Image Processing Reference
In-Depth Information
facial parameters to be used for interpolating the preceding keyframe towards the present one
( τ <0). The latter regards the dominance of the next viseme on the parameters used morph the
present keyframe towards the next one ( τ >0). Our implementation doesn't make use of an
animation engine to control the facial parameters (labial opening, labial protrusion and so
on) but the interpolation process acts on the translation of all the vertexes in the mesh. The
prosodic sequence S of time intervals [ t i− 1 , t i [ associated to each phoneme can be expressed as
follows:
S = { f 1 ∈ [ 0, t 1 [ ; f 2 ∈ [ t 1 , t 2 [ ; . . . ; f n ∈ [ t n− 1 , t n [} (4)
A viseme is defined “active” when t falls into the corresponding time interval. The preceding
and the following visemes are defined as “adjacent visemes”. Due to the negative exponential
nature of the dominance function, just the adjacent visemes are considered for computing
weights. For each time instant, 3 weights must be computed on the basis of the respective
dominance functions of 3 visemes at a time. The weights are computed as follows:
w i (t) = D i (t) = α i exp (− θ i |t − τ i |)
(5)
where τ i the mid point of the i -th time interval. The w i must be normalized:
w i (t)
w
i (t) =
(6)
+ 1
j =− 1
w i−j (t)
so that for each time instant the coordinates of the interpolating viseme vertexes v (l)
int ( t ) ∈
{V int (t)} will be computed as follows:
i+ 1
k = i 1
v ( l )
w i (t) v ( l k (t)
int (t) =
(7)
where the index l indicates corresponding vertexes in all the involved keyframes.
Our implementation simplifies also this computation. It is sufficient to determine the result of
the coarticulation just for the keyframes, because the interpolation is obtained using directly
the morphing engine with a linear control function. Once the dominance functions are
determined, each coarticulated keyframe is computed and its duration is the same as in the
corresponding phoneme.
4.2.2 Diphthongs and dominant visemes
A sequence of two adjacent vowels is called diphthong. The word “euro” contains one
diphthong. The vowels in a diphthong must be visually distinct as two separate entities.
The visemes belonging to the vowels in a diphthong mustn't influence each other. Otherwise,
both the vowel visemes wouldn't be distinguishable due to their fusion. In order to avoid this
problem, the slope of the dominance function belonging to each vocal viseme in a diphthong
must be very steep (see Fig.2). On the contrary, the sequence vowel-consonant requires a
different profile of the dominant function. Indeed, the consonant is heavily influenced by the
preceding vowel: a vowel must be dominant with respect to the adjacent consonants, but not
with other vowels. As shown in Fig.3, the dominance of a vowel with respect to a consonant
is accomplished with a less steep curve than the consonant one.
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