Patent Grant
8175121
The present application claims priority for Japanese application serial No. JP 2006-353700, filed on Dec. 28, 2006, the content of which is hereby incorporated by reference into this application.
(1) Field of the Invention
The present invention relates to an image processor and an image display apparatus; and, more particularly, to an image processor that includes a configuration for converting frame rate of an input image signal and an image display apparatus.
(2) Description of the Related Art
A technique called frame rate conversion has been recently introduced for offering better moving image performance. It uses frames included in an image signal and a motion vector of the input image signal and combines interpolation frames created in an apparatus to generate a signal of a new frame sequence. Accordingly, it becomes possible to offer better moving image performance by improving unnatural motion such as a feeling of afterimage or unstable image in a display of moving images.
In order to improve moving image performance more, it is necessary to generate an interpolation frame with high accuracy. This in turn requires raising the detection accuracy for a motion vector used to generate an interpolation frame. A prior art technique related to the improvement of the detection accuracy for a motion vector is disclosed in Japanese Patent Application Publication No. 2006-165602, for example. The technique involves inserting an interpolation frame between two temporally successive frames included in an input image signal. In detail, plural frame pixel pairs at point symmetric positions about an interpolated pixel that constitutes the interpolation frame are extracted from a predetermined area, and a pixel direction having a minimum difference between the pixels is designated as a motion vector.
As aforementioned, the prior art technique designates a motion vector in a direction where a difference between previous and subsequent frame pixels (blocks) at point symmetric positions about the interpolated pixel (hereinafter, this process is called a matching process) is minimal. Therefore, in the case that only one or two edge components of a certain level appear in the predetermined area, the number of pixel pairs having the minimum difference is small so it is relatively easy to specify the motion of an object. However, as for an image pattern where change in brightness such as a wire-netting (mesh) pattern, a stripe pattern, a grid pattern, telop characters, etc., appears periodically or non-periodically, edge components having the same level often appear in a designated direction (horizontal or vertical direction) in the predetermined area. Consequently, a number of pixel pairs having the minimum difference appear in the predetermined area, causing erroneous detection of a motion vector.
For example, in the case that a certain stripe pattern moves throughout two temporally successive frames, i.e., from a previous frame to a subsequent frame, one of the sticks or even an adjacent stick in the subsequent frame has the same pixel value as one of the sticks in the previous frame. Therefore, a detection error occurs by regarding one of the sticks in the previous frame has moved to the adjacent stick in the subsequent frame.
To elaborate this phenomenon in reference to
Suppose that a target area 503 of the first frame actually moved to an area 506 in the subsequent frame. If a motion vector is detected accurately from the matching process and an interpolation frame is composed accordingly, it appears in the lower left corner. That is, a stripe pattern 508 in the previous frame of the target area 503 moves in the direction of the motion vector 520 to a stripe pattern 509 in the subsequent frame, a stripe pattern 510 is generated as an interpolation frame by the matching process. At this time, an enlarged image in the area 506 is denoted by reference numeral 530. However, since a stripe pattern, as shown in an enlarged image 531, has the same pixel value as that of the enlarged image 530, it is erroneously detected that the stripe pattern has moved to the area 507, not the area 506. In this case, as shown in the lower right corner of
When such a detection error of the motion vector occurs, an image of no relationship with motion or an image with low relationship appears on an interpolation frame, resultantly producing a broken, jitter image.
The above related art does not consider erroneous detection of a motion vector in an image pattern where such change in brightness appears periodically or non-periodically, or a broken, jitter image due to an interpolation frame generated by the erroneously detected motion vector.
In view of the foregoing disadvantages and problems, it is, therefore, an object of the present invention to provide a frame conversion technology in use of an interpolation frame, capable of displaying a high picture quality image with less jitter or break-ups even for an image pattern where change in brightness such as a grid pattern, a stripe pattern and so on appears periodically or non-periodically.
To achieve the above object, the present invention is characterized in the configuration described in the claims.
There is provided an image processor, which designates an area with an image pattern having a periodically or non-periodically appearing brightness change such as a grid pattern, a stripe pattern, telop characters, etc., as a specific area and acquires a motion vector for the specific area by using an amount of relative misalignment of a predetermined edge component between two temporally successive frames in an input image signal.
In detail, the matching process described above assigns a motion vector to other areas except for the specific area. Meanwhile, a motion vector for the specific area is given by the following process. That is, two frames in a predetermined pixel unit shift relatively in a horizontal and/or vertical direction, and a frequency where the predetermined edge components coincide with each other between the two frames is detected per shift to compose a histogram. Then, a shift quantity (an amount of misalignment) with the highest frequency where the predetermined edge components coincide with each other is designated as a motion vector for the specific area.
Accordingly, the image processor of the invention incorporating a frame conversion technology in use of an interpolation frame is capable of displaying a high picture quality image with less jitter or break-ups even for an image pattern where change in brightness such as a grid pattern, a stripe pattern and so on appears periodically or non-periodically.
These and other features, objects and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings wherein:
A preferred embodiment of the present invention will now be described with reference to the accompanying drawings. In addition, the embodiment may be applied not only to convert the frame rate of an input signal from 60 Hz to its twice, 120 Hz, for example, but also to convert a 2-3 pull down mode input image signal having the frame rate of 60 Hz to a 60 Hz non-pull down mode image signal by substituting an interpolation frame for several frames in the input image signal.
[Embodiment I]
A first embodiment of the invention will be described with reference to
An image signal is inputted to an input terminal 1, and frame data corresponding to at least two of the input image signals is stored in a frame buffer 2. Accordingly, the frame buffer 2 outputs a previous frame 12 that is temporally ahead and a subsequent frame 13 that is temporally behind the previous frame. The previous frame 12 and the subsequent frame 13 are respectively inputted to a first motion vector acquisition section 31, a second motion vector acquisition section 32, and a frame interpolation circuit 5.
The first motion vector detection section 31 detects a motion vector by the matching process described above from the previous and subsequent frames 12 and 13 being inputted. The matching process is now described with an example shown in
As shown in
Meanwhile, the second motion vector acquisition section 32 is the key feature of this embodiment and acquires a motion vector by a different approach from the matching process used for the first motion vector acquisition section. For instance, the second motion vector acquisition section 32 carries out a process for giving a second motion vector 14 that is different from the first motion vector 16 to a specific area having an image pattern with a periodically appearing brightness change such as a grid pattern, a stripe pattern, etc., among images in one frame. In detail, a predetermined edge component is detected or filtered from the subsequent frame signal 21 and the previous frame signal 22, respectively. And while these frames are moving (shifting) relatively in the horizontal and/or vertical direction, the number of times (the frequency of appearance) a coincidence point where the predetermined edge components of the subsequent and previous frame signals 21 and 22 coincide with each other occurs is counted per shift. Based on the count number, a histogram showing the distribution of coincidence points per shift is plotted, and a shift with the highest frequency appearance of the coincidence point is designated as a motion vector for the specific area. More details on this will follow later. In the meantime, referring back to
Moreover, the second motion vector acquisition section 32 of this embodiment discriminates a specific area from the filtered predetermined edge components to output an area identification signal 15. Here, the area identification signal 15 outputs a signal “1” if a certain pixel is included in the specific area for example. Otherwise, the area identification signal 15 outputs “0” for the other cases. The discrimination of a specific area will also be explained further.
The first motion vector 16 outputted from the first motion vector acquisition section 31, and the second motion vector 14 and the area identification signal 15 both being outputted from the second motion vector acquisition section 32 are inputted to a selector 4. The selector 4 selects one of the first motion vector 16 and the second motion vector 14, according to a value of the area identification signal 15. In other words, if the area identification signal 15 is “1”, the selector 4 selects the second motion vector 14 for pixel interpolation in a specific area; otherwise selects the first motion vector 16 for pixel interpolation in areas other than the specific area, and outputs a selected motion vector 17.
The selected motion vector 17 from the selector 4 is inputted to a frame interpolation circuit 5. As mentioned earlier, the frame interpolation circuit 5 also receives signals from the previous and subsequent frames 12 and 13. And the frame interpolation circuit 5 computes a pixel value of the target block (or pixel) 34 among the interpolation frames 33 shown in
The signal 18 in a new frame sequence is provided to a display panel 7 via a timing control circuit 6. The display panel 7 may be an LCD panel or a PDP. The timing control circuit 6 feeds an output signal from the frame interpolation circuit 5 to the display panel 7 according to horizontal or vertical scan timing, so that a frame rate converted image may be displayed on a screen of the display panel 7.
The first motion vector acquisition section 31 and the frame interpolation circuit 5 may be configured as two separate units as in this embodiment, or in one unit. For instance, the first motion vector acquisition section 31 may be built in the frame interpolation circuit 5 to extract a first motion vector by the above-described matching process from the previous frame signal 12 and the subsequent frame signal 13 being inputted to the frame interpolation circuit 5. In this case, the selector 4 is not required, and the first motion vector is substituted with the second motion vector 14 according to the area identification signal 15 (i.e., for a pixel with the area identification signal “1”).
Next, further details on the second motion vector acquisition section 32 of this embodiment are now provided with reference to
A previous frame signal 12 and a subsequent frame signal 13 in the predetermined area the application area judgment circuit 107 has extracted are inputted to a first edge filtering circuit 109 and a second edge filtering circuit 111, respectively, so that an edge component in the predetermined area is filtered by each circuit. First, the first edge filtering circuit 109 detects a difference between a target pixel and its adjacent pixel (in any direction, horizontal or vertical) for each of the previous frame signal 12 and the subsequent frame signal 13 and filters an edge if the difference is greater than a first threshold stored in a first threshold table 108 and if brightness level of the target pixel is greater than a second threshold. In order for the first edge filtering circuit 109 to detect a second motion vector from its successive circuit, a threshold stored in the first threshold table 108 is set slightly high so that an infinitesimal change in brightness may not be detected as noise.
Similarly to the first edge filtering circuit 109, the second edge filtering circuit 111 detects a difference between a target pixel and its adjacent pixel (in any direction, horizontal or vertical) for each of the previous frame signal 12 and the subsequent frame signal 13 and filters an edge if the difference is greater than a first threshold stored in a second threshold table 110 and if brightness level of the target pixel is greater than a second threshold. In order for the second edge filtering circuit 111 to detect a specific area having telop characters, a grid pattern or a stripe pattern from its successive circuit, a threshold stored in the second threshold table 110 is set relatively lower than the threshold stored in the first threshold table 108.
Even though this embodiment suggested two edge filtering systems, it is for illustrative purposes only so only one of them may be used. That is, one edge filtering circuit and one threshold table may be sufficient to detect an edge component for detecting a motion vector and an edge component for deciding a specific area.
An edge component filtered by the first edge filtering circuit 109 is inputted to a motion vector detection circuit 115. The motion vector detection circuit 115 detects a motion vector of an image in an area designated by the application area judgment circuit 107, by performing a different process from the above-described matching process on the basis of respective edge components of the previous frame signal 12 and the subsequent frame signal 13. Further details on this motion vector detection are provided below, with reference to
In this embodiment, the target pixel 902 (edge component) on the first frame screen 906 is sequentially compared with 143 pixels (13×11) (edge component) within the search domain 904 in order to search for a point having the same level as the target pixel 902 in the search domain 904. Suppose that a coincidence point 908 as a pixel both sides coincide is detected in the search domain 904. Then, the search for a coincidence point within the search domain 904 is conducted for the entire pixels on the first frame screen 906 (as mentioned earlier, these are pixels within a specific area designated by the application area judgment circuit 107). In this manner, a rough motion vector is obtained. In the example shown in
Information about a motion vector detected by the motion vector detection circuit 115, i.e., information about a coincident point and information about an amount of misalignment (shift quantity) from a target pixel to a coincidence point, is inputted to a first histogram generation section 114. The first histogram generation section 114 plots a histogram similar to one shown in
The histogram of shift quantity of
In addition, a mask set-up circuit 120 receives the mask signal 11 mentioned earlier and sets up a mask area displaying an OSD screen such as a control board or menu screen. A mask processing circuit 117 operates in a manner that a histogram value for the mask area being set up by the mask set-up circuit 120 is excluded. Accordingly, the count number of coincidence points on the OSD screen does not affect the generation of a second motion vector. The mask set-up circuit 120 and the mask processing circuit 117 are not absolutely required but used optionally if needed.
Histogram information outputted from the mask processing circuit 17 is inputted to a shift error judgment circuit 118. It was described earlier that a shift quantity having the largest count number of coincidence points on the histogram of shift quantity was designated as a second motion vector. However, a second motion vector of interest can be determined more accurately by the following process in the shift error judgment circuit 118.
For example, in the case that the accuracy of an input image increases or decreases with respect to the input image to be matched with the resolution of the display panel 7, the input image undergoes a filtering operation to compose an interpolation pixel to be inserted to the input image. Therefore, pixels of the input image may become shaky before and after shift. Moreover, characters may be made transparent and blended with a background image. In these cases, edges of a character may not be filtered or filtering locations may be changed. Also, an image motion is not limited to the expression of an integer multiple number of pixel units such as 2 pixels, 3 pixels, etc., but can be expressed in a decimal number of pixel units such as 1.5 pixel, 3.7 pixels, etc. These pose a problem that an accurate motion vector cannot be obtained simply by obtaining a maximum value of histogram of shift quantity.
Even under these circumstances, however, the process by the shift error judgment circuit 118 as shown in
For example, note a point with the shift quantity−3 in
Referring again to
Note a point with the shift quantity+3 this time. The count number of coincidence points with the shift quantity has a value of “170”. The count value of the shift quantity+2 and the count value of the shift quantity+4, each being adjacent to the shift quantity+3, are “50” and “140”, respectively. Because neither count value exceeded the threshold, an addition value with respect to the shift quantity+3 becomes “360” as shown in the addition table 181.
Therefore, referring to the addition table 181, the shift error judgment circuit 118 determines a second motion vector in the specific area to be the shift quantity+3, not the shift quantity−3, because the addition value corresponding to the shift quantity+3 is the largest. This second motion vector indicates that an image of the specific area has shifted to right by three pixels from the previous frame over the subsequent frame.
The second motion vector thusly obtained is inputted to a continuity judgment circuit 121 that monitors a change in the second motion vector by 5 to 6 frames for example. If there are almost no changes in the size of the second motion vector throughout five frames, the continuity judgment circuit 121 decides that the motion vector is valid, and sends an output from the shift error judgment circuit 118 to the frame interpolation circuit 5 as the second motion vector 14 (see
Next, the description on extraction of a specific area continues, referring back to
The left side of
Compared with adjacent images, an image in a specific area that has a periodically or non-periodically appearing pattern (telop characters, a grid pattern, or a stripe pattern) in a direction with a change in brightness (this is regarded as a problem in this embodiment) has a clear edge (contour) and the color occupying the area is often of the same series. That is, the specific area exhibits a large gradation difference between pixels near the edge, while the inside the area exhibits a small gradation difference. This embodiment utilizes such a nature of the specific area to make the area generation circuit 119 generate a specific area 142. Here, a gradation difference between pixels sometimes varies by screen size or resolution. A gradation difference between two adjacent pixels changes because an image signal expands or reduces if image size or resolution between the image signal and the display signal 6 varies, thereby interpolating pixels or gradation levels. For example, when an image is enlarged twice by bilinear filtering, an interpolated pixel has a horizontal or a vertical mean value and the gradation difference tends to become narrow.
In order to cope with the above problem, the first and the second threshold hold data being stored in the second threshold table 110 may be set in advance based on the information that has a direct influence on pixels forming the edge such as the screen size or resolution. Such information includes an external input such as DVD image, broadcasting system or type such as digital Hi-Vision broadcast, and kind of an image filter being performed in a TV set. For example, if a high-resolution image of digital Hi-Vision broadcast needs to be displayed on a high-resolution display, the first threshold stored in the second threshold table 110 is set high (e.g., if an image signal has 256 gradation levels, the threshold is set to 100). On the other hand, if a low-resolution image of general broadcast needs to be magnified for display, the first threshold is preferably set lower (e.g., 50) than the previous one.
The specific area 142 thus extracted by the area generation circuit 119 is inputted to the area judgment circuit 123. The area judgment circuit 123 uses the second histogram generation section 122 to compose a histogram in terms of spatial position of a specific area. An example of this histogram is shown in
The following describes a process flow in the second motion vector acquisition section 32, referring to
In the following S300, the second threshold table 110, the second edge filtering circuit 111 and the area generation circuit 119 generate a specific area. Lastly in S104, as shown in
In this manner, the second motion vector 14 and a specific area to which the second motion vector 14 is applied are obtained.
Hereafter, an example of calculating the motion vector by expressions is described with reference to
dS=Sk(x+dx,y+dy)−Sk-1(x,y) (1)
Here, a displacement vector (dx,dy) having a minimum dS is a motion vector of interest.
Because this embodiment obtains a motion vector based on the coincidence of gradation difference using a pixel having a fixed gradation range [S0,S1] and a pixel having a gradation between two adjacent pixels thereof above a threshold (E0), the following equations (2) to (4) can be obtained.
dE=Ek((x+dx,y+dy)−Ek-1(x,y)) (2)
Ei(x,y)=Pi(x,y)grad Si(x,y) (3)
Pi(x,y) =H(Si(x,y)−S0)H(S1−Si(x,y))H(grad|Si(x,y)|−E0) (4)
wherein, H denotes the heavyside function (step function), and H(x) is 1 when x≧0; otherwise, 0.
Here, the equation (3) is a function expressing an edge of a specific area (hereinafter, this function is referred to as an edge function) and a motion vector of pixels in which a displacement vector (dx,dy) having the minimum dE forms an edge of a specific area.
In the case of satisfying |dE|<δE for any small threshold (δE (>0)) (hereafter, this condition is called ‘Condition 1’), an edge at the position of the (k−1)th frame (x+dx,y+dy), coincides with an edge at the position of the k-th frame, (x,y).
The number of pixels counted in a frame by condition 1 becomes the number of pixels for a specific area, and the conditions of the equations (2) to (4) can be quantized by a substitution into the following equation (5).
N(dx,dy)=Σx,yH(δE−|dE|) (5)
where, a displacement (dx,dy) having the maximum N is a motion vector of interest.
The following describes an example of the second motion vector acquisition flow by an area histogram, with reference to
Here, if, in S201-S205, a pixel adjacent to the position (x,y) satisfies a gradation range [S0,S1] and if gradation between two pixels adjacent to the δx or δy side is greater than a threshold (E0) (hereinafter, these conditions are called ‘Condition 2’), the equation (3) can be expressed as the following equation (6).
Ei(x,y)=grad Si(x,y) =({Si(x+δx)−Si(x)}/δx, {Si(y+δy)−Si(y)}/δy) (6)
where, Pi(x,y)=1
In addition, the equation (2) can be substituted into the following equation (7) by using the equation (6).
dE=grad Sk(x+dx,y+dy)−grad Sk-1(x,y) (7)
Further, it is judged in S206 whether a calculation result from S205 is greater than a designated value, and if Yes, the process proceeds to S207.
Suppose that a present position (x+i,y+j) exists within the search domain 904 (x0,y0)−(x1,y1)(x0≦x≦x1, y0≦y≦y1). As the equation (7) shows a gradation difference between the position (x+i,y+j) of the (k−1)th frame and the position (x,y) of the k-th frame, a difference between the two frames is calculated in S207 to judge whether the difference is below a designated value. If the judgment result turns out to be Yes, the process proceeds to S208 to compose an area histogram. This is accomplished by performing the calculation based on the equation (5) on every pixel that satisfies the condition 2. As a result, an area histogram N(i,j) exhibiting the count number of edge coincidence points for every (i,j) pair within a bin is calculated. In this manner, the area histogram N(i,j) is calculated in S208, and a maximum is determined out of all (i,j) pairs to obtain a motion vector (i,j). Here, the area histogram N(i,j) satisfies 1≦i≦n and 1≦j≦m (n and m are integers).
Following the operation in S208, the process proceeds to S209 to shift the position (of a searching target pixel) within the search domain 904. In S210, it is judged whether the operations from S201 through 209 have been carried out for every pixel in the search domain 904, and repeats the operations of S201 to S209 until they are performed on all pixels. Meanwhile, if the judgment results in S204, S206, and S207 are No, the process proceeds to S209 to perform an operation corresponding to the step.
When a 2-D histogram is implemented into a real apparatus, (i×j) tables in rows and columns only increase the circuit size and the memory capacity, so it is better to approximate the 2-D histogram by the following equation (8).
N(n,m)≈N(n,1)·N(1,m) (8)
Next, an example of the creation of a specific area signal 15 is explained with reference to
If the judgment result of S303 is Yes, it is judged in S305 whether a gradation difference between the position (x+i,y+j) of the second frame and the position (x,y) of the first frame is small. If Yes, the process proceeds to S306. On the other hand, if the judgment results of S301 and S303 are No, the process proceeds to S304 to decide whether the judgments in previous operations of S301 to S303 have been clear (i.e., whether the judgments results of S301 and S303 were Yes). If this judgment result turns out to be Yes, the process proceeds to S306. It is judged in S306 whether degradation of the pixel at the position (x+i,y+j) of the second frame falls within a designated range. If Yes, the process proceeds to S309 to decide that the pixel of this position exists in the specific area and assigns the value “1” to the area judgment signal M(x,y).
Meanwhile, if the judgment result of S304 is No, the process proceeds to S307 to judge whether there exists a pixel within the stretched area (a designated expansion range). If Yes, the process proceeds to S309. If No, however, the process proceeds to S308. It is judged in S308 whether a mask area exists within the designated expansion range. If No, the process proceeds to S310. In S310, the pixel is regarded as being outside the specific area, and the value “0” is assigned to the area judgment signal M(x,y). These operations in S304, S307, and S308 are for stretching (expanding) the specific area, provided that the conditions of each step have been satisfied.
As has been explained in this embodiment, if a specific area has an image pattern with a periodically or non-periodically appearing change in brightness, an interpolation frame is composed by using a histogram and a motion vector being detected. In this way, an erroneous detection rate of a motion vector in the specific area can be lowered, leading to a decrease in image degradation.
[Embodiment II]
A second embodiment of the invention will be described with reference to
As mentioned above, the second embodiment features the use of a plural number (four to be specific) of the second motion vector acquisition sections 32a to 32d, and a mask synthesis circuit 303 is additionally provided to synthesize and output an output from each. This embodiment enables to generate a second motion vector 14 and an area identification signal 15 for a plurality of specific areas. Even though the second motion vector acquisition sections used in the first and the second embodiment are substantially the same in configuration, they differ as follows. That is, each of the second motion vector acquisition sections 32a to 32d of the second embodiment outputs a second motion vector 14 being obtained to one of the second motion vector acquisition sections 32a to 32d at a subsequent stage in processing through one of converters 301 to 302c. This feature makes the second motion vector acquisition sections distinctive from the second motion vector acquisition section 32 of the first embodiment. Also, it is assumed for this embodiment that the first motion vector acquisition section 31 and the frame interpolation circuit 5 are built in one unit.
In detail, the second motion vectors 14 in this embodiment are converted by the converters 301a to 302c into mask signals 302a to 302c, respectively, so that a histogram of shift quantity composed by a certain second motion vector acquisition section is not combined with another histogram of shift quantity composed by a different second motion vector acquisition section. As such, each of the second motion vector acquisition sections 32a to 32d provides a second motion vector 14a to 14d that corresponds to a predetermined area having been assigned thereto and an area designation signal 15a to 15d to the mask synthesis circuit 303. Then, the mask synthesis circuit 303 generates from these signals a synthesized second motion vector 304 and a synthesized area identification signal 305 and feeds them to the interpolation frame circuit 5.
The following describes an example of the area designation implemented into the second embodiment, with reference to
In addition, as shown in
As discussed earlier, the area judgment signals 15a to 15d outputted from the respective second motion vector acquisition sections are one-bit signals for designating an area to which a motion vector is applied, or an area to which a motion vector is not applied. Therefore, the synthesized mask area signal 305 generated by the mask synthesis circuit 303 have the same number of bits as the count number of the second motion vector acquisition sections. In this embodiment, since there are four of the second motion vector acquisition sections, the synthesized mask signal 305 is a four-bit signal. Since the areas shown in
As has been explained so far, according to this embodiment, an area including an image pattern with a periodically or non-periodically appearing change in brightness such as a grid pattern, a stripe pattern, telop characters, etc., is designated as a specific area, and a motion vector is acquired for the specific area not by the matching process but based on a relative misalignment of edge components between two frames. Further, a histogram showing a relation between shift quantity and motion vector is plotted, and a motion vector having the highest number of times or occurrence on the histogram is set as a motion vector for the specific area. Therefore, according to this embodiment, the frame conversion technology in use of an interpolation frame makes it possible to display a high picture quality image with less jitter or break-ups even including an image pattern where change in brightness such as a grid pattern, a stripe pattern and so on appears periodically or non-periodically.
While we have shown and described several embodiments in accordance with our invention, it should be understood that disclosed embodiments are susceptible of changes and modifications without departing from the scope of the invention. Therefore, we do not intend to be bound by the details shown and described herein but intend to cover all such changes and modifications that fall within the ambit of the appended claims.
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