Television – Image signal processing circuitry specific to television – Transition or edge sharpeners
Reexamination Certificate
1999-09-13
2002-05-07
Hsia, Sherrie (Department: 2614)
Television
Image signal processing circuitry specific to television
Transition or edge sharpeners
C348S629000, C348S630000, C348S910000, C348S619000
Reexamination Certificate
active
06384872
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to image processing. More particularly, the present invention relates to a method and apparatus for interlaced image enhancement.
2. Background
Because video resolution images are decimated versions of high resolution images, the video images can contain more detail than can be displayed using conventional TV display, reproduction signals for which typically originate with NTSC or PAL format video cameras. Typical high-resolution images can be expected to contain a much greater amount of vertical high spatial frequencies than conventional TV images. This additional vertical high spatial frequency content can cause a raster scan display artifact known as“interlace flicker”, where the vertical edge details (from sharp horizontal lines, for example) will flicker visibly at a 30 Hz rate on normal interlaced NTSC displays. This interlace flicker occurs because the TV monitor displays every other line of the image during the first {fraction (1/60)} second field time, and then displays the lines in between during the next {fraction (1/60)} second field time, as shown in FIG.
1
. In regions of an image having significant vertical detail, the lines of the first field will be quite different from the lines of the second field. This results in the observation of“movement” in the line or edge at the 30 Hz field rate. This perceived movement is called flicker. The amount of flicker in an image depends upon the image content and the way in which the image was created.
Interlace flicker can be reduced by lowpass filtering the image in the vertical direction, namely in a direction effectively transverse to the raster scan direction, thus reducing the vertical sharpness of the image. Generally, the higher the order of the filter the better the overall quality of the resultant image. Flicker filters of order
3
to
6
are typically used to mitigate the effects of flicker. Unfortunately, the filters also distort small font lettering such that the fonts become unreadable or nearly so. Moreover, higher order filters typically require significantly more hardware and memory bandwidth.
Turning again to
FIG. 1
, a block diagram that illustrates an interlaced display is presented. Odd lines
10
,
12
,
14
and
16
are written first, followed by even lines
18
,
20
and
22
. Without filtering, pixel
24
is output only when even line
20
is output, thus creating the appearance of flicker. Filtering the pixel values spreads the pixel across even and odd lines as represented by pixels
26
-
36
. While this method reduces the appearance of flicker, it also creates a blurred image.
The effect of filtering on the readability of small font sizes is illustrated in
FIGS. 2A
to
3
B.
FIG. 2A
is a block diagram that illustrates an unfiltered image of the character “g”. For the purposes of illustration, the lines
40
comprising the character image are one pixel wide.
FIG. 2B
is a block diagram that illustrates the contrast levels present in the
FIG. 2A
image. Only two contrast levels are present in the image. One contrast level is used for the background and another contrast level is used for the lines comprising the character image. The small number of contrast levels and the difference between contrast levels provides a relatively clear image.
FIG. 3A
is a block diagram that illustrates a filtered image of the character in
FIG. 2A
As a result of filtering the character image, the image
42
now appears to be more than one character wide, having luma or chroma values between that of the background and that of the original character image.
FIG. 3B
is a block diagram that illustrates the contrast levels present in the filtered image of a character in FIG.
3
A. As shown by
FIG. 3B
, there are many contrast levels in the filtered image
42
. This smoothing of the character image results in a character that is more fuzzy and consequently less readable.
Further improvements are made possible by analyzing the high frequency content of an image and storing with the image a “flicker code” representative of the extent, if any, to which the vertical dimension of the image is to be subjected to a low pass filtering operation in the course of preparing the image for display. However, this method is not performed real-time. Furthermore, the flicker code typically applies to a whole image and thus fails to recognize that an image may be comprised of some regions that would benefit from filtering and other regions that would not benefit.
There is a trend in the computer field for brighter displays and increased resolution. For example, increasing numbers of computer applications such as e-mail and many Windows™-based applications use relatively small font sizes such as 10 point. Also, many Web pages have brightly colored bars and boxes to attract attention. Additionally, the popularity of High definition and large screen projection TVs continues to increase. Unfortunately, the perceived effects of flicker increase with increasing display brightness and with increasing display resolution.
Accordingly, a need exists in the prior art for a method and apparatus for image enhancement for interlaced displays that mitigates the effect of flicker in real-time and provides enhanced readability of relatively small fonts.
BRIEF DESCRIPTION OF THE INVENTION
A method for image enhancement for an interlaced display includes receiving a first group of pixels aligned about an axis, detecting a second group of pixels within the first group of pixels, each pixel of the second group of pixels having a luminous disparity between adjacent pixels less than a first threshold, determining whether each pixel within the second group of pixels is part of a line or edge, the determination including a comparison of luma or chroma disparities between neighboring pixels and filtering each pixel determined to be part of a line or edge. An apparatus for image enhancement for an interlaced display includes an interface for receiving video data, a vertical filter coupled to the video data interface to detect inter-line correlation between pixels, a horizontal filter coupled to the output of the vertical filter to detect line correlation between pixels when inter-line correlation is detected and a flicker filter coupled to the output of the horizontal filter to filter pixels having horizontal correlation.
REFERENCES:
patent: 5025315 (1991-06-01), Johary et al.
patent: 5136385 (1992-08-01), Campbell
patent: 5428456 (1995-06-01), Parulski et al.
patent: 5724067 (1998-03-01), Atchley et al.
patent: 5874937 (1999-02-01), Kesatoshi
patent: 6094226 (2000-07-01), Ke
Leong Mei Kuen
Tsui Ernest Tinyork
Hsia Sherrie
Schaub John P.
Thelen Reid & Priest LLP
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