A = bwlookup(BW,lut) performs
a 2-by-2 or 3-by-3 nonlinear neighborhood filtering operation on binary
or grayscale image BW and returns the results
in output image A. The neighborhood processing
determines an integer index value used to access values in lookup
table lut. The fetched lut value
becomes the pixel value in output image A at the targeted position.

A is the same size as BW

A is the same data type as lut

This function supports code generation (see Tips).

gpuarrayA = bwlookup(gpuarrayBW,lut) performs
the filtering operation on a GPU. The input image and output image
are gpuArrays. lut can be
a numeric or gpuArray vector. This syntax requires
the Parallel Computing Toolbox™.

Perform an erosion along the edges of a binary
image using a 2-by-2 neighborhood. Vector lut is
constructed such that the filtering operation places a 1 at the targeted
pixel location in image A only when all 4 pixels
in the 2-by-2 neighborhood of BW are set to 1.
For all other 2-by-2 neighborhood combinations in BW,
the targeted pixel location in image A is set to 0.

Construct lut so it is true only
when all four 2-by-2 locations equal 1.

Input image transformed by nonlinear neighborhood filtering
operation, specified as either a grayscale or binary (logical) image.
In the case of numeric values, non-zero pixels are considered true which
is equivalent to logical 1.

Lookup table of output pixel values, specified as a 16- or 512-element
vector. The size of lut determines which of the
two neighborhood operations is performed.

If lut contains 16 data elements,
then the neighborhood matrix is 2-by-2.

If lut contains 512 data elements,
then the neighborhood matrix is 3-by-3.

Output image, returned as a grayscale or binary image whose
size matchesBW, and whose distribution of pixel
values are determined by the content of lut.

This function supports the generation of C code using MATLAB^{®} Coder™.
Note that if you choose the generic MATLAB Host Computer target
platform, the function generates code that uses a precompiled, platform-specific
shared library. Use of a shared library preserves performance optimizations
but limits the target platforms for which code can be generated. For
more information, see Code Generation Using a Shared Library.

When generating code, specify an input image of class logical.

The first step in each iteration of the filtering operation
performed by bwlookup entails computing the index into
vector lut based on the binary pixel pattern
of the neighborhood matrix on image BW. The value
in lut accessed at index, lut(index),
is inserted into output image A at the targeted
pixel location. This results in image A being
the same data type as vector lut.

Since there is a 1-to-1 correspondence in targeted pixel locations,
image A is the same size as image BW. If the
targeted pixel location is on an edge of image BW and
if any part of the 2-by-2 or 3-by-3 neighborhood matrix extends beyond
the image edge, then these non-image locations are padded with 0 in
order to perform the filtering operation.

The following figures show the mapping from binary 0 and 1 patterns
in the neighborhood matrices to its binary representation. Adding
1 to the binary representation yields index which
is used to access lut.

For 2-by-2 neighborhoods, length(lut) is
16. There are four pixels in each neighborhood, and two possible states
for each pixel, so the total number of permutations is 2^{4} =
16.

To illustrate, this example shows how the pixel pattern in a
2-by-2 matrix determines which entry in lut is
placed in the targeted pixel location.

Create random 16-element lut vector
containing uint8 data.

scurr = rng; % save current random number generator seed state
rng('default') % always generate same set of random numbers
lut = uint8( round( 255*rand(16,1) ) ) % generate lut
rng(scurr); % restore

Create a 2-by-2 image and assume for this example
that the targeted pixel location is location BW(1,1).

BW = [1 0; 0 1]

BW =
1 0
0 1

By referring to the color coded mapping figure above,
the binary representation for this 2-by-2 neighborhood can be computed
as shown in the code snippet below. The logical 1 at BW(1,1) corresponds
to blue in the figure which maps to the Least Signficant Bit (LSB)
at position 0 in the 4-bit binary representation (,2^{0}=
1). The logical 1 at BW(2,2) is red which maps
to the Most Significant Bit (MSB) at position 3 in the 4-bit binary
representation (2^{3}= 8) .

% BW(1,1): blue square; sets bit position 0 on right % BW(2,2): red square; sets bit position 3 on left
binNot = '1 0 0 1'; % binary representation of 2x2 neighborhood matrix
X = bin2dec( binNot ); % convert from binary to decimal
index = X + 1 % add 1 to compute index value for uint8 vector lut
A11 = lut(index) % value at A(1,1)

index =
10
A11 =
246

The above calculation predicts that output image A
should contain the value 246 at targeted position A(1,1).

A = bwlookup(BW,lut) % perform filtering

A =
246 32
161 231

A(1,1) does in fact equal 246.

Note:
For a more robust way to perform image erosion, see function imerode.

For 3-by-3 neighborhoods, length(lut) is
512. There are nine pixels in each neighborhood, and two possible
states for each pixel, so the total number of permutations is 2^{9} =
512.

The process for computing the binary representation of 3-by-3
neighborhood processing is the same as shown above for 2-by-2 neighborhoods.