Shutter Fix
The Shutter Fix node can be used to reduce the effect of CMOS shutter roll on fast moving footage.
Shutter roll occurs because the scanlines of the image are captured at different times by the CMOS sensor. When the camera is moving quickly (for example, a whip-pan or rapid translation), the resulting image contains an offset between the top and bottom keyframes.
This offset must be removed because the camera model used throughout the application assumes that all scanlines were captured at the same point in time. The node has one input and one output.
Note: Currently the rolling shutter fix does not work for independently moving objects, or scenes with a wide range of parallax motion.
Usage
To correct for shutter roll, the clip's motion must first be analysed. Then, the scan direction and exposure parameters can be adjusted to make sure vertical lines in the image are straight, as would occur when both the top and bottom scanlines are captured at the same time. Enabling the Compensate for shutter roll option will display the resulting image after the effect of shutter roll has been reduced.
Recommended practice is to shoot a calibration clip viewing straight vertical lines first to determine a suitable exposure time parameter and create a preset for the CMOS camera. That preset can then be re-loaded for other clips that do not view straight vertical lines, or the same exposure time parameter entered. Motion analysis still needs to be run on each clip, even when the exposure time parameter is known beforehand.
The global image motion curve is available in the curve editor.
Controls
Motion Analysis
Accuracy: Options are Low accuracy, which can be used when the motion in the image is relatively simple and will result in faster analysis; High accuracy will increase analysis time, but may produce better quality results; and Normal as a trade-off between higher accuracy than Low, and faster result than High.
Channels: The red, green and blue channels of the image are used for motion analysis. If certain channels contain significant noise or other image defects, removing these channels from the motion analysis can often improve accuracy.
Smoothness: The overall smoothness of motion analysis. Increasing the smoothness parameter will provide a smoother overall motion field that is less influenced by image noise or uncertain motion, but may result in incorrect motion vectors in areas of the image that exhibit complex motion (for example, around the boundaries of moving objects).
Analyse: Start motion analysis, displaying a progress window. Once finished, the image will be corrected according to the current scan direction and exposure time controls, provided the Compensate for shutter roll option is enabled. If the Shift key is held whilst clicking this button, the motion analysis will be performed in the background.
Rendering
Preset: The preset for the scan direction and exposure time parameters. Presets are stored on disk as XML files.
Store Preset: Store the current scan direction and exposure parameters as a new preset. Clicking the button will display a dialog box allowing a preset name to be specified.
Scan direction: This option controls whether the top image scanline was captured before the bottom scanline, or vice-versa.
Exposure: The fraction of frame time that was spent scanning from the top scanline to the bottom (or vice-versa if scan direction is set to Bottom To Top). This parameter can be adjusted interactively whilst examining the resulting image to determine a value suitable for the CMOS camera that captured the clip. If vertical edges are visible in the clip, this parameter should be adjusted until those edges become straight lines.
Show Grid: Display a grid overlay to assist with adjusting the exposure time parameter to straighten vertical lines in the image. The grid can be translated, scaled and rotated by clicking and dragging any of the control handles.
Compensate for shutter roll: When enabled, the image displayed in the main window (and passed down the tracking tree) will be adjusted to compensate for shutter roll according to the results of the motion analysis, the scan direction and exposure time parameters.
Default Keyboard Shortcuts
Keyboard shortcuts can be customised in the Preferences.
Analyse |
Shift+A |
Store Preset |
Shift+P |
Show Grid |
Shift+G |
Compensate for shutter roll |
Shift+C |