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490 10. Image-Based Effects
10.14 Motion Blur
For interactive applications, to render convincing images it is important
to have both a steady (unvarying) frame rate that is also high enough.
Smooth and continuous motion is preferable, and too low a frame rate is
experienced as jerky motion. Films display at 24 fps, but theaters are dark
and the temporal response of the eye is less sensitive to flicker in the dark.
Also, movie projectors change the image at 24 fps but reduce flickering by
redisplaying each image 2–4 times before displaying the next image. Per-
haps most important, each film frame normally contains a motion-blurred
image; by default, interactive graphics images are not.
In a movie, motion blur comes from the movement of an object across
the screen during a frame. The effect comes from the time a camera’s
shutter is open for 1/40 to 1/60 of a second during the 1/24 of a second
spent on that frame. We are used to seeing this blur in films and consider
it normal, so we expect to also see it in videogames. The hyperkinetic
effect, seen in films such as Gladiator and Saving Private Ryan,iscreated
by having the shutter be open for 1/500 of a second or less.
Rapidly moving objects appear jerky without motion blur, “jumping”
by many pixels between frames. This can be thought of as a type of aliasing,
similar to jaggies, but temporal, rather than spatial in nature. In this sense,
motion blur is a form of temporal antialiasing. Just as increasing display
resolution can reduce jaggies but not eliminate them, increasing frame rate
does not eliminate the need for motion blur. Video games in particular are
characterized by rapid motion of the camera and objects, so motion blur
can significantly improve their visuals. In fact, 30 fps with motion blur
often looks better than 60 fps without [316, 446]. Motion blur can also be
overemphasized for dramatic effect.
Motion blur depends on relative motion. If an object moves from left to
right across the screen, it is blurred horizontally on the screen. If the camera
is tracking a moving object, the object does not blur—the background does.
There are a number of approaches to producing motion blur in computer
rendering. One straightforward, but limited, method is to model and render
the blur itself. This is the rationale for drawing lines to represent moving
particles (see Section 10.7). This concept can be extended.
Imagine a sword slicing through the air. Before and behind the blade,
two polygons are added along its edge. These could be modeled or gener-
ated on the fly by a geometry shader. These polygons use an alpha opacity
per vertex, so that where a polygon meets the sword, it is fully opaque, and
at the outer edge of the polygon, the alpha is fully transparent. The idea is
that the model has transparency to it in the direction of movement, simu-
lating blur. Textures on the object can also be blurred by using techniques
discussed later in this section. Figure 10.32 shows an example.