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Digital Camera Patent Abstract
A method of storing an image in a digital camera, comprising the
steps of: capturing the image using the selected quantization table.
A method of storing an image in a digital camera, wherein the step
of selecting a quantization table comprises the steps of: selecting
a quality setting; compressing the image using a quantization table
corresponding to the selected quality setting; decompressing the
image; evaluating the decompressed image with the image quality
metric; and, adjusting the quantization table such that the quality
metric matches the selected quality setting.
Digital Camera Patent Claims
1. A method of storing an image in a digital camera, comprising
the steps of: a) capturing an image; b) selecting a quantization
table based on an image quality metric that is dependent upon the
captured image; and c) compressing the image using the selected
quantization table.
2. The method claimed in claim 1, wherein the step b) of selecting
a quantization table comprises the steps of: i) selecting a quality
setting; ii) compressing the image using a quantization table corresponding
to the selected quality setting; iii) decompressing the image; iv)
evaluating the decompressed image with the image quality metric;
and v) adjusting the quantization table such that the quality metric
matches the selected quality setting.
3. The method claimed in claim 2, wherein the compressing step
ii) is a JPEG compression step involving block DCT, quantization
of DCT coefficients, and entropy coding of the quantized coefficients.
4. The method claimed in claim 2, wherein the decompressing step
iii) is a JPEG decompression step involving entropy decoding, dequantization
of DCT coefficients, and inverse block DCT.
5. The method claimed in claim 3, wherein the step ii) of compressing
is performed without entropy coding and the step iii) of decompressing
is performed without entropy decoding.
6. The method claimed in claim 4, wherein the step ii) of compressing
is performed without entropy coding and the step iii) of decompressing
is performed without entropy decoding.
Digital Camera Patent Description
FIELD OF THE INVENTION
[0001] The present invention relates to digital image capture,
and more particularly to a digital camera having a picture quality
setting.
BACKGROUND OF THE INVENTION
[0002] Digital cameras such as the Kodak DC 280 apply JPEG compression
prior to storing captured images in a camera memory card. JPEG compression
involves performing a discrete cosine transform (DCT) on blocks
of pixels (e.g. 8.times.8) of the image. The DCT coefficients are
quantized to compress the image, and the quantized coefficients
are entropy encoded (e.g. Huffman encoding) to produce the compressed
image file. Such cameras may include a picture quality setting feature
that allows the operator to select a quality option, for the image
that is stored in the camera. The quality selection chooses a quantization
table scale factor or quantization table used to quantize the DCT
coefficients.
[0003] Generally a smaller image file results from a lower quality
setting, and vice versa. One problem with this approach is that
for a given quality setting images having different amounts of image
detail will have different apparent quality. For example a very
busy image compressed at an intermediate quality setting will have
a low quality appearance, whereas an image with very little detail
may have a high quality appearance even if it is compressed at the
lowest quality setting.
[0004] There is a need therefore for an improved technique for
accurately adjusting image quality in a digital camera.
SUMMARY OF THE INVENTION
[0005] The need is met according to the present invention by providing
a method of storing an image in a digital camera, comprising the
steps of: capturing an image; selecting a quantization table based
on an image quality metric; and compressing the image using the
selected quantization table.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 shows a typical digital camera architecture.
[0007] FIG. 2 shows a block diagram of one embodiment of the current
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0008] Memory cards (i.e., CompactFlash card) are widely used by
digital capture devices such as digital cameras to store captured
images before they are transferred to other storage mediums. Due
to the limited capacity of a memory card, digital cameras such as
the Kodak DC 280 apply JPEG compression prior to storing captured
images in a camera memory card. JPEG compression involves performing
a discrete cosine transform (DCT) on blocks of pixels (e.g. 8.times.8)
of the image. Image compression is achieved when the DCT coefficients
are quantized with a quantization table. The quantized coefficients
are entropy encoded (e.g. Huffman encoding) to produce the compressed
image file.
[0009] Different levels of compression can be achieved with different
quantization tables, and in general, the more an image is compressed,
the lower the quality the image has, and vice versa. Therefore,
digital cameras such as the Kodak DC 280 normally include a picture
quality setting feature that allows the operator to select a quality
option, essentially trading off between image quality and the number
of images that can be stored in a memory card. Within the camera
design, the quality selection actually chooses an appropriate quantization
table used to quantize the DCT coefficients.
[0010] The current invention adds intelligence to the quantization
table selection process by adding an image analysis step using certain
image quality metric(s). With this image-dependent approach, consistent
apparent quality may be achieved for images having different amounts
of details.
[0011] Referring first to FIG. 1, a typical digital camera architecture
is shown. Lights from a scene pass through camera lens 10, and are
collected by the sensor 12. The sensor image is then sent to a signal
processor 14 where various image processing steps may take place
(i.e., CFA interpolation, color correction). A typical digital camera
also has other peripherals such as RAM 18 where intermediate images
are stored, memory card 20 where the captured images are finally
stored. All these peripherals are normally controlled by a microprocessor
controller 16 acting as a coordinator.
[0012] For camera settings (i.e., quality, date) as well as for
image preview, a LCD 22 is also included in a typical digital camera
along with buttons 24 26 28 for navigating within the LCD. For example,
underneath the Quality Setting, a camera may let users select from
three choices (as shown in FIG. 1):
[0013] "Best", "Better" and "Good",
respectively. A user may press up arrow button 24 or down arrow
button 28 to switch among these three settings, and then press the
selection button 26 for final quality selection. All these actions
are coordinated by the microprocessor controller, and the quality
selection by a user will eventually be feedback to the microprocessor
as well.
[0014] Referred to FIG. 2, which illustrates the flowchart for
one embodiment of the current invention. A user selects a quality
setting 40, then presses the button to capture an image 42. The
digital camera processes the captured sensor image to produce a
processed digital image for compression before storing it in the
memory card. A copy of the processed digital image is stored in
RAM. Based on the quality setting of the user, a default quantization
table is selected 44 and is used to compress the digital image 46
to generate a compressed image. Assume the digital camera has three
quality setting choices as shown in FIG. 2, three quantization tables
(table-best, table-better, and table-good) are pre-selected to be
the default tables for the corresponding quality setting choices.
In another approach, one quantization table (table-best) is pre-selected
as default quantization table for the "Best" quality setting
selection, along with two scale factors (scale-better and scale-good).
These two scale factors will be used to generate default quantization
tables for "Better" and "Good" quality setting
selections on the fly by scaling the entries of table-best. These
default quantization tables as well as the scaling factors can be
pre-determined through studying a large number of consumer type
images as well as consumer preferences. A copy of the compressed
image is stored in RAM.
[0015] The compressed image is then decompressed 48 to reconstruct
the processed digital image. Image quality metric(s) is(are) further
applied 50 to evaluate the quality of the reconstructed processed
digital image. After that, a decision has to be made whether the
quality of the reconstructed processed digital image meets the user
requirement 52. If the image quality indicated by the image quality
metric for the specific reconstructed processed digital image is
appropriate for the user selected quality settings, then the copy
of the compressed image in RAM is sent to the memory card for storage
54. Otherwise, a more appropriate quantization table is selected,
the copy of the processed digital image is then retrieved from RAM
and it goes through compression 46, decompression 48, and image
quality evaluation 50 steps again, followed with another decision
step 52. Recursive loops of step 44, 46, 48, 50 and 52 might be
necessary until a satisfying result can be achieved.
[0016] For a three-level quality setting of "good", "better"
and "best" with three quantization tables, namely "coarse",
"medium" and "fine", respectively, the number
of recursive loops is limited to 2, incurring additional but reasonable
computation. If the initially selected quantization table produces
a compressed image with consistent quality to the user selected
quality setting, the compressed image is sent to the memory card
for storage and no further processing is needed; if the initially
selected quantization table produces a compressed image with lower
quality than the user selected quality setting, a finer quantization
table, if still available, is used to compress the processed digital
image, until a satisfying result is achieved; if the initially selected
quantization table produces a compressed image with higher quality
than the user selected quality setting, a coarser quantization table,
if still available, is used to compress the processed digital image,
until a satisfying result is achieved. Alternatively, all the available
quantization tables can be used to compress the processed digital
image, and a satisfying result is selected according to the image
quality metric; however this procedure is computationally inefficient
compared to the recursive procedure.
[0017] Major image quality issues associated with JPEG compression
are the severity of blocking and contouring artifacts in a JPEG
compressed image. Therefore, any image quality metric that correlates
with visual perception of blocking and contouring artifacts may
be used in the image quality evaluation step. In the embodiment
of the current invention, the metric used is the one described in
the commonly assigned U.S. patent application Ser. No. ______ (Docket
No. 81593), filed on even date herewith in the names of Q. Yu and
J. Luo and entitled "A Method of Detecting the Extent of Blocking
and Contouring Artifacts in a Digital Image", which is incorporated
herein by reference. This metric measures both the amount of blocking
and contouring artifacts within a JPEG compressed image, and predicts
the image quality that will be perceived by consumers. More specifically,
this metric is based on a digital image processing method that includes
the steps of: forming a column difference image; averaging the values
in the columns in the column difference image to produce a column
difference array; computing the average of the values in the column
difference array that are separated by one block width to produce
a block averaged column difference array; locating the peak value
in the block averaged column difference array; calculating the mean
value of the block averaged column difference array excluding the
peak value to produce a column base value; computing the ratio between
the peak value and the base value to produce a column ratio; repeating
steps the above steps in the row direction to produce a row ratio;
and employing the column and row ratios as a measure of the extent
of blocking artifacts in the digital image. In an additional series
of steps the extent of contouring artifacts is determined by the
steps of: locating block boundaries based on the locations of peak
values of column and row difference arrays; calculating a DC value
for each block; generating a histogram of the block DC values; calculating
the Fourier transform of the histogram; locating the first non-DC
peak in the Fourier transform domain; calculating a DC quantization
step size based on the frequency of the first non-DC peak; and employing
the DC quantization step size as a measure of the extent of the
contouring artifacts in the digital image.
[0018] Note that the entropy encoding process in the compression
step and the entropy decoding process in the decompression step
cancel each other and they do not affect the image quality of the
reconstructed processed digital image. Therefore, for implementation
efficiency, these two steps are not included in the embodiment of
current invention.
[0019] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will
be understood that variations and modifications can be effected
within the spirit and scope of the invention.
PARTS LIST
[0020] 10 camera lens [0021] 12 image sensor [0022] 14 signal processor
[0023] 16 microprocessor controller [0024] 18 RAM [0025] 20 memory
card [0026] 22 LCD [0027] 24 push button to increase quality setting
[0028] 26 push button to select quality setting [0029] 28 push button
to decrease quality setting [0030] 40 quality selection step [0031]
42 image capture step [0032] 44 quantization table selection step
[0033] 46 image compression step [0034] 48 image decompression step
[0035] 50 image quality evaluation step [0036] 52 image quality
checking step [0037] 54 image storage step |