inverting an image in a preexisting code

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I have a code that was created to analyze white particles on a black background, and I need to alter it to analyze black particles on a white background.

RGB=imread('CW5000.jpg');
s_wavelength=4; ; %use every 4th pixel for the perimeter calculation
I = rgb2gray(RGB);
threshold=.4 %black and white threshold
bw = im2bw(I,threshold);
imshow(bw)
bw = bwareaopen(bw,350);
se = strel('disk',2);
bw = imclose(bw,se);
bw = imfill(bw,'holes');
[B,L] = bwboundaries(bw,'noholes');
imshow(label2rgb(L, @jet, [.5 .5 .5]))
hold on
for k = 1:length(B)
boundary = B{k};
plot(boundary(:,2), boundary(:,1), 'y', 'LineWidth', 1)
end
stats = regionprops(L,'Area','Centroid','MajorAxisLength','MinorAxisLength')
for k = 1:length(B)
boundary = B{k};
A = size(boundary)
boundary2=boundary(s_wavelength:s_wavelength:A(1,1),:);
delta_sq = diff(boundary2).^2;
perimeter = sum(sqrt(sum(delta_sq,2)));
area2 = stats(k).Area;
area=polyarea(boundary2(:,1),boundary2(:,2));
metric = 4*pi*area/perimeter^2;
metric_string=sprintf('%2.2f',metric);
text(boundary(1,2)+0,boundary(1,1)+50,metric_string,'Color','k',...
'FontSize',12,'FontWeight','bold');
%text(boundary(1,2)+40,boundary(1,1)+100,metric_string2,'Color','c',...
%'FontSize',12,'FontWeight','bold');
%text(boundary(1,2)+30,boundary(1,1)+150,metric_string3,'Color','g',...
%'FontSize',12,'FontWeight','bold');
%text(boundary(1,2)+30,boundary(1,1)+200,metric_string4,'Color','k',...
%'FontSize',12,'FontWeight','bold');
end

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Miranda Raven 2018-1-10

This is what an image looks like. I am wondering what I should adjust the threshold to in order to get the best results?

Image Analyst 2018-1-10

BGCorrect_demo.m

First thing should be to do a background correction. You have a lot of lens shading going on there and that will prevent a global threshold from working well. See attached demo.

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