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GPU PRO 3 by Wolfgang Engel

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5
II
Inexpensive Antialiasing of
Simple Objects
Mikkel Gjøl and Mark Gjøl
5.1 Introduction
In the age of mobile devices, every algorithm gets a second chance. This article
explores a use of discontinuity edge overdraw [Sander et al. 01] for antialiasing
simple objects on 3D-capable mobile devices. The essence of this technique is to
render a “smooth” line on top of aliasing primitive edges in order to cover the
aliasing edge.
The method is trivially useful for rendering objects whose outline is geomet-
rically defined or easily derived. This applies to everything from GUI-elements
to 2D objects positioned in 3D, or even simple 3D-objects—anything where the
outline is defined by geometry. For textured 2D-elements, the usual way to deal
with antialiasing is to use textures with a translucent edge, relying on texture-
sampling to produce a smooth transition at the boundary. While this is a good
solution, it requires careful construction of the mip-chain to avoid artifacts during
scaling or when viewed at steep angles.
5.2 Antialiasing via Smoothed Lines
The founding observation of [Sander et al. 01] was that aliasing in 3D scenes
appears mostly at geometric silhouette edges and crease edges, and so only pixels
along these lines require antialiasing. As these boundaries can be described as
lines, and since line smoothing is widely available in hardware, aliasing can be
reduced by rendering smooth lines on top of the aliasing edges. The main contri-
bution of [Sander et al. 01] was an algorithm to find the aliasing edges. For some
applications, the potentially aliasing edges are trivially available, allowing us to
use this method easily. It is worth noting that the resulting algorithm does not
work for translucent objects, and does nothing to improve upon texture-aliasing,
169
170 II Rendering
Figure 5.1. Basic rendering setup.
including aliasing when rendering 2D sprites. Hardware texture filtering mostly
solves this issue however, and is available on the majority of modern mobile GPUs.
Applying the algorithm consists of just two steps (see Figure 5.1):
1. Determine the geometric edges that are causing aliasing.
2. Render aliasing edges as smooth lines.
In the general case it is not trivial to determine which edges cause aliasing, and
we refer readers to the exhaustive discussion in [Sander et al. 01]. In summary,
three sets of geometric edges should be considered for any visible object:
silhouette edges,
discontinuous edges, e.g., “hard” edges or texture seams,
intersection-edges between geometric objects.
While discontinuous edges are typically static for a triangle mesh and can
be precomputed, silhouette edges change with the viewer. Dealing with scenes
constructed entirely of triangles, a geometric approach to finding silhouette edges
is to locate all edges where one of the adjacent triangles faces towards the viewer
and the other faces away. Computing exact intersections between geometric ob-
jects is computationally expensive, and a solution was not presented in [Sander
et al. 01].
When rendering lines to cover the detected aliasing edges, “over” alpha-
blending is applied (src_alpha,one_minus_src_alpha), and depth buffering is
used to achieve correct occlusion. While the rendered lines are potentially sub-
ject to z-fighting, the fighting pixels have the same color as the object on which
they are rendered. Since the main contribution of the smooth lines is outside
the aliasing object, z-fighting causes minimal issues. Rendering multiple layers
of alpha-blended pixels necessitates composition in a back-to-front manner. This

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