1. Field of the Invention
The present invention relates to a filtering circuit for displaying an image, and in particular, to a letter-box filtering circuit and a method using the same for sampling an image.
2. Background of the Related Art
A filter coefficient used for a letter-box filtering circuit is experimentally determined. To easily structure the letter-box filtering circuit hardware, filter coefficients such as 0.25, 0.5, 0.75, etc. are used as shown in FIG. 1. When the filter coefficient 0.5 is multiplied by a pixel value (e.g., pixel value A), the pixel data is shifted to the right for 1 bit. Similarly, when the coefficient 0.25 is multiplied by the pixel value, the pixel data is shifted 2 bits to the right. When 0.75 is multiplied by the pixel value, the pixel data is obtained by adding the previous two resultant values. That is, 0.75A=0.5A+0.25A. In FIG. 1, A, B, C, and D are pixels before filtering, and xcex1, xcex2, and xcex3 are pixels after filtering.
When A is a pixel data of 8 bits in a related art letter-box filtering circuit, FIG. 2 is a diagram showing a circuit diagram for obtaining a value (e.g., 0.75A) in which the pixel A is multiplied by 0.75 of a filter coefficient. As shown in FIG. 2, an operand of 0.75 is made by adding an operand OP21 of 0.5A, which is obtained by shifting an inputted pixel data 1 bit to the right, and an operand OP22 of 0.25A, which is obtained by shifting the inputted pixel data 2 bits to the right.
In the case of 0.25 and 0.5, each one of the filtering data is obtained by right shifting the pixel data. In the case of 0.75, the filtering data is obtained by adding the two (e.g., 0.25 and 0.5) right shifted filtering data.
Down sampling data is obtained by adding two data that are lengthwise adjacent to each other.
Tables 1-7, A and B are two pixel data that are lengthwise adjacent to each other. Also, a value illustrated in brackets ( ) shows an error when a value 0.75A is erroneously corrected. A value designated by an xe2x80x9c*xe2x80x9d shows a case where correction by rounding is also required.
In Tables 8-9, B[O] and C[O] are the lowest bits of the two pixel data that are lengthwise adjacent to each other. Also, a value designated by an xe2x80x9c*xe2x80x9d in Table 9 shows a case where correction by rounding is also required.
A shifting operation to the right in order to multiply 0.5 or 0.25 to a pixel data value deletes 1 bit or 2 bits of the lowest bits (i.e., the least significant bits (LSB)). Accordingly, when a value, of the deleted bit is 1, a value 0.75 that is multiplied by the pixel data value may have an error up to 1.25 Refer to Tables 1, 2, and 3).
Since an error occurred by the shifting operation is accumulated in the addition for filtering, an error up to 2 may be generated in an operation of 0.75A+0.25B (refer to Tables 4-7 and FIG. 3). Also, an error can be produced up to 1 in an operation of 0.5B+0.5C (see Tables 8 and 9 and FIG. 4). An operation of 0.25C+0.75B is as same as the result of 0.75A+0.25B. Again, the A, B, C, and D data is assumed to be a pixel data of 8 bits that is lengthwise adjacent to each other.
An object of the present invention is to provide a filtering circuit and method that US=: substantially obviates at least one of the problems or limitations of the related art.
Another object of the present invention to provide a letter-box filtering circuit and a method using the same that corrects a round-off error that occurs in a process of la multiplying a prescribed filter coefficient such as 0.25, 0,5, or 0.75, etc.
Another object of the present invention is to provide a letter-box filtering circuit and a method using the same that corrects errors in a down sampling addition or the like.
Another object of the present invention is to provide a letter-box filtering circuit and a method using the same that minimizes round-off error.
Another object of the present invention is to provide a letter-box filtering circuit and a method using the same that uses an additional circuit to correct both round-off and down sampling addition errors.
Another object of the present invention is to provide a letter-box filtering circuit and a method using, the same that displays an image having, an aspect ratio of 16:9 on a display unit having an aspect ration of 4:3 in a digital screen process.
Another object of the present invention is to provide a filtering circuit and a method using the same that down samples an aspect of NTSC 720*480 to an aspect of 720*360 (in case of PAL, 720*576 to 720*432) and expects and corrects a round-off error that occurs in multiplying a prescribed filter coefficient by a pixel value and in a down sampling addition to minimize or reduce an overall round-off error.
To achieve at least the above objects in a whole or in parts, there is provided a letter-box filtering circuit, according to the present invention, that includes a preprocessing correcting circuit that combines first data deleted when a pixel data is right-shifted when multiplying a filter coefficient by the pixel data and selectively inputting a first resultant value to an adder, and a filtering correcting circuit that combines second data deleted when the pixel data is right-shifted in a filtering addition and selectively inputting a second resultant value to the adder.
To further achieve the above objects in a whole or in parts, there is provided a letter-box filtering method according to the present invention that includes a preprocessing correcting step for correcting a first error by combining data that are deleted when multiplying a filter coefficient by a pixel data for a down sampling, and a filtering correcting step for correcting a second error by combining data that are deleted when a filtering addition when correction is required.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.