METHOD OF MOVING A PIXEL A SUBPIXEL DISTANCE

Abstract: The invention relates to a method of moving a pixel a subpixel distance and is intended to be applied in picture coding methods to determine the value of a pixel located between the fixed pixels on the screen. The pixel value is calculated using a known motion vector and the pixel values located in the vicinity of the corresponding pixel in the previous picture. According to the invention at least 3x3 pixels from the previous picture are used and the pixel value is calculated as a sum of the previous pixel values weighted by coefficients depending of the motion vector. The coefficients are preferrably calculated as polynomials of the subpixel part of the motion vector.

Description: TITLE OF INVENTION: METHOD OF MOVING A PIXEL A SUBPIXEL DISTANCE FIELD OF THE INVENTION The present invention relates to a method of moving a pixel a subpixel distance. Subpixel distance refers to a distance shorter than the distance between two adjacent pixels (pixel = picture element) . The inve"' ;ion is intended to be applied in the picture coding methods for determining the value of a pix located between the fixed pixels on the screen. This situation may occur in moving pictures where the pixels are moved arbitrary distances and thus, may end up between defined positions. The situation may also occur in conversion between different picture formats, so-called standard conversion. STATE OF THE ART According to the prior art usually 2x2 filters with so- called bilinear interpolation are used. The drawback with this technique is that it provides blurry pictures, especially in positions between two pixels. The reason is that the 2x2 filter has strong low- s characteristics . This drawback is overcome to a great extent with the invention utilizing 3x3-filters or greater. 4x4 filters are also previously known, however, not with functions for calculating coefficients of arbitrary positions but only for half pixel distances. However, the present invention provides a method for arbitrary pixel distances . SUMMARY OF THE INVENTION Thus, the present invention provides a method of moving a pixel a subpixel distance, wherein the value of the pixel is calculated using a known motion vector and values of pixels located in the vicinity of the corresponding pixel in the previous picture. According to the invention at least 3x3 pixels from the previous picture are used and the pixel value is calculated as a sum of the previous pixel values weighted by coefficients depending on the motion vector. The coefficients are calculated as functions of the subpixel part of the motion vector, preferrably as polynomials . Further features of the invention are set forth in the accompanying claims. The invention will now be described in detail referring to the enclosed drawings . BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. is a diagram illustrating a 3x3 filter in accordance with the present invention. Figure 2. is a diagram illustrating a 4x4 filter in accordance with the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The purpose of the present invention is to assign a value to a pixel which is to be calculated from the previous picture. It is assumed that a vector indicating the position in the old picture corresponding to the actual pixel is available. This situation occurs for instance in picture coding with prediction and standard conversion between different picture formats. In figure 1, the positions of nine pixels in the pre- vious picture are shown as dots and the pixel of which the value is to be calculated, is marked with a circle. Thus, in the new picture the circle corresponds to pixel in a fixed barpattern, similar to the nine dots. The motion vector (not shown) may be of arbitrary length. In calculating the new pixel value the integer parts of the motion vector are not used but only the fractional parts from the centre pixel, in the figure denoted by f and g for respective directions . For both f and g -1/2 < f < 1/2 and -1/2 < g < 1/2. In accor¬ dance with the invention the pixel value q is calculated as 1 1 q (n, k) = X ∑aιr j p (n+x+i, k+y+j ) i— 1 j— 1 where a^ and r j are the filter coefficients, n, k are the coordinates in the new picture, x+f, y+g is the motion vector, x and y being the integer part and f and g being the fractional part, p is the pixel value in the previous picture. Thus, referring to figure 1, the pixel value of the pixel at the left top should be multiplied with a-i and r-i et cetera. The problem is separable horisontally and vertically meaning that the coefficient aj . r j is a product of two co¬ efficients ai and r j . The following coefficients for a 3x3- filter has been found: a _ 1 .. 2 - 3f 2 + f 3 0 2f + 3f 2 'f3 ai = and r j are identical functions of g. In figure 2 the correspondning situation of a 4x4 filter is illustrated. For f and g 0 < f < 1 and 0 < g < 1 For a 4x4 filter in accordance with the invention the following coefficients are obtained: -If. +12f 2 - 5f3 a - ! = 15 15 - 3f - 27f 2 + 15f 3 a 0 = 15 12f + 18f 2 - 15f 3 aι = 15 -2f - 3f 2 + 5f 3 a 2 - ^ and r j are identical functions of g. The greater filter is chosen, the more information is obtained from the previous picture. In orders greater than four, however, the difference is hardly perceptible to the eye with the present screen technology. The functions used to calculate the coefficients are chosen in a suitable way. The condition is that ∑ ai = 1 and Σ r j = 1. It should be possible to find other functions as well, preferrably polynomials, using iteration, which would work acceptably. The coefficients are preferrably stored in a look-up table for fast retrieval for all possible values of f and g. Thus, the present invention solves the problem with the prior art, because the 3x3 filters and the 4x4 filters do not have the low-pass characteristics of the 2x2 filter. It is also possible with the present invention to move a pixel an arbitrary subpixel distance. The invention is only limited by the claims below.

Claims: CLAIMS 1. Method of movirg a pixel a subpixel distance, wherein the pixel value (q(n,k)) is calculated using the known motion vector (x+f, y+g) and pixel values (p(n', k 1 )) located in the vicinity of the corresponding pixel in the previous picture, characterized in that at least 3x3 pixels from the previous picture is used, that the pixel value is calculated as a sum of previous pixel values weighted with coefficients depending on the motion vector. 2. Method according to claim 1, characterized in that the coefficients are calculated as functions of the motion vector subpixel part (f, g) . 3. Method according to claim 1, characterized in that the coefficients are calculated as polynomials of the motion vector subpixel parts (f, g) . 4. Method accordin g to claim 1 or 2, characterized in that the coefficients a:.. stored in a look-up table. 5. Method according to anyone of the preceding claims chraracterized in that 3x3 pixels from the previous picture are used, the value of the pixel being calculated as 1 1 q(n,k) = ∑ ∑aιr j p(n+x+i,k+y+j) i=-l j—1 where the motion vector is (x+f, y+g) ; x and y are integers, -1/2 < f < 1/2 and -1/2 < g < 1/2, p(n+x+i, k+y+j) are pixel values in the previous picture, and the coefficients being calculated as -2f + 3f 2 - |f 3 | a-i = 2 - 3f 2 + f 3 a n = 2f + 3f 2 - f 3 ai = 4 -2g + 3g : 2 _ r-l 2 - 3g 2 + | g3| r Q = 2g + 3g 2 _ ri = 6. Method according to claim 1-3, characterized in that 4x4 pixels from the previous picture are used, the pixel value being calculated as 2 2 q (n, k) = ∑ ∑a±r j p (n+x+i, k+y+j ) i=-l j=-l where the motion vector is (x+f, y+g) / x and y are integers, 0 < f <_1 and 0 < g < 1, p(n+x+i, k+y+j) is the pixel value in the previous picture and the coefficients being calculated as -7f + 12f 2 - 5f 3 15 -2σ - 3cr 2 + 5cr » __2 15

Inventors: BRUSEWITZ HARALD, BRUSEWITZ, HARALD
Applicants: TELEVERKET
Application year: 1991
Main classification: G06F15
IPC codes: G06F17, H04N7