Computer graphics processing and selective visual display system – Computer graphics processing – Attributes
Reexamination Certificate
1999-10-08
2002-09-24
Razavi, Michael (Department: 2672)
Computer graphics processing and selective visual display system
Computer graphics processing
Attributes
Reexamination Certificate
active
06456294
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims priority from prior French Patent Application No. 98-12692, filed Oct. 9, 1998, the entire disclosure of which is herein incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to displays, and more specifically to a method and circuit for improving the chromatics of digital on-screen displays overlaid on a video image by increasing the number of colors available to create the overlays.
2. Description of Related Art
A video image is formed by a sequence of lines of pixels. The pixels are elementary picture elements. In color television systems, each pixel is a carrier of luminance information and chrominance information that makes it possible to specify the luminosity and color of each image point. In the most commonly used television systems, the image develops according to a principle of horizontal scanning of the screen. Whether it is the NTSC television system, the PAL television system, or the SECAM television system, each image displayed results from the alternating display of two distinct partial images known as frames. In general, there is a first frame, which is known as an odd-parity frame, and a second frame which is known as an even-parity frame.
The very nature of television images (i.e., the interlacing of two partial images) gives rise to problems of image quality and flicker. In an extreme case, a line of black pixels on a white image background gives the human eye the unpleasant impression of appearing and disappearing, especially if this line is observed from a close distance. Furthermore, it is increasingly common to present on-screen displays (OSDs) that are overlaid on video images. Each pixel of an OSD is characterized by three values Y, U, and V that determine luminosity and color and a value MW for transparency. For example, OSDs are commonly used to provide a permanent or non-permanent indication of the progress of a sporting event, such as the score of a match. OSDs are also used by certain television channels to show their logo so as to indicate to the viewer the television channel that is being watched.
FIG. 1
shows a conventional structure for the code of an OSD. A binary train
10
containing the codes needed to display an OSD typically consists of three sets of bits that have different functions. A first set of bits
12
forms the OSD header. The OSD header
12
has information on the number of bits needed to define a pixel, and information on the coordinates of certain characteristic points of the OSD. A second set of bits
14
is used to define the panel of colors that will be used during the display of the OSD. All of the colors that will be used are stored in a memory known as a color look-up table (CLUT).
Generally, the CLUT is limited to
256
memory lines, with each memory line containing the values of the bits corresponding to a color programmed in a table through the second set of bits
14
. The choice of the size of the color look-up table results from a compromise between the access speed of the table and the number of colors needed to provided accurate quality for the OSD. Additionally, a third set of bits
16
is formed by the addresses of the memory lines of the table of the available colors containing the appropriate color for each pixel of the OSD.
While flicker problems exist, in purely analog television pictures they are small as compared with the flicker phenomena that can appear when digital on-screen displays are presented on a video image background. In particular, the systematic attenuation of contrasts in video images, which is caused by the presence of cameras in the image generation cycle, is not encountered during the creation of an OSD. As a result, two patterns of colors that are distant from each other in the spectrum of visible frequencies may be juxtaposed. Thus, major contrasts may appear between two consecutive lines when digital on-screen displays are overlaid on the video image. Further, when an OSD is created, the background of the video image on which the OSD will be displayed is not generally known. Thus, in extreme cases, a blue OSD may be displayed when the background of the video image is red. Additionally, nothing prevents the designer of an OSD from juxtaposing two lines with high contrast with respect to each other in the OSD itself.
FIGS. 2
a
and
2
b
illustrate problems that can be encountered during the display of an OSD on a video image background.
FIG. 2
a
shows a television screen
10
that is displaying an OSD
21
on a video image background
20
. The OSD
21
shows a zone with a white background from which there emerges a thin black line with the width of a pixel. This is typically the case where the flicker effect is the most visible. Due to the interlacing of the even-parity and odd-parity frames, the thin black line
22
appears and disappears at a speed that is low enough for this phenomenon to be detected by the human eye.
FIG. 2
b
shows another problem related to the display of an OSD on a video image background. On the background of a video image
20
, an OSD
21
is formed by a first pattern
24
that is displayed with the even-parity frame and a second pattern
25
that is displayed with the odd-parity frame. This gives an impression of flutter
26
. More specifically, the OSD seems to rise and fall the amplitude of a pixel at a rate dictated by the refresh frequency of the frames of the image. This flutter problem has the same cause as the flicker problem. The phenomenon of flutter appears when the same image is displayed by the even-parity frame and by the odd-parity frame. Depending on the television system used, the even-parity frame appears every 1/25 seconds or every 1/30 seconds in alternation with the odd-parity frame. The human eye detects a flutter motion due to the appearance of the OSD alternately on one set of lines and then on the set of directly neighboring lines.
These phenomena of flicker and flutter are mainly perceived when long horizontal lines are displayed on the screen, which is frequent in OSDs. There are several conventional approaches to overcoming these problems of vertical transition in the definition of an image having one or more OSDs. For example, it is possible to apply mathematical filters that use the values of neighboring pixels and weighting to compute new values for these pixels. Furthermore, there are circuits that enable automatic detection of excessive color contrasts between succeeding lines. These circuits then automatically carry out the weighting operations associated with the appropriate mathematical filter.
To avoid the sudden transitions of color that give rise to the phenomena explained above, it is also possible to act on the transparency of the pixels of an OSD. This approach is applied to the lines or groups of lines that constitute the boundary zone between an OSD and a video image. When action is taken on a single boundary line between the OSD and the video image, the transparency of the pixel of the OSD is usually 50%. Thus, each pixel of the final image that is located on the boundary has a balanced contribution of the pixels of the video image and of the pixels of the OSD. When several successive lines of the boundary zone are concerned, the transparency of the pixels of the OSD increases when approaching the pixels of the video image.
FIG. 3
shows an example of such a technique. In
FIG. 3
, an OSD
30
is constituted by portions of pixel lines. Three first lines of the OSD L
1
, L
2
, and L
3
have the same values Y, U, and V characterizing the luminance and chrominance of each pixel. These lines of pixels differ only by their transparency MW. By way of example, the transparency of the pixels of line L
3
is about 25%, the transparency of the pixels of line L
2
is about 50%, and the transparency of the pixels of line L
1
is about 70%. These transitions are visible in a zone
31
that corresponds to a portion of the screen that is enlarged for clarity. The pixels of the OSD (symbolize
Bongini Stephen
Cunningham G. F.
Fleit Kain Gibbons Gutman & Bongini P.L.
Jorgenson Lisa K.
Razavi Michael
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