Motion blur

From Academic Kids

Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie or animation.

When an image of moving objects or from a moving camera is created it does not always merely represent a single instant of time. Often because of technological constraints or artistic requirements the image must represent the scene over a period of time. As objects move in a scene over time an image of that scene must represent an integration of all positions of a camera's viewpoint and object positions over the period of exposure determined by the shutter speed. Any moving object with respect to the camera in such an image will look blurred or smeared along the direction of relative motion, this smearing may occur on an object that is moving or on a static background if the camera is moving. In a film or television image this looks natural because the human eye behaves in much the same way.

Because the effect is caused by the relative motion of the camera, object and scene being imaged, the camera may be used to track moving objects by panning the camera such that even with long exposure times motion blur is avoided on the moving object but instead appears on the background.

In televised sports, where conventional cameras expose pictures 25 or 30 times per second, motion blur can be inconvenient because it obscures the exact position of a projectile or athlete in slow motion. For this reason special cameras are often used which eliminate motion blurring by taking rapid exposures on the order of 1/1000 of a second, and then transmitting them over the course of the next 1/25 or 1/30 of a second. Although this gives sharper slow motion replays it can look strange at normal speed because the eye expects to see motion blurring and does not.

Similarly, in real-time computer animation each frame shows a perfect instance in time (analogous to a camera with an infinitely fast shutter), with zero motion blur. This is why a video game with a frame rate of 25-30 frames per second will seem 'jumpy' and strange, while natural motion filmed at the same frame rate appears continuous. To compensate for this, much higher frame rates are desirable, of 60 frame/s or more. In pre-rendered computer animation, such as CGI movies, simply raising the frame rate is not possible, but realistic motion blur can be drawn because the renderer has much longer to draw each frame. Temporal anti-aliasing produces frames as a composite of many instants.


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