# Fast Image Thresholding-will a mex file improve speed?

조회 수: 2 (최근 30일)
Joel 2011년 8월 15일
I have some image/video processing code where on each frame I have to threshold it about 3 different times. Right now I simply use...
while(1)
im = getsnapshot; %returns a uint8 640X480 array
im1 = im>50&im<100; %example thresholds
im2 = im >100&im<150;
im3 = im>150&im<200;
%lots more image processing
end
______
I have made the remainder of the code so efficient that at this point, most of the time is spent on the thresholding lines. Any tips to make this faster? (FYI im2bw is much slower).
Is this a situation where a Mex File could improve performance? I know a lot has changes in the last few versions of matlab to make it much faster so I am wondering if its worth mex-ing?
Thanks, Joel

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### 채택된 답변

Jan 2011년 8월 15일
Yes, this will be faster as C-Mex.
im1 = im > 50 & im < 100
Here at first a 640x480 LOGICAL matrix is calculated in "im > 50". Then a 2nd matrix is created for "im < 100". But is a pixels is <= 50, the test for > 100 is a waste if time. Most of all if the different intervals are not overlapping a lot of time can be saved by avoiding unnecessary tests.
Before I post a C-code, it would be helpful to know, if the intervals are continous, in opposite to your example. E.g. 50 < im < 100, 100 <= im < 150, 150 <= im < 200? Or is im==100 exluded on purpose?
[TEST-IMPLEMENTATION UNTIL QUESTIONS ARE CLEARED]: I assume, that the intervals are dense here.
#include "mex.h"
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
uint8_T *im, limit1, limit2, limit3, limit4, p;
mxLogical *im1, *im2, *im3;
mwSize m, n, len;
mwIndex i;
// Check inputs:
if (nrhs != 5 || nlhs != 3) {
"Need 5 inputs and 3 outputs.");
}
if (!mxIsUint8(prhs[0])) {
"1st input must be an UINT8 matrix.");
}
// Get size of input and pointer to data:
m = mxGetM(plhs[0]);
n = mxGetN(plhs[0]);
im = (uint8_T *) mxGetData(prhs[0]);
len = m * n;
limit1 = (uint8_T) mxGetScalar(prhs[1]);
limit2 = (uint8_T) mxGetScalar(prhs[2]);
limit3 = (uint8_T) mxGetScalar(prhs[3]);
limit4 = (uint8_T) mxGetScalar(prhs[4]);
// Create outputs:
plhs[0] = mxCreateNumericMatrix(m, n, mxLOGICAL_CLASS, mxREAL);
im1 = (mxLogical *) mxGetData(plhs[0]);
plhs[1] = mxCreateNumericMatrix(m, n, mxLOGICAL_CLASS, mxREAL);
im2 = (mxLogical *) mxGetData(plhs[1]);
plhs[2] = mxCreateNumericMatrix(m, n, mxLOGICAL_CLASS, mxREAL);
im3 = (mxLogical *) mxGetData(plhs[2]);
// The calculations:
for (i = 0; i < len; i++) {
p = im[i];
if (p > limit1) {
if (p < limit2) {
im1[i] = 1;
} else if (p < limit3) {
im2[i] = 1;
} else if (p < limit4) {
im3[i] = 1;
}
}
}
return;
}
##### 댓글 수: 8이전 댓글 6개 표시이전 댓글 6개 숨기기
Joel 2011년 8월 16일
function [imb1, imb2, imb3] = FastThresholdThreeColor(im, t1, t2, t3)
% Quick image thresholding of three colors
% im is an uint8 color image (YCbCr)
% t1, t2, t3 are 6 element vectors that list the low and high threshold on
% each channel ex t1 = [Ylo, Yhi, CbLo, CbHi, CrLo, CrHi];
% I decided to make thresholds mutually exclusive
s = size(im);
imb1 = false(s(1), s(2));
imb2 = false(s(1), s(2));
imb3 = false(s(1), s(2));
for i = 1:s(2) % each column...going down a column is supposed to be faster?
for j =1:s(1) % down a row
% note that intentionally make channel 1 last to exploit
% shortcircuit (its usually the least restrictive)
if im(j,i,2)>=t1(3) && im(j,i,2)<=t1(4) && ...
im(j,i,3)>=t1(5) && im(j,i,3)<=t1(6)&&...
im(j,i,1)>=t1(1) && im(j,i,1)<=t1(2)
imb1(j,i) =1;
elseif im(j,i,2)>=t2(3) && im(j,i,2)<=t2(4) && ...
im(j,i,3)>=t2(5) && im(j,i,3)<=t2(6)&& ...
im(j,i,1)>=t2(1) && im(j,i,1)<=t2(2)
imb2(j,i) =1;
elseif im(j,i,2)>=t3(3) && im(j,i,2)<=t3(4) && ...
im(j,i,3)>=t3(5) && im(j,i,3)<=t3(6)&& ...
im(j,i,1)>=t3(1) && im(j,i,1)<=t3(2)
imb3(j,i) =1;
end
end
end
Joel 2011년 8월 22일
Any more thoughts on how the color version shoudl be structured to optimize performance?

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### 추가 답변 (1개)

Chirag Gupta 2011년 8월 15일
You can also use manual trigger mode on your videoinput object to reduce the overhead associated with getsnapshot.
Check the "Acquiring a Single Image in a Loop" demo in the documentation
##### 댓글 수: 3이전 댓글 1개 표시이전 댓글 1개 숨기기
Joel 2011년 8월 16일
While I do that currently, I am not convinced it is the festeest possible way. Of course its an order of magnitude better that using the default trigger setting but have you experimented with setting teh frames per trigget to infinite and then using peekdata?
What about using a callback function?
Joel 2011년 8월 22일
Any more thoughts?

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