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;;;
;;; Thresholding code contributed by Ivan Chernetsky in 2011.
;;; See COPYRIGHT file for details.
(in-package :opticl)
(defun threshold-image (image threshold)
"Performs simple thresholding of grayscale image and returns a
binarized image of type 1-bit-gray-image. Before thresholding
threshold is coerced to type of image's elements.
An error of type type-error is signaled if image is not of
gray-image type."
(etypecase image
(gray-image
(with-image-bounds (height width) image
(let ((binary-image (make-1-bit-gray-image height width :initial-element 0))
(threshold (coerce threshold (array-element-type image))))
(declare (type gray-image image)
(type 1-bit-gray-image binary-image))
(do-pixels (i j) image
(when (>= (pixel image i j) threshold)
(setf (pixel binary-image i j) 1)))
binary-image)))))
;; Below is an implementation of minimum error thresholding
(defconstant +8-bit-values-count+ 256
"Number of different values an element of 8-bit-gray-image
image can be of.")
(defconstant +8-bit-max-value+ 255
"Maximum value an element of 8-bit-gray-image image can be.")
(defun compute-histogram (image)
"Computes a normalized histogram of 8-bit-gray-image image,
i.e. an estimate of the probability density function of gray
levels, and returns it."
(declare (type 8-bit-gray-image image))
(let ((histogram (make-array +8-bit-values-count+
:element-type 'double-float
:initial-element 0d0)))
(do-pixels (i j) image
(incf (aref histogram (pixel image i j))))
(with-image-bounds (height width) image
(let ((size (* height width)))
(loop for i below +8-bit-values-count+
do (setf (aref histogram i)
(/ (aref histogram i) size)))))
histogram))
(defconstant +j-fn-max-value+ most-positive-double-float
"A constant that is used as a default value of J(T) function,
if its value cannot be calculated for a given T.")
(defun sample-j-fn (histogram)
"Returns an array of values of J(T) for T in [0; 255].
The elements at indexex 0 and 255 are set to +j-fn-max-value+."
(declare (type (simple-array double-float (256)) histogram))
(let* ((histlen (length histogram))
(samples (make-array histlen :element-type 'double-float
:initial-element +j-fn-max-value+))
(apriori-1 (aref histogram 0)))
(declare (type double-float apriori-1))
(loop for idx from 1 upto (- histlen 2) do
(progn
(incf apriori-1 (aref histogram idx))
(if (zerop apriori-1)
(setf (aref samples idx) +j-fn-max-value+)
(let* ((apriori-2 (- 1d0 apriori-1))
(mean-1 (/ (loop for i upto idx
summing (* (aref histogram i) i))
apriori-1))
(mean-2 (/ (loop for i from (1+ idx) to (- histlen 1)
summing (* (aref histogram i) i))
apriori-2))
(variance-1 (/ (loop for i upto idx
summing (* (expt (- i mean-1) 2)
(aref histogram i)))
apriori-1))
(variance-2 (/ (loop for i from (1+ idx) to (- histlen 1)
summing (* (expt (- i mean-2) 2)
(aref histogram i)))
apriori-2)))
(declare (type double-float apriori-2 mean-1 mean-2 variance-1 variance-2))
(if (or (zerop variance-1) (zerop variance-2))
(setf (aref samples idx) +j-fn-max-value+)
(let ((deviation-1 (sqrt variance-1))
(deviation-2 (sqrt variance-2)))
(declare (type double-float deviation-1 deviation-2))
(setf (aref samples idx)
(+ 1
(* 2 (+ (* apriori-1 (log deviation-1))
(* apriori-2 (log deviation-2))))
(* -2 (+ (* apriori-1 (log apriori-1))
(* apriori-2 (log apriori-2))))))))))))
samples))
(defun min-error-threshold-image (image)
"Binarize 8-bit-gray-image image with an automatically guessed
threshold. Returns multiple values: 1-bit-gray-image image and a guessed
threshold.
For further details, please refer 'Minumum error thresholding' by
J. Kittler and J. Illingworth."
(etypecase image
(8-bit-gray-image
(let* ((histogram (compute-histogram image))
(j-fn-samples (sample-j-fn histogram))
(min-sample (reduce #'min j-fn-samples))
(threshold (position min-sample j-fn-samples)))
(values (the 1-bit-gray-image (threshold-image image (truncate threshold)))
threshold)))))
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