Method and apparatus for video filtering

A method and apparatus for video filtering filters video frames by determining a set of frames to be used to generate a single filtered frame. This set of frames is then combined to generate a single combined image. In one embodiment, proportions of the luminances for each of four fields from two consecutive frames in an NTSC video signal are combined to generate the single combined frame.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to filtering processes for video images. 
More particularly, this invention relates to reducing noise in and 
improving compression ratios for video images. 
2. Background 
As computer technology advances, computer systems are finding expanded uses 
in a wide variety of personal, academic and business fields. One such use 
is for the storage, display and transmission of video images. The storage 
and display of video images is very beneficial, for example, in a wide 
variety of multimedia and video conferencing applications. Additionally, 
transmitting video images allows video conferencing environments to exist, 
which allows physically separated individuals to observe one another while 
talking. 
Storing and transmitting video images, however, typically requires a large 
amount of storage space. For example, one common video conferencing format 
provides a display area with 120.times.160 resolution. The display area 
contains 19,200 pixels, each of which can have any one of the available 
colors. If the display has a color resolution of 256 colors (which can be 
represented using 8 bits), then the storage size of an entire-display 
image is approximately 19 kilobytes, which is the amount of memory 
necessary to store the image. Furthermore, video typically operates at a 
rate of 30 frames per second, when showing motion. Thus, under the 
example, five seconds of video requires approximately 2.8 Megabytes of 
storage space. Similarly, transmission of the video image sequence 
requires a transfer rate of 570 kilobytes per second. 
Compression algorithms are often used to reduce the storage space required 
for video images. A wide variety of compression algorithms are known to 
those skilled in the art. However, problems can occur with compression 
algorithms when noise exists in the image. Noise represents spatial 
differences within an image and temporal differences between multiple 
images which do not exist in the actual image input to the system. Noise 
can result, for example, due to camera jitter while receiving the image or 
inaccuracies resulting from an analog to digital conversion. 
There are several disadvantages to noise in video images. One such 
disadvantage is the visual perception of noise. Noise results in a 
difference between the image being displayed by the system and the image 
as it was input to the system, thereby resulting in an inaccurate image 
being displayed as representative of the original input image. 
Additionally, noise degrades the performance of the compression algorithm. 
That is, the compression algorithm needlessly attempts to account for the 
differences in an image or between images created by the noise. Thus, it 
would be beneficial to provide a system which reduces the amount of noise 
in video images. 
Typically, three types of filtering have been used prior to compressing an 
image in order to reduce noise: spatial filtering, temporal filtering, and 
spatio-temporal filtering. In spatial filtering, each image is filtered 
individually, without taking into account the motion between frames. 
Images are filtered by taking into account the values of surrounding 
pixels in determining the value for a particular pixel. Spatial filtering 
typically results in smoothing out the edges of objects being displayed, 
and blurring details in the objects. 
In temporal filtering, an attempt is made to identify the motion between 
image frames. Typically, two frames are separated into blocks and an 
attempt is made to identify the motion of the blocks between the frames. 
Then, the data for the block is stored once along with a motion vector 
indicating the motion of the block. Temporal filtering, however, can 
result in the unnecessary expenditure of computational resources when a 
significant amount of motion is occurring in the images or noise exists in 
the images. This is due to the fact that, regardless of the actual detail 
of an object, an object is perceived as being blurry by the human eye if 
the object is moving fast enough. Thus, a large amount of computational 
effort may be spent to maintain accurate detail in a series of frames, 
even though when the series of frames is actually displayed the observer 
is unable to perceive the detail. 
Temporal filtering typically performs poorly when there is a significant 
amount of temporal noise present. For example, noise in a series of images 
may be interpreted by the temporal filtering process as movement. Thus, 
the noise would remain part of the image, even though it should be 
removed. Furthermore, in some cases, temporal filtering algorithms may 
confuse temporal noise with the motion and thus generate erroneous values 
of motion vectors. 
Spatio-temporal filtering combines aspects of both spatial filtering and 
temporal filtering. In spatio-temporal filtering, an attempt is made to 
identify motion between image frames, as is done in temporal filtering. 
Additionally, images are filtered by taking into account the values of 
surrounding pixels in determining the value for a particular pixel, as is 
done in spatial filtering. Although spatio-temporal filters can be an 
improvement over temporal filtering, spatio-temporal filters still tend to 
work poorly in situations where temporal filters work poorly. 
Additionally, spatio-temporal filters typically require significantly more 
computational effort than either spatial filters or temporal filters 
alone. 
Thus, it would be beneficial to provide a system which efficiently filters 
images to reduce noise in the images. Additionally, it would be beneficial 
to provide a system which improves the compression ratios which can be 
obtained by a compression algorithm, thereby reducing the amount of 
storage space needed to store the images and reducing bandwidth 
requirements for transferring images. 
One additional concern in computer systems is the performance or speed of 
the system. The display, storage and transmission of a large number of 
video images requires substantial computational effort, with faster 
computer systems being able to manipulate video images more quickly. This 
computing power, however, can be very expensive. Therefore, if the amount 
of time spent on displaying, storing or transmitting video images is 
reduced, the system would be able to manipulate video images more 
efficiently. Thus, it would be beneficial to provide a system which 
reduces the computational energy used to filter images. 
The present invention provides for these and other advantageous results. 
SUMMARY OF THE INVENTION 
A method and apparatus for video filtering is described herein. The present 
invention determines a set of frames to be used to generate a single 
filtered frame. This set of frames is then combined to generate a single 
combined image. In one embodiment, proportions of the luminances for each 
of four fields from two consecutive frames in an NTSC video signal are 
combined to generate the single combined frame.

DETAILED DESCRIPTION 
In the following detailed description numerous specific details are set 
forth in order to provide a thorough understanding of the present 
invention. However, it will be understood by those skilled in the art that 
the present invention may be practiced without these specific details. In 
other instances well known methods, procedures, components, and circuits 
have not been described in detail so as not to obscure the present 
invention. 
Some portions of the detailed descriptions which follow are presented in 
terms of algorithms and symbolic representations of operations on data 
bits within a computer memory. These algorithmic descriptions and 
representations are the means used by those skilled in the data processing 
arts to most effectively convey the substance of their work to others 
skilled in the art. An algorithm is here, and generally, conceived to be a 
self-consistent sequence of steps leading to a desired result. The steps 
are those requiring physical manipulations of physical quantities. 
Usually, though not necessarily, these quantities take the form of 
electrical or magnetic signals capable of being stored, transferred, 
combined, compared, and otherwise manipulated. It has proven convenient at 
times, principally for reasons of common usage, to refer to these signals 
as bits, values, elements, symbols, characters, terms, numbers, or the 
like. It should be borne in mind, however, that all of these and similar 
terms are to be associated with the appropriate physical quantities and 
are merely convenient labels applied to these quantities. Unless 
specifically stated otherwise as apparent from the following discussions, 
it is appreciated that throughout the present invention, discussions 
utilizing terms such as "processing" or "computing" or "calculating" or 
"determining" or "displaying" or the like, refer to the action and 
processes of a computer system, or similar electronic computing device, 
that manipulates and transforms data represented as physical (electronic) 
quantities within the computer system's registers and memories into other 
data similarly represented as physical quantities within the computer 
system memories or registers or other such information storage, 
transmission or display devices. 
In general, a computer system used by one embodiment of the present 
invention is illustrated in block diagram format in FIG. 1. The system 
comprises a bus 100 for communicating information and a central processor 
(CPU) 101 coupled with bus 100 for processing information and 
instructions. In one embodiment, CPU 101 is an Intel.RTM. architecture 
compatible microprocessor; however, the present invention may utilize any 
type of microprocessor, including multiple processors of different types. 
The system also includes a random access memory (RAM) 102 coupled with bus 
100 for storing information and instructions for processor 101, a read 
only memory (ROM) 103 coupled with bus 100 for storing static information 
and instructions for processor 101, a data storage device 104 such as a 
magnetic disk and disk drive or CD-ROM and drive coupled with bus 100 for 
storing information (such as video data) and instructions, a display 
device 105 coupled to bus 100 for displaying information to the computer 
user, an alphanumeric input device 106 including alphanumeric and function 
keys coupled to bus 100 for communicating information and command 
selections to processor 101, a cursor control device 107 coupled to bus 
100 for communicating user input information and command selections to 
processor 101, and a signal generating device 108 coupled with a signal 
transformation device 110 which is coupled to bus 100 for communicating 
data to processor 101 or for displaying images on display device 105. 
In one embodiment of the present invention signal generation device 108 
includes, as an input device, a standard video camera to input video data 
to be processed and stored by the computer system. Signal generation 
device 108 includes an analog to digital converter to transform analog 
video data to digital form which can be processed by the computer system. 
In one embodiment, signal generation device 108 also includes a video 
cassette player to input stored video data to processor 101 and the 
remainder of the system over bus 100. 
In one embodiment, signal transformation device 110 is a Philips SAA716 
digital video to PCI bus interface, available from Philips Semiconductors 
of Englewood, Colo. However, it is to be appreciated that the present 
invention can use any of a wide variety of conventional signal 
transformation devices capable of transforming video data from the signal 
generation device format to digital format and transferring the data to 
the bus. In one implementation, signal generation device 108 also includes 
well-known video processing hardware to transform digital video data to 
display signals for display device 105, thereby creating a visual output. 
In one embodiment of the present invention, data transmission device 109 is 
also coupled to bus 100. Data transmission device 109 operates to send and 
receive data (such as video and/or audio data) to and from other computer 
systems. In one implementation, data transmission device 109 is a 
conventional modem. Alternatively, data transmission device 109 may be a 
conventional ISDN interface coupled to an ISDN line. 
The display device 105 utilized with the computer system and the present 
invention may be a liquid crystal device, cathode ray tube, or other 
display device suitable for creating graphic images and alphanumeric 
characters (and ideographic character sets) recognizable to the user. 
Cursor control device 107 allows the computer user to dynamically signal 
the two dimensional movement of a visible symbol (e.g., a pointer) on a 
display screen of the display device 105. Many implementations of the 
cursor control device are known in the art including a trackball, 
trackpad, mouse, joystick or special keys on alphanumeric input device 105 
capable of signaling movement of a given direction or manner of 
displacement. It is to be appreciated that the cursor may also be directed 
and/or activated via input from the keyboard using special keys and key 
sequence commands. It is to be appreciated that the input cursor directing 
device or push button may consist any of those described above and 
specifically is not limited to the mouse cursor device. 
Certain implementations of the present invention may not require nor 
include all of the above components. For example, the system may not 
include cursor control device 107. Alternatively, certain implementations 
of the present invention may include additional processors or other 
components. For example, a second processor identical to processor 101 may 
be included, or a graphics co-processor may be added to the computer 
system. By way of another example, the computer system may include an I/O 
bus and a bridge between the I/O bus and bus 100. Processor 101, RAM 102 
and ROM 103 may be coupled to bus 100, and all other devices may be 
coupled to the I/O bus. 
In one embodiment, the present invention is implemented as a series of 
software routines run by the computer system of FIG. 1. In one 
implementation, these software routines are written in the C programming 
language. However, it is to be appreciated that these routines may be 
implemented in any of a wide variety of programming languages. In an 
alternate embodiment, the present invention is implemented in discrete 
hardware or firmware. 
FIG. 2 is a block diagram showing the functional components used in 
filtering and compressing video images according to one embodiment of the 
present invention. FIG. 2 shows a system 200 which includes Analog/Digital 
(A/D) transformation logic 220, digital video decoder and scaler logic 
(DESC) 230, filtering process 240, and coder/decoder (codec) logic 250. 
System 200 receives as input a video signal 210. Video signal 210 is a 
signal which represents one or more video frames. For example, video 
signal 210 can be a series of video frames which, when displayed 
sequentially, display a movie. Alternatively, video signal 210 can be a 
series of video frames which are the real-time visual images of the 
individual(s) of one side of a video conference. In one embodiment of the 
present invention, video signal 210 is a conventional video signal 
conforming to the National Television Standard Committee (NTSC) format. 
Video signal 210 can be provided to system 200 in any of a wide variety of 
conventional manners. In one embodiment, video signal 210 is an analog 
signal input from a conventional video camera. In one implementation, the 
video camera is a Proshare.TM. video camera. Alternatively, video signal 
210 may be input from a storage device, such as a videocassette player or 
CD-ROM. It should be noted that if the video signal from a storage device 
is in a digital format, A/D transformation logic 220 and DESC 230 can be 
bypassed, as discussed in more detail below. 
Video signal 210 is input to A/D transformation logic 220. When video 
signal 210 is received in analog form, AND transformation logic 220 
transforms video signal 210 to a digital form which can be processed by 
the rest of system 200. The conversion of signals in analog form to 
signals in digital form is well-known to those skilled in the art, and 
thus will not be discussed further. 
DESC 230 provides color decoding and scaling logic for the digitized input 
signal. In one embodiment of the present invention, DESC 230 is the CMOS 
circuit SAA7194 available from Philips Semiconductors. The SAA7194 circuit 
combines the functions of a digital multistandard decoder (Philips 
SAA7191B) and a digital video scaler (Philips SM7186). The DESC allows a 
user of system 200 with controls to adjust luminance, brightness, 
contrast, chroma gain, and chroma saturation. A further discussion of 
these circuits can be found in the Desktop Video Data Handbook, pp. 
259-310, Philips Semiconductors, 1993. 
DESC 230 outputs the video signal to filtering process 240. In one 
embodiment, filtering process 240 reduces digital noise in the video 
signal and performs temporal filtering according to the present invention 
in order to improve the compression ratio when the image is later 
compressed. Filtering process 240 generates and outputs a series of 
filtered video frames, as discussed in more detail below with reference to 
FIG. 3. 
In an alternate embodiment of the present invention, video signal 210 is 
received in digital format. For example, video signal 210 may originate 
from data stored on a CD-ROM, digital video tape, or a magnetic disk. When 
video signal 210 is in digital form, system 200 receives the video input 
as one or more scaled and digitized frames of video at a typical rate of 
between 10 and 30 fps. Thus, in this embodiment, A/D transformation logic 
220 and DESC 230 are not necessary and video signal 210 is input directly 
to filtering process 240. 
The filtered video frames are then output to the codec 250. Codec 250 
compresses the frames which are received from filtering process 240. In 
one embodiment, codec 250 is the Indeo.TM. codec, available from Intel 
Corporation of Santa Clara, Calif. It is to be appreciated, however, that 
any of a wide variety of codecs using any of a wide variety of compression 
algorithms can be used as codec 250. 
In an alternate embodiment of the present invention, the filtered video 
frames from filtering process 240 are not input to codec 250. In this 
alternate embodiment, the filtered video frames are transferred to another 
device in the computer system, such as a data storage device, in an 
un-compressed format. 
Codec 250 outputs a series of compressed video frames 260. The video frames 
260 are a filtered and compressed representation of the input video signal 
210. The compressed video frames 260 can be output to a wide variety of 
devices. For example, video frames 260 may be stored in a storage device, 
such as a magnetic disk. Alternatively, video frames 260 may be 
transferred to a second computer, such as the other computer in a video 
conferencing system. 
FIG. 3 is a flowchart showing the steps followed to filter video images 
according to one embodiment of the present invention. The filtering 
process 240 first selects the fields for filtering, step 310. Each video 
frame comprises two fields, typically referred to as even and odd fields. 
The even field contains even lines and the odd field contains odd lines. 
The fields exist due to the method used by many conventional television 
and video systems to generate their displays. Typically, a conventional 
television system generates its display by using an electron gun(s) to 
illuminate each line on the display screen. The electron gun illuminates a 
single line at a time and typically starts at the top of the screen. It 
illuminates every other line as it travels down the screen, then resets 
itself to the top of the screen once it reaches the bottom. Thus, two 
vertical passes (that is, fields) of the electron gun generate a single 
frame. Video signals which use two fields to generate a single frame are 
often referred to as "interlaced" signals. Video input devices (e.g., 
conventional video cameras and video cassette players) usually generate 
interlaced video signals. 
FIG. 4 shows an example of a series of video frames and fields. A series of 
video frames 400 is shown. The series of frames 400 includes multiple 
fields, such as 430a, 430b, 431a, 431b, etc. In the example of FIG. 4, 
labels with "a" (e.g., 430a) refer to odd fields, and labels with "b" 
(e.g., 430b) refer to even fields. A single frame is comprised of two 
consecutive fields (one odd and one even), such as frames 430 and 431. 
Returning to FIG. 3, in step 310 the present invention selects a number of 
fields from the video frames. In one embodiment of the present invention, 
an even number of consecutive fields is selected so as to have an equal 
number of even and odd fields. In one implementation, four consecutive 
fields are selected, corresponding to two consecutive frames. However, it 
is to be appreciated that any number of fields can be selected in step 
310. 
The filtering process then determines a proportion of the luminance of each 
of the selected fields, step 320. The luminance of a given pixel within a 
frame represents the light intensity or brightness of that pixel. In one 
embodiment, the luminance of a color pixel is determined by taking 
proportions of the red, green and blue video signals for that pixel. In 
one implementation, the proportions are 0.30 of the red video signal, 0.59 
of the green video signal, and 0.11 of the blue video signal. However, it 
is to be appreciated that other conventional methods can also be used to 
determine the luminance. The luminance of pixels within a video image is 
well-known to those skilled in the art, and thus will not be discussed 
further. 
For the ease of notation, the luminance value of a pixel in column i and 
line j of a display sampled at the moment t can be called I.sub.FR 
(i,j,t). We can use similar notation for a pixel in column i, line p of 
the odd field at time t; that is, I.sub.a (i,p,t). We can also use similar 
notation for a pixel in column i, line n of the even field at time t; that 
is, I.sub.b (i,n,t). 
In step 320, a proportion for each of the fields is determined. That is, a 
coefficient is applied to each of the luminance values for every pixel in 
the field. The resulting values are then combined to generate a filtered 
output frame. 
In one implementation, steps 320 and 330 are performed according to the 
following calculations. 
For a sequence of frames with even numbers of lines, j in I.sub.FR (i,j,t) 
varies from 1 to 2.multidot.N (that is, j=1, 2, 3, . . . , 2.multidot.N), 
p in I.sub.a (k,p,t) varies from 1 to N (that is, p=1, 2, 3, . . . , N), 
and n in I.sub.b (m,n,t) varies from 1 to N (that is, n=1, 2, 3, . . . , 
N). 
For the pixels in odd lines the luminance in the filtered frame is equal 
to: 
EQU I.sub.FR (i,2.multidot.n-1,t)=1/8.multidot.I.sub.a 
(i,n,t-1)+3/8.multidot.I.sub.b (i,n,t-1)+3/8.multidot.I.sub.a 
(i,n,t)+1/8.multidot.I.sub.b (i,n,t) (1) 
For the pixels in even lines, the luminance in the filtered frame is equal 
to: 
EQU I.sub.FR (i,2n,t)=1/8.multidot.I.sub.a (i,n+1,t-1)+3/8.multidot.I.sub.b 
(i,n,t-1)+3/8.multidot.I.sub.a (i,n+1,t)+1/8.multidot.I.sub.b (i,n,t)(2) 
For the pixels in the last line (that is, j=2.multidot.N), the luminance in 
the filtered frame is equal to: 
EQU I.sub.FR (i,2.multidot.N,t)=1/2.multidot.I.sub.b 
(i,N,t-1)+1/2.multidot.I.sub.b (i,N,t) (3) 
In calculations (1), (2) and (3), I.sub.a and I.sub.b are as defined above, 
t refers to the current frame and t-1 refers to the previous frame. 
Thus, the fields of two consecutive frames of video are combined to 
generate the combined frame. That is, as shown in calculation (1), the 
pixels of the even and odd fields of the current frame (that is, at time 
t) and the previous frame (that is, at time t-1) are combined to generate 
the odd lines of the combined frame. Similarly, as shown in calculation 
(2), the pixels of the even and odd fields of the current frame the 
previous frame are combined to generate the even lines of the combined 
frame. The lines of the even or odd field used to generate the luminance 
of a particular pixel for the combined frame are as shown in calculations 
(1), (2), and (3). 
In one embodiment, the set of coefficients applied to the fields is {1/8, 
3/8, 3/8, 1/8}, as shown above. In an alternative embodiment, the set of 
coefficients applied to the fields is {1/4, 1/4, 1/4, 1/4}. It is to be 
appreciated, however, that different combinations of the coefficients can 
also be used. 
Thus, in steps 320 and 330, multiple fields are combined to produce a 
filtered frame. The visual effect of this combination is dependent on what 
action, if any, is taking place in the series of frames. For example, a 
particular object may be displayed in the frames which includes screen 
location A. If this object is not moving, then the luminance for location 
A should be the same for each of the frames. Any differences in the 
luminances would be due to noise. Thus, the combining of luminances of 
different frames results in filtering out noise. Furthermore, the 
filtering does not introduce a blurring effect because the combining 
process produces the same luminance for location A as in the frames being 
combined. However, if this object is in a different location in the series 
of frames, then the averaging of luminances has a blurring effect. That 
is, the combined frame is not as clear as the original frames because some 
detail in the figure is lost due to the combining process. However, the 
magnitude of this blurriness is dependent on the speed of the action in 
the original frames. Slow-moving action is blurred a little, whereas 
fast-moving action is blurred more. 
It should be noted, however, that the blurring of the image in the combined 
frame does not necessarily result in an abnormally-blurred image being 
perceived by a viewer. This is due to the fact that individuals perceive 
fast moving objects as being blurred. For example, if a pen with a name 
engraved on it is moved quickly, an individual watching the pen would not 
be able to actually read the engraved name. That is, an individual does 
not actually see all of the details of a fast moving object. Thus, even 
though the combined frame may not contain all of the detail of the 
original frames, some, if not all, of this detail would not have been 
perceived by the individual even if it were present due to the speed of 
the motion. 
It should also be noted that the combining process of the present invention 
does not require significant computational effort. That is, the combining 
of fields as done in the present invention is a relatively quick and 
efficient process--the process does not require that time or computational 
effort be spent in an attempt to identify any motion within the series of 
frames. The luminances of fields can be combined in an efficient manner 
according to calculations (1) and (2), without expending time and energy 
to distinguish between movement and non-movement between the frames. 
Furthermore, the combining process of the present invention filters the 
series of images before it is output to the codec. This filtering improves 
the compression ratios which can be achieved by the codec. Compression 
ratios are increased due to the combining process. That is, by combining 
multiple frames together, fewer changes will occur between frames being 
compressed. Fewer changes will occur because the details of moving objects 
have been blurred to some extent, thereby reducing the magnitude of the 
change which occurs between the frames. 
Additionally, as discussed above, a certain amount of noise can exist in 
the video frames. This noise results in changes in the values of luminance 
being displayed even though the object which includes the noise does not 
change or move. Thus, the codec expends additional computational effort to 
compress the noise as if it represents the actual changes in the frame. 
The combining process of the present invention, however, filters out some 
of this noise, thereby improving compression ratios. 
It should also be noted that, in some instances, not all of the fields are 
used by the present invention in generating the combined frame output. For 
example, data can be captured by the present invention from the input 
video stream at a rate of 30 frames per second, which is 60 fields per 
second. As discussed above, in one embodiment the combined frame is 
generated by combining four fields. Thus, if the present invention were 
outputting combined frames at a rate of 15 combined frames per second, 
then each of the four fields would be incorporated into one of the 
combined frames. However, if the present invention were outputting 
combined frames at a rate of 10 combined frames per second, then only four 
of every six fields would be incorporated into one of the combined frames. 
In embodiments of the present invention where fewer than all of the fields 
are incorporated into the combined frames, certain fields are dropped from 
the filtering process. In one implementation which outputs combined frames 
at a rate of 10 combined frames per second, the fields are divided into 
contiguous groups of six fields each and the first four fields of each 
such group are incorporated into the combined frame in steps 320 and 330. 
For example, if only four of every six fields of the frames in FIG. 4 were 
incorporated into one combined frame, then fields 430a, 430b, 431a and 
431b would be combined using calculations (1) and (2) above, and fields 
432a and 432b would be dropped. The next combined frame would be generated 
starting with field 433a. 
In alternate embodiments of the present invention, the output rate of 
combined frames of the present invention may be high enough that certain 
fields can be incorporated into multiple combined frames. In one 
implementation, this is accomplished by overlapping the frames. For 
example, if the present invention were outputting combined frames at a 
rate of 20 combined frames per second from an input video stream received 
at a rate of 30 frames per second, then there are 60 fields per second to 
combine. The overlapping can be shown by way of example in FIG. 4. The 
first combined frame would incorporate fields 430a, 430b, 431a and 431b. 
The second combined frame would then incorporate fields 431b, 432a, 432b 
and 433a, thereby resulting in an overlap of one field (that is, field 
431b). 
Once the combined frame is generated, the frame is output, step 340. In one 
embodiment, combined frames are output at a rate of 15 combined frames per 
second. The combined frame can be output to any of a wide variety of 
devices. In one embodiment, the combined frame is output to a codec. In an 
alternate embodiment, the combined frame is transferred directly to a 
storage device or a signal transmission device (such as a modem). 
Thus, the present invention efficiently filters frames of video. The frames 
are combined such that portions of the images where motion is occurring 
are blurred slightly, with the amount of blurring being dependent on the 
rate of motion. This blurring both reduces noise and improves compression 
ratios. Additionally, noise is reduced in areas of the frames where no 
motion is occurring, thereby providing improved compression ratios and an 
image with reduced noise. 
Whereas many alterations and modifications of the present invention will be 
comprehended by a person skilled in the art after having read the 
foregoing description, it is to be understood that the particular 
embodiments shown and described by way of illustration are in no way 
intended to be considered limiting. Therefore, references to details of 
particular embodiments are not intended to limit the scope of the claims, 
which in themselves recite only those features regarded as essential to 
the invention. 
Thus, a method and apparatus for video filtering has been described.