to a pixel or sample of the image. As light particles bombard each photosite they knock electrons from one part of the sensor to another and create a small pocket of electrical charge. The sensor chip has the power to:

 

• Release collected charge from each photosite and start fresh.
  
• Measure collected charge on each photosite and thus measure the amount of incoming light.
  
• Transmit that measurement to the other circuitry of the camera so that it can be reconstructed as a digital image.

The activity of the entire sensor chip can be divided into four parts.

1. The sensor scans across the image and resets each individual sensor so that it begins freshly collecting charge.
   
2. The sensor waits as charge is collected. The length of this delay determines the shutter speed.
   
3. The sensor scans across the image and reads each individual sensor and transmits the information.
   
4. The sensor waits until the appropriate time to begin a new exposure.

These four actions effectively simulate the action of a mechanical shutter. The time and order in which pixels are reset is the same as the that of reading them and transmitting their data. This ensures that the exposure time of each pixel is equal. In reality the chip is always actively collecting charge. It is only that information collected during the inactive periods is ignored.

 

Unlike a film camera, it is possible to keep a digtal shutter open for the entire duration of each frame. This is sometimes called a 360 degree shutter. This is achieved by combining steps 1 and 3 from above into a single step and eliminating step 4.

Shutter Artifacts

Rolling shutter artifacts occur because the camera must transmit the measurement of light from each photosite one at a time. This process happens so instantaneously that it is normally imperceptable. Strobes however, create a burst of light that can start and stop faster than the sensor can transmit an image. When a strobe occurs during those short periods that the sensor is reading or reseting itself (stages 1 or 3 from above) the light from the strobe will have a different effect on pixels that have been read than it will on those that have not. This difference creates a visible horizontal line, a rolling shutter artifact.

Hybrid digital mechanical cameras such as the Dalsa Origin and the Arri D20 do not suffer from rolling shutter artifacts because the readout process is hidden behind a mirror.


 

This diagram shows three examples of strobes interacting with a rolling shutter. These images were recorded on a RED One camera in September 2007. Images are from Die Mannequin used by permission of Marc Bachli and Fini Films Inc.
Because the readout time of the sensor is fixed changing the shutter speed does not change the probability that a shutter artifact will occur (it only changes the probability that the strobe will occur during the active or inactive period. Changing the frame rate, does change this probability. Lower frame rates have a lower likelyhood of shutter artifacts.

A 360 degree shutter does not eliminate rolling shutter artifacts but instead splits them evenly between consecutive frames.

Using the sensor in a windowed mode (such as 2K Windowed RAW or 720p RGB) can reduce the probability of catching a shutter artifact. In windowed mode the sensor must only read or reset one quarter of the pixels. The readout time is therefore one quarter of the time of the full chip.

Syncing Strobes with the Shutter

This can be achieved by connecting the Genlock output of the camera to a strobe controller.