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1 In three dimensions we would like the same control over abstraction as in two dimensions but there are several additional requirements We ll focus on animated and interactive applications since if we only care about still images we can just use the techniques Doug described For animation we need temporal or frame to frame coherence between consecutive frames For interactive applications we need both temporal coherence and good performance A desirable though not strictly necessary quality is flexibility in the types of lines that the method can handle 2 Line density is one of the most important cues for abstraction in line drawings In the image shown the central building is obviously emphasized while the surrounding buildings are not and the only variation between the two is the line density However the basic pipeline described previously hasno natural control over screen space line density lines are drawn wherever the extraction and visibility algorithm determine they should be 3 The simplest way to deal with inordinate line density is to nip it in the bud by creating an abstraction of the model itself such as by smoothing The lines are then extracted from this smooth model and rendered as normal 4 So how do you make an abstraction of a model One way is to create a specialized method tailored to a specific type of model such as bushy trees In this example we represent each leaf with a disk and vary the radius of these disks to control the level of abstraction This method has the nice effect of not only controlling density but also varying the shape of the lines themselves 5 More generally we can create a set of simplified meshes using some low pass filtering method We then interpolate between these meshes at runtime giving precidence to the smooth mesh in areas where low density are desired and the detail mesh for areas of high density The major issue is that this method is restricted to smooth models that can be effectively filtered without losing their character

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