Digital cameras abstract
A focusing method and apparatus, for use with digital cameras having
an electronic viewfinder with less display resolution than in the
image generated by the camera's photocell array, uses a uniformly
subsampled representation of the entire image for focusing, rather
than displaying a selected portion of the higher resolution image.
The focusing is assisted by the exaggerated discontinuities produced
by subsampling. Introducing flicker enhances focusing sensitivity
by repetitively displaying, on the electronic viewfinder, a prescribed
set of different reduced-resolution images obtained by subsampling
the same high-resolution image at different sampling locations.
Each subsampled image of the set of reduced resolution images uses
a different set of substantially uniformly distributed pixels.
Digital cameras claims
What is claimed is:
1. A dynamic focusing method for a high-resolution digital camera
having an adjustable focusing mechanism and a reduced-resolution
electronic viewfinder display, the focusing method comprising: (a)
dividing a high-resolution image generated by a high-resolution
image sensor into a plurality of blocks of pixels, each block having
a plurality of pixels, selecting one pixel from a pixel position
within each block, and displaying a reduced-resolution image, with
substantially uniform vertical and horizontal scaling, obtained
from the selected pixel within each block for at least one display
frame on the reduced-resolution viewfinder display; (b) repeating
(a) for a prescribed number of times using a different pixel position
each time, the prescribed number of times being fewer than the number
of pixels in at least one block of pixels; and (c) repeating (b)
to produce a periodic sequence of reduced-resolution images.
2. The method of claim 1 wherein (a) comprises dividing the high-resolution
into a set of equal-size square blocks of pixels.
3. The method of claim 2 wherein the square blocks are 3 pixels
by 3 pixels.
4. The method of claim 2 wherein the square blocks are 4 pixels
by 4 pixels.
5. The method of claim 1 wherein the using a different pixel position
each time is by sequentially selecting a different pixel from one
of four pixel positions arranged as corners of a square.
6. The method of claim 1 wherein the using a different pixel position
each time is by sequentially selecting a different pixel from one
of eight pixel positions forming the perimeter of a square.
7. The method of claim 1 further comprising determining the prescribed
number of times by selecting a number that results in a desired
8. The method of claim 7 wherein the desired flicker rate is approximately
in the range of 3 to 5 cycles per second.
9. The method of claim 1 further comprising adjusting the focusing
mechanism to produce a desired focusing result while watching the
periodic sequence of reduced-resolution images on the reduced-resolution
electronic viewfinder display.
10. A digital electronic camera with a live viewfinder to aid in
focusing, the digital electronic camera comprising: a pixel sensor
array having a plurality of pixel positions for sensing a sequence
of images; a row and column address generator for subsampling the
pixel sensor array, at pixel positions with substantially uniform
intervals, for each image in the sequence of images, said row and
column address generator configured to use a periodic cycle of row
and column start addresses for subsampling successive images of
the sequence of images, thereby generating a sequence of reduced
resolution images consisting of data from addressed pixel positions,
wherein the number of addressed pixel positions of all images in
the sequence of reduced resolution images are fewer than the number
of pixel positions in the plurality of pixel positions; and an electronic
display for displaying the sequence of images as the sequence of
reduced resolution images.
11. The digital electronic camera of claim 10 wherein the row and
column address generator selects a different set of pixels for at
least one reduced resolution image in the sequence of reduced resolution
12. The digital electronic camera of claim 11 wherein the row and
column address generator repeats the sequence of reduced resolution
images for producing a periodic sequence of reduced resolution images
as an aid to focusing.
13. The digital electronic camera of claim 12 wherein the row and
column address generator selects a different set of pixels, for
at least one reduced resolution image, in the sequence of reduced
resolution images used for producing a periodic pattern that corresponds
to a closed cycle of displacement over a total displacement that
is less than the smallest interval between subsamples.
14. The digital electronic camera of claim 13 wherein the periodic
pattern is a cycle of four shifts of equal magnitude that forms
a square closed cycle of displacement.
15. The digital electronic camera of claim 13 wherein the periodic
pattern is a cycle of eight shifts of equal magnitude that forms
a square closed cycle of displacement.
Digital cameras description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of digital electronic
cameras and more specifically to a method for focusing by dynamic
subsampling of the electronic image when using a reduced resolution
2. The Prior Art
All cameras, except the simplest cameras that use small aperture
or pin-hole lenses, require focusing of the lens in order to optimize
the useable camera-to-subject range for a given lens aperture. As
the lens aperture is increased to provide greater light gathering,
the depth of field (range over which a subject is acceptably in
focus) decreases causing the focus adjustment to be more critical.
A wide lens aperture is also used as a means for emphasizing selected
portions of an image by limiting the depth of focus range to include
only the selected portion. For this reason, the photographer needs
to be able to sharply focus a selected portion of the subject scene
so that the portion of interest is within the depth of field for
the lens aperture in use.
The lens focusing is normally done while watching the image sharpness
directly or through some kind of focusing aid using eye-to-hand
feedback control. Conventional film cameras employ a variety of
optical focusing aids such as magnifiers and split-image focusing
prisms. Digital electronic cameras (hereinafter simply referred
to as "digital cameras" can also use the same type of
optical viewing and focusing devices. Digital cameras that incorporate
an electronic display with a resolution that equals (or exceeds)
the full resolution of the electronic image sensor can be focused
by directly viewing the image sharpness while adjusting the focus.
The present invention uses a focusing method for digital cameras
with electronic viewfinders that have an electronic display, such
as a liquid-crystal-display (LCD), with a resolution that is less
than the resolution of the internal electronic image sensor array.
Image sensor array resolution is usually expressed in terms of the
number of "pixels" used to represent the image or, equivalently,
the number of photocells in the image sensor array. Because the
number of pixels in the image sensor array is greater than the number
of pixels that can be displayed, focusing is typically done only
on a selected subset of pixels by using a zoom (or magnifier) mode.
The zoom mode displays a full resolution image by selecting a subset
of contiguous pixels with a pixel count equal to the electronic
The present invention provides a focusing aid and method for use
on an electronic display with reduced resolution that allows the
photographer to interactively focus on any region of the image while
viewing the full image.
BRIEF DESCRIPTION OF THE INVENTION
A dynamic image display method for high-resolution digital cameras,
when viewing is done with a reduced-resolution electronic viewfinder
display, includes the steps of: (a) selecting a subset of pixels
for producing a reduced resolution image with uniformly reduced
horizontal and vertical scales; (b) displaying the reduced resolution
image pixels on a reduced resolution electronic viewfinder display
for at least one display frame; (c) generating a dynamic display
by using a different subset of uniformly spaced pixels each time
a new lower resolution image frame is displayed; and (d) producing
a periodic sequence of such reduced resolution images by cycling
through a prescribed set of pixel subsets.
The dynamic focusing method of the present invention produces a
periodic display of a set of reduced resolution images that exhibit
the effects of spatial frequency aliasing, which provides a useful
aid to focusing.
A digital electronic camera is also disclosed that has a live viewfinder
to aid in focusing that includes a row and column address generator
for subsampling the image sensor pixel array for display in a viewfinder
with a lower resolution. The subsampling is accomplished at substantially
uniform intervals by a programmable row and a column address generator
so that scaling is substantially uniform in the horizontal and vertical
directions of the image. The live viewfinder displays a dynamic
periodic sequence of different subsampled images of the entire image
as an aid to focusing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1. is a simplified drawing of a digital camera.
FIG. 2. shows subsampling using contiguous 4.times.4 pixel blocks
for a N.times.M resolution image.
FIGS. 3(a) and 3(b) are examples of subsampling 1 out of 9 pixels
selected from a 3.times.3 pixel block.
FIG. 3(c) shows an example of subsampling 1 out of 16 pixels selected
from a 4.times.4 pixel block.
FIGS. 4(a)-4(g) show examples of periodic focusing images, produced
by subsampling, as seen in a reduced resolution electronic viewfinder.
FIG. 5 is a table showing a method for computing the coordinates
of non-integer pixel blocks.
FIG. 6 shows the partitioning of an image into pixel blocks for
non-integer resolution reduction.
FIG. 7 is a flow diagram showing a method for computing pixel addresses
for use in producing subsampled images.
FIG. 8 is a block diagram of a digital camera employing scanning
circuitry of the present invention.
FIG. 9 is a block diagram of the main components of scanning circuitry
for an active pixel sensor array of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Persons of ordinary skill in the art will realize that the following
description of the present invention is illustrative only and not
in any way limiting. Other embodiments of the invention will readily
suggest themselves to such skilled persons.
Digital cameras record an image by placing an electronic image
sensor array of N.times.M photocells at the focal plane of the camera
lens. Exposing the photocells to the image captured by the lens
creates a picture image as a set of stored charges. The digital
camera's associated electronic display accesses the image stored
in the photocell array for producing a visual image of the contents
of the photocell array. Thus, the image is stored as a N.times.M
sampled image with each photocell corresponding to a sample of the
FIGS. 1 (a) and 1(b) are drawings of a prior-art digital camera
100. FIG. 1(a) is an external view that includes a lens assembly
101 for projecting an image of a subject onto a photocell array,
an LCD view finder 102 for viewing the image captured by lens assembly
101, and external controls 103 for camera operating mode and shutter
control. The external controls 103 are used for controlling the
lens and for inputting ancillary parameters (such as exposure mode,
compression quality, and aperture size).
FIG. 1(b) shows an electronics assembly 104 that is located at
the focal plane of camera 100 and includes control electronics 105,
storage subsystem 107, and image sensor module 106. Image sensor
module 106 typically has at least one charge-coupled-device (CCD)
or photodiode type pixel sensor array for capturing images as arrays
of charges which can be read out as voltage signals. Multiple pixel
sensor arrays are used for capturing color images after separating
the image by means of a set of filters into a set of color component
images (such as red, green, and blue). The length of time for which
the pixel array is exposed determines the image signal intensity
formed in the pixel array. Either a conventional mechanical shutter
controls the exposure time or an electronic shutter controls the
amount of time from initialization of the array of pixels to the
transfer of the pixel charge to a storage or readout circuit.
Because the pixel charge is sensed as an analog voltage, each pixel
voltage is converted to a binary quantized signal representative
of the charge for storage, read out, or viewing on electronic viewfinder
102 of FIG. 1(a).
If the associated electronic display has a resolution equal to
the N.times.M pixel image stored in the image sensor array (i.e.
capable of electronically resolving the N.times.M pixels on the
display screen) then the photographer can interactively focus the
image on the display screen by adjusting the focus control of the
lens while observing the full resolution image on the display screen.
However, if the electronic display can not provide the full N.times.M
resolution of the stored image, it is customary to average adjacent
image pixels together to form the value of a representative display
pixel. In such a reduced resolution display, the image is blurred
so that the sharpness at the photosensor can not be effectively
judged by eye.
In order to avoid blurring the image, a smaller number of pixels
(N'.times.M') must be selected for display. The prior-art zoom method
selects a pixel array of N'.times.M' contiguous pixels as a representation
of a magnified portion for display. The N'.times.M' pixel array
is chosen to be compatible with the electronic display resolution
capability. The focusing operation, performed on the magnified portion,
does not allow the photographer to view the effects on the overall
image without sequencing through all relevant portions of the image.
As an alternative to the averaging and zooming methods of displaying
the N.times.M image on the reduced resolution N'.times.M' display,
a selection of a subset of uniformly spaced pixels from the original
image can be used. This kind of subsampling without averaging is
known in the art to cause spatial frequency aliasing, an effect
wherein high spatial frequency signal components in the original
image are converted to unwanted signal components of lower spatial
frequency, or components of larger size scale. These effects of
aliasing are often described as "jaggies". The present
invention takes advantage of these aliasing artifacts for providing
information that is a useful aid to focusing.
A simple focusing method related to the present invention is to
adjust the camera to maximize jaggies that result where crisply
focused edges in the original image are aliased into staircase-like
jaggies. At a particular depth, in any region of the image, the
best focus (i.e. maximum sharpness) will correspond to a maximum
jaggieness (i.e. maximum amount of local variance or contrast in
the display). However, the effect is subtle, and difficult to maximize
Subsampling is typically done by taking every n.sup.th pixel value
from every n.sup.th row or, equivalently, by taking a pixel value
from one particular location from every contiguous n.times.n pixel
block 201 that makes up the original N.times.M pixel array 200 as
shown in FIG. 2. This also results in the subsampled image having
the same horizontal and vertical scale reduction. From the example
shown in FIG. 2, for n.times.n=4.times.4, it can be seen that there
are n.sup.2 =16 choices of which pixel to choose as representative
of an n.times.n block of pixels. A choice of a particular identically
positioned pixel in each of the n.times.n blocks results in a unique
uniformly subsampled representation of the original image. For each
particular pixel position within the 4.times.4 block, a different,
but equally valid, reduced resolution representation of the higher
resolution image is obtained.
The present invention provides an improved focusing method that
takes advantage of the previously noted fact that subsampling by
choosing 1 out of n.sup.2 pixel positions as the representative
pixel position allows n.sup.2 different and useful uniformly sampled
images to be created by subsampling. By sequentially displaying
all, or some, of the n.sup.2 subsampled images, the resulting dynamic
display results in a periodic pattern of animated jaggies that displays
more of the original pixel data. The periodic pattern corresponds
to a closed cycle of displacement over a total displacement that
is less that the interval between displayed samples. This dynamic
display provides a live viewfinder display that makes focusing over
the entire data field easier than focusing on a static single subsampled
frame that is repetitively displayed. This results because the human
eye is exquisitely sensitive to very small temporal changes in an
image, so choosing different sampled pixel alignments has a much
greater visual effect on aliased image components than on low spatial
A variety of periodic patterns has been investigated for the purpose
of determining which subsampling schemes produce the most effective
periodic patterns for focusing. Because human vision has maximum
sensitivity to flicker in the 3 to 5 Hertz (Hz) frequency region,
and because image capture and display rates are in the range of
12 to 30 images per second, decimation factors ranging from 3 to
8 result in flicker intensified images in, or near, the preferred
flicker rate range of 3 to 5 Hz.
Preferred subsampling schemes result in the selection of pixels
that are separated horizontally and vertically by the same prescribed
distance so that the resulting change of scale in the horizontal
and vertical directions is the same. FIGS. 3(a)-3(c) show examples
of suitable subsampling schemes in which 1 out of 9 pixels is chosen
from 3.times.3 pixel blocks 201 of FIGS. 3(a) and 3(b), and 1 out
of 16 pixels is selected from 4.times.4 pixel block 201 in FIG.
3(c). In FIG. 3(a), the image is sampled sequentially, starting
at pixel 1 of each 3.times.3 blocks 201 and then sequentially resampling,
clockwise, each 3.times.3 block 201 of sequential image frames 200
for the remaining pixel positions 2-8. Because the sequence is periodic,
the sequence repeats every 8 display frames. This causes the flicker
rate to bed 1/8.sup.th of the display frame rate (e.g. 1.5 to 3.75
Hz for frame rates of 12 to 30 frames per second). The sampling
pattern of FIG. 3(b) sequences through four pixel positions (1-4)
for each 3.times.3 block 201 in sequential frames 200 before repeating
the sequence. This causes the flicker rate to be 1/4.sup.th of the
frame rate and typically results in flicker rates of 3 to 7.5 Hz.
Similarly, the pattern shown in FIG. 3(c) samples 1 out of 16 pixels
of each 4.times.4 block 201 for pixel positions 1-4 before repeating
and thus produces a flicker rate equal to 1/4.sup.th of the frame
rate. The resulting flicker rate would typically be in the range
of 3 to 7.5 Hz. The subsampling patterns that are preferred are
periodic patterns of 4 or 8 different offsets generated in 3.times.3
or 4.times.4 pixel blocks, such that the offset moves in a 4 pixel
small square pattern, or in an 8 pixel large square pattern. Although
a clockwise subsampling sequence is used in FIGS. 3(a) through 3(b),
it should be noted that a counterclockwise sequence or any sequence
through the selected pixel positions can be used to produce the
desired animation of aliased image components.
FIGS. 4(a)-4(g) illustrate an example of a periodic image sequence
produced by subsampling and as displayed on an electronic viewfinder.
In FIG. 4(a), a portion of an image frame 200 is shown. Each full
resolution frame 200 is to be subsampled using 3.times.3 pixel blocks
201. Pixel positions within each pixel block 201 that are to be
used for creating four subsampled images are labeled 1 through 4.
The shaded pixels represent a sharp brightness edge in the discrete
sampled image created by the photocell array of a digital camera.
A row and column coordinate, (r, c) respectively identifies each
pixel block. If one pixel position (of 1-4) is used in every pixel
block 201 of FIG. 4(a) to produce a reduced resolution image 250,
a different image, with a 3-to-1 scale reduction, is created for
each of the four pixel positions. Thus, FIGS. 4(b) through 4(e)
respectively show the subsampled images corresponding to sampling
pixel positions 1 through 4. The indices for the rows and columns
of FIGS. 4(b)-4(e) corresponds to the pixel block coordinates of
FIG. 4(a) from which the subsampled pixels were taken. If all four
subsampled images of FIGS. 4(b)-4(e) are sequentially displayed,
the image in 4(f) would result and have a flicker rate of 1/4 of
the display frame rate. The relative jaggieness of the resulting
image in FIGS. 4(f) is also increased because a discontinuity of
one pixel in the scaled subsampled image corresponds to a 3 pixel
discontinuity in the original image. The degree of shading in FIG.
4(f) indicates a variation in intensity due to the number of shaded
pixels in the set of subsampled images that are superimposed. FIG.
4(g) shows the light-dark (or on-off) time history of selected pixels
(0, 6), (0, 7), (1, 2), and (1, 3) as a function of both frame intervals
and sample pixel number from which it can be seen that a flicker
period of four frame intervals is created.
The important visual feature that distinguishes this inventive
viewfinder image from that of the prior-art method of averaging
of corresponding frames is the use of motion and flicker, which
are readily apparent in image regions that are sharply focused.
The above descriptions were limited to specific examples for clarity
of explanation of the invention. For example, subsampling, which
was limited to scaling factors of 3-to-1 and 4-to-1 (or decimation
factors of 9 and 16), may not be appropriate because specific differences
between a digital camera resolution and the viewfinder resolution
may require other scaling factors that can include non-integer reduction
factors. However, the principles described above can be readily
adapted to accommodate the general non-integer case.
For example, consider a non-integer resolution reduction factor
of 2.75. Because fractional pixels do not exist in the full resolution
image, the pixel array 200 of FIG. 4(a) can not be partitioned into
2.75.times.2.75 pixel blocks 210. FIG. 5 is a table that shows how
the method is adapted for the non-integer case. Column A is a sequence
of uniform horizontal and vertical pixel addresses (decimal) at
which an edge of a pixel block would be located if fractional pixels
could be used. Column B is the binary coded equivalent of column
A. Column C is a truncated version of column B where the fractional
part of the column B entries have been dropped so that an integer
approximation of column B results. The average pixel block interval
asymptotically approaches the desired non-integer interval as the
size of the high-resolution image pixel array increases. Because
of the substantially uniform subsampling interval, substantially
uniform horizontal and vertical scaling of the image results.
FIG. 6 shows the results of using the values of FIG. 5, column
C. The full resolution image array is shown partitioned into 3.times.3,
3.times.2, 2.times.3, and 2.times.2 pixel blocks 201 in proper proportion
to produce a subsampled image with an average decimation factor
of 2.75.times.2.75. (If the values of column B were rounded before
truncation, the distribution of pixel block sizes for large size
image arrays would have been the same. Hence, the preferred implementation
does not include rounding before truncation.)
The location of the pixels to be displayed within each pixel block
should preferably be chosen so that all pixel locations will fit
within all pixel blocks, including the smallest (2.times.2 for the
example of FIG. 6). As a result, a closed cycle of displacement
over a total displacement that is less than the smallest interval
between samples. The number of pixel locations selected for sequential
display determines the flicker rate. For example, in FIG. 6, four
unique pixel locations are indicated for each pixel block so that
the flicker rate is one-fourth of the frame display rate if each
subsampled image corresponding to a selected unique pixel location
is displayed once during a flicker period. The flicker period can
be increased either by increasing the number of unique pixel locations
or by sampling one or more of the unique pixel locations more than
once during a flicker period.
Dashed line boundaries 202 in FIG. 6 show that samples are still
taken from equal-size square blocks, but that these blocks are no
longer necessarily contiguous since they are sub-blocks of the unequal
FIG. 7 is the flow diagram of preferred method 700 for determining
the coordinates (addresses) of the pixels that are required to achieve
a given integer or non-integer resolution reduction factor, m. Step
701 sets initial sample coordinates Y=Y.sub.0 . . . Step 702 sets
X=X.sub.0. In step 703, the pixel value at coordinates X.sub.int,
Y.sub.int, where the subscript represents the floor function or
integer part, is read from the high resolution image. In step 704,
the next possibly non-integer horizontal address, is computed using
its previous value and m. If, in step 705, it is determined that
X does not exceed the horizontal pixel range, the process returns
to step 703. Otherwise, step 706 is used to compute the next possibly
non-integer row address, Y. If, in step 707, it is determined that
Y is not greater than the row limit of the high-resolution limit,
the process returns to step 703. Otherwise, the process ends and
the subsampling is complete.
By repeating the process for a selected set of initial pixel locations
(X.sub.0, Y.sub.0), method 700 can be used to generate a periodic
sequence of reduced resolution images for display.
FIG. 8 is a block diagram of a digital camera 800 employing scanning
circuitry for subsampling high resolution pixel sensor array 802
for display on lower resolution viewfinder display 804 that may
be used in accordance with the methods disclosed herein. The addresses
and control signals, generated by flexible address generator 806,
provides all of the signals necessary to control the reading of
pixel data out of pixel sensor array 802. Flexible address generator
806 is used to read the high-resolution image out of pixel sensor
array 802 for storage in storage system 808. Also, flexible address
generator 806 is used to subsample the high-resolution image generated
by pixel sensor array 802 for display on viewfinder display 804
so that the captured image can be adjusted and focused at the reduced
resolution display of viewfinder 804.
FIG. 9 is an illustrative block diagram showing in more detail
the relationship between the flexible address generator and pixel
sensor array of FIG. 8 with N rows and M columns. Flexible address
generator 900 includes row address generator 902, row decoder 904,
column address generator 906, column selector 908, and controller
910. Row address generator 902 and column address generator 906
are loadable counters under the control of controller 910. Controller
910 provides clock signals, counting interval (scale factor) in,
and an initial offset address, (X.sub.0,Y.sub.0), to row and column
address generators 902 and 906, and receives status signals from
row and column address generators 902 and 906. The readout of a
subsampled image from pixel sensor array 912 begins with the loading
of the initial offset coordinates, (X.sub.0,Y.sub.0), as respective
initial addresses to row address generator 902 and column address
generator 906. The column address counter is then clocked to increment
by m for producing the non-truncated coordinates, (X, Y) of which
only the integer part bits are respectively supplied to row decoder
904 and column selector 908 for selecting the row and column of
the pixel that is to be readout on output line 914 for display on
viewfinder 804 of FIG. 8. When the last subsampled pixel of a given
row is read out, the column address generator activates line EQ
to indicate that the row has been subsampled. The counter of row
address generator 902 is incremented by m for producing a next Y
value, and the column address generator 906 is reset to X.sub.0.
The previously described operation for reading the selected columns
is repeated. When the last row and column is readout, a scan-complete
signal (EQ) is sent to controller 910 by row and column address
generators 902 and 906. The controller produces a new subsampled
image display by initializing the process with a new set of prescribed
initial coordinate offsets.
A more detailed description of the circuitry shown in and described
with reference to FIGS. 8 and 9 may be found in co-pending application
Ser. No. 09/120,491, filed Jul. 21, 1998. This co-pending application
is expressly incorporated herein by reference.
While embodiments and applications of this invention have been
shown and described, it would be apparent to those skilled in the
art that many more modifications than mentioned above are possible
without departing from the inventive concepts herein. The invention,
therefore, is not to be restricted except in the spirit of the appended