Multiplication Symmetric Convolution Property for Discrete Trigonometric Transforms

The symmetric-convolution multiplication (SCM) property of discrete trigonometric transforms (DTTs) based on unitary transform matrices is developed. Then as the reciprocity of this property, the novel multiplication symmetric-convolution (MSC) property of discrete trigonometric transforms, is developed.


Introduction
Shen et al .[1] developed fast DCT-domain convolution for time/spatial-domain multiplication using DCT type-2.They exploited symmetry and orthogonality for the fast algorithm.Logo-keying operation in the spatial domain can be done in the DCT domain for compressed image/video editing.However, they have not derived the convolution from the symmetric-convolution multiplication (SCM) property of discrete trigonometric transforms (DTTs), which will be the focus of this paper.
Time-domain symmetric convolution for a linear phase filtering application has been developed by Martucci [2] and is called the symmetric-convolution multiplication property of DTTs.Based on the SCM property, Zou et al. [3] have developed a symmetric convolution for linear phase FIR filtering.Filter coefficients consist of symmetric and asymmetric parts.
Based on those earlier works, Reju et al. [4] have finally developed fast circular convolution using DTTs.The input sequences to be convolved need not be symmetric or asymmetric.Thus fast DCTs

OPEN ACCESS
and DSTs can be used instead of the FFTs for FIR filtering.Generalized fast convolution using numerous transforms as well as the DFT and DTTs has been studied by Korohoda et al. in [5].
In this paper we show that swapping the forward and inverse transforms in the SCM property [2] yields a multiplication-convolution (MSC) property.That is, the convolution of transformed sequences gives the same results as the forward transform after element-by-element multiplication of the data sequences.The necessary scaling factor M for these new properties has been described in the third line below Eq. (20) in [2], saying "it is possible to swap the usage of the forward and inverse transform; in that case, an extra scaling factor may be required".Here M is the size of the generalized DFT (GDFT) when a DTT is derived from an M-point GDFT.We can get 40 types of MSCs corresponding to 40 types of SCMs in Tables VI, VII of [2].
In (1) of [6], the formulation of the SCM for the unitary matrices is presented, with only the exception of some of the 40 options mentioned by Martucci [2], because they cannot be expressed with the assumed tools.Here, we fill that gap.Consider the following matrix relationship between convolutional/unnormalized DTTs (denoted with lower-case subscript like C 1e ) and unitary DTTs (denoted with capital-letter subscript like C IE ).
Here the fact that scalars commute with matrices is used for C 3e and others.
, where m, n = 0, 1, …, N , where m, n = 0, 1, …, N  1 (10) where diag(a 11 , a 11 , …, a NN ) implies a diagonal matrix with the diagonal elements as (a 11 , a 11 , …, a NN ).I N is the identity matrix of size N  N. Let matrices J, K, Q, and R be defined as: where O 1N is the zero row-vector, with N elements, whose entries are all zero.Multiplying one of those matrices with an input vector appends a single zero on the top of the first or under the last element of the vector.Multiplying the transpose of one of those matrices with an input vector discards the first or last element of the vector.

Symmetric-Convolution Multiplication Property
One of the forty cases of symmetric convolution is proven as an example. Property.
Proof.Equation (A.1) in the appendix of [2] can be rewritten as: where e C 1 ˆ is the kernel.
From ( 16), ( 17) and ( 18): From ( 1) and ( 10): Let x and w be input column vectors in time domain.Let y be an output column vector in time domain.Let  C2e be a diagonal matrix defined as  C2e = diag([C 2e w] T ), where superscript T denotes the transpose operator.We rewrite the 8th property in Table VI of [2] as: The matrix J shows up in the last line of (22) for zero padding.Since  C2e and [ ] N are diagonal matrices, they can commute.Thus: Another example is shown in (11) of [6].Equation (12) of [6] is expanded to full for our derivation and only results are listed in the first column of Tables 3 and 4 in the Appendix.

Multiplication Symmetric-Convolution Property
Let X and Y be transformed input and output data vectors.Since there are one-to-one correspondences between unitary discrete trigonometric transforms (DTTs), we can exchange the forward transform for inverse one and vice versa as follows.In other words, a pair has the same matrix but has different names.Define h C3e as: where C 3e will be defined in (27).That is,  C2e and h C3e are the same matrix with different names (thus names of DTTs need to change).
Then from (23): Notice the forward transform matrix C IIE is replaced by the inverse transform matrix C IIIE 1 since they are the same matrix, and vice versa.Now this equation represents a SCM of DTTs.A key point of this new property is that we need to redefine convolution forms of DCTs and DSTs.The factor of M is divided for the inverse DCT of the convolution form in [2] whereas it is divided for the forward DCT of the new convolution form.M is 2N for even and 2N -1 for odd.Now new convolution form for DCT 2 is denoted as C 2e : The rest of DTTs can be readily obtained from the appendix of [2].MSC properties can be described in terms of convolutional / unnormalized DTTs to obtain similar results: Only results are listed.Matrices J, K, Q, and R are required for different index ranges between operands.Link between the index range-based maneuvers described by Martucci [2] and the introduction of the J, K, Q and R matrices is presented in Tables 1 and 2 for M = 2N and M = 2N  1, respectively.

Applications
For an image resizing (filter) application, one of 40 MSC properties is used.The definition of a normalizing parameter F(k) in [7] needs a minor change as: Then we can derive one of MSC properties, which is (4) in [7], from our equation as follows: where the symbol  denotes symmetric convolution.Since By the associativity of (continuous and discrete) convolution: Convolution has the property of associativity with scalar multiplication.Let F and G be any real sequences.Then: for any real (or complex) number a.
For a numerical example, let: where:

3
,  in [8, p. 2635], and W t and W h are a symmetric Toeplitz matrix and a Hankel matrix [9].Symmetric convolution is represented in matrix multiplication form of (31) using (32).Since the expression inside the square brackets of (30) is the matrix Y defined in (31): Equation (34) corresponds to (30).Thus Y can be computed by using either (33) or (31).In other words, the symmetric convolution of DCT coefficients, Figure 1(b) is an alternative method to computing the DCT of multiplication of two time sequences, Figure 1(a).

Conclusions
For logo-keying operation or alpha blending, one image is scaled by  and the other is scaled by (1  ), where  is a value between zero and one, and then two images are added up.For compressed image / video editing, multiplication operation in logo-keying operation in the spatial can be done in the DTT domain by convolving the unitary DTT coefficients of the two images.For those applications, we have developed 40 types of the MSC properties for the unitary DTTs (DCTs and DSTs [11] [13]).
where A is a diagonal matrix.

Appendix
Table 1.Link between the index range-based maneuvers described by Martucci [2] and the introduction of the J, K, Q and R matrices, M = 2N.
Table 2. Link between the index range-based maneuvers described by Martucci [2] and the introduction of the J, K, Q and R matrices, M = 2N  1.

SCM (
This is shown in block diagram format in Fig.1(b).Since G(k) and F(k)

Figure 1 .
Figure 1.For compressed image / video editing, logo-keying operation (alpha blending) can be done in (a) the spatial and (b) transform domains [1].The symbol  denotes the element-by-element multiplication of the two vectors and  denotes the symmetric convolution of the two vectors.

Table 3 .
20 of 40 types of SCM and MSC properties for the DTTs, M = 2N.