Real-Time GPU Silhouette Refinement using adaptively blended ...

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and shading normals of each triangle in Ω to construct a corresponding cubic Bézier patch. The end result of our construction is a set of triangular patches that constitutes a piecewise smooth surface. Our construction is a variant of the one in [7], see also [22]. For each edge e ij in Ω, we determine a cubic Bézier curve based on the edge endpoints v i , v j and their associated shading normals: C ij (t) = v i B 3 0(t) + c ij B 3 1(t) + c ji B 3 2(t) + v j B 3 3(t), where Bi 3(t) = ( 3 i) t i (1 − t) 3−i are the Bernstein polynomials, see e.g. [8]. The inner control point c ij is determined as follows. Let T s and T t be the two triangles adjacent to e ij and let n si and n ti be the shading normals at v i belonging to triangle T s and T t respectively. If n si = n ti , we say that v i is a smooth edge end and we determine its inner control point as c ij = 2vi+vj 3 − (vj−vi)·nsi 3 n si . On the other hand, if n si ≠ n ti , we say that v i is a sharp edge end and let c ij = v i + (vj−vi)·tij 3 t ij , where t ij is the normalized cross product of the shading normals. We refer to [7] for the rationale behind this construction. Next, we use the control points of the three edge curves belonging to a triangle [v i , v j , v k ] to define nine of the ten control points of a cubic triangular Bézier patch of the form F = ∑ b lmn Blmn. 3 (3) l+m+n=3 Here B 3 lmn = 6 l!m!n! ul v m w n are the Bernstein-Bézier polynomials and u, v, w are barycentric coordinates, see e.g. [8]. We determine the coefficients such that b 300 = v i , b 210 = c ij , b 120 = c ji and so forth. In [22] and [7] the center control point b 111 was determined as b 111 = 3 12 (c ij + c ji + c jk + c kj + c ki + c ik ) (4) − 1 6 (v i + v j + v k ) . We propose instead to optionally use the average of the inner control points of the three edge curves, b 111 = 1 6 (c ij + c ji + c jk + c kj + c ki + c ik ) . (5) This choice allows for a significantly more efficient implementation at the cost of a slightly “flatter” patch, see Section 6.4 and Figure 4. This example is typical in that the patches resulting from the two formulations are visually almost indistinguishable. 0 β ij v i v j ij [ , ] φ 0
espect to T . Then p = u i p i + u j p j + u k p k and S m [f](p) = u i f(p i ) + u j f(p j ) + u k f(p k ). (7) Given two tessellations P s and P m and two integers s ≤ m, the set of vertices of P s is contained in the set of vertices of P m and a triangle of P m is contained in a triangle of P s . Since both maps are linear on each triangle of S m [ Ss [f] ] and agrees at the corners, the two maps must be equal in the whole of P 0 . This implies that a tessellation can be refined to a finer level without changing its geometry: Given a map f : P 0 → R d , we have a corresponding tessellation S m [ Ss [f] ] = S s [f]. (8) We say that S m [ Ss [f] ] has topological refinement level m and geometric refinement level s. From the previous result we can define tessellations for a non-integer refinement level s = m + α where m is an integer and α ∈ [0, 1). We refine S m [f] to refinement level m+1 and let α control the blend between the two refinement levels, S m+α [f] = (1 − α)S m+1 [S m [f]] + αS m+1 [f]. (9) See Figure 5 for an illustration of non-integer level tessellation. The sampling operator S m is linear, i.e. S m [α 1 f 1 + α 2 f 2 ] = α 1 S m [f 1 ] + α 2 S m [f 2 ] for all real α 1 , α 2 and maps f 1 , f 2 . As a consequence, (8) holds for non-integer geometric refinement level s. Our objective is to define for each triangle T = [v i , v j , v k ] a tessellation T of the corresponding patch F <strong>adaptively</strong> with respect to the silhouetteness of the edges e ij , e jk , e ki . To that end we assign a geometric refinement level s ij ∈ R + to each edge, based on its silhouetteness as computed in Section 3. More precisely, we use s ij = Mα ij where M is a user defined maximal refinement level, typically M = 3. We set the topological refinement level for a triangle to be m = ⌈max{s ij , s jk , s ki }⌉, i.e. our tessellation T has the same topology as P m . Now, it only remains to determine the position of the vertices of T . We use the sampling operator S s with geometric refinement level varying over the patch and define the vertex positions as follows. For a vertex p of P m we let the geometric refinement level be s(p) = { s qr if p ∈ (p q , p r ); max{s ij , s jk , s ki } otherwise, Draft Draft (10) Figure 4: The surfaces resulting from the center control point rule (4) (left) and (5) (right), applied to a tetrahedron with one normal vector per vertex. The difference is marginal, although the surface to the right can be seen to be slightly flatter. where (p q , p r ) is the interior of the edge of P 0 corresponding to e qr . Note that the patch is interpolated at the corners v i , v j , v k . The vertex v of T that corresponds to p is then defined as v = S s(p) [F](p) = S m [ Ss(p) [F] ] (p). (11) Note that s(p) is in general a real value and so (9) is used in the above calculation. The final tessellation is illustrated in Figure 6. The topological refinement level of two neighboring patches will in general not be equal. However, our choice of constant geometric refinement level along an edge ensures that neighboring tessellations match along the common boundary. Although one could let the geometric refinement level s(p) vary over the interior of the patch, we found that taking it to be constant as in (10) gives good results. 6 Implementation We next describe our implementation of the algorithm. We need to distinguish between static meshes for which the vertices are only subject to affine transformations, and dynamic meshes with more complex vertex transformations. Examples of the latter are animated meshes and meshes resulting from physical simulations. We assume that the geometry of a dynamic mesh is retained in a texture on the <strong>GPU</strong> that is updated between frames. This 5
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