Problem in plotting the matrix over an image.

Question

Prachi Sharma 2016-11-16

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https://ww2.mathworks.cn/matlabcentral/answers/312459-problem-in-plotting-the-matrix-over-an-image

回答： Image Analyst 2016-11-17

I am required to read an image,find out its rgb channels and then in those channels separately,mark the pixels whose value exceeds above a certain threshold value.I wrote the following code for marking the pixels but they are not getting marked and I am getting the error that it exceeds matrix dimensions.What am I doing wrong in the code? Below is the code-

img=imread('test.jpg');

imshow(img)

redchannel=img(:,:,1);

greenchannel=img(:,:,2);

bluechannel=img(:,:,3);

rs=zeros(size(redchannel));

r1=cat(3,redchannel,rs,rs);

figure

imshow(r1)

gs=zeros(size(greenchannel));

g1=cat(3,gs,greenchannel,gs);

figure

imshow(g1)

bs=zeros(size(bluechannel));

b1=cat(3,bs,bs,bluechannel);

figure

imshow(b1)

[r,c,~]=size(img);

indx=zeros(r,c);

for i=1:r

    for j=1:c
        if ((redchannel(i,j))>=180)
            indx(i,j)=1;
        end
    end

end

for i=1:r

for j=1:c

imshow(r1)

hold on

plot(indx(:,1),indx(:,2),'b*');

hold off

end

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Image Analyst 2016-11-16

Do you want to change the actual pixel values, like make them red or something? OR do you want to leave the pixel values the same and just change how they are displayed, like use a colormap where those pixels are red, or use a binary overlay ?

Prachi Sharma 2016-11-17

I need to take an image,find its 3 channels and then in every channel I need to check if any pixel is exceeding a certain threshold value.If a pixel exceeds a certain threshold value then those pixels should be marked or highlighted in the image of that channel.

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Answer 1

KSSV 2016-11-16

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https://ww2.mathworks.cn/matlabcentral/answers/312459-problem-in-plotting-the-matrix-over-an-image#answer_243489

在 MATLAB Online 中打开

clc; clear all ;
img=imread('Prachi Sharma image');
imshow(img)
%
redchannel=img(:,:,1);
greenchannel=img(:,:,2);
bluechannel=img(:,:,3);
%
rs=zeros(size(redchannel));
r1=cat(3,redchannel,rs,rs);
figure
imshow(r1)
%
gs=zeros(size(greenchannel));
g1=cat(3,gs,greenchannel,gs);
figure
imshow(g1)
%
bs=zeros(size(bluechannel));
b1=cat(3,bs,bs,bluechannel);
figure
imshow(b1)
%
[r,c,~]=size(img);
indx=NaN(r,c);
indx(redchannel>=180) = 1 ;
%
imshow(r1) ;
x = 1:c ;
y = 1:r ;
[X,Y] = meshgrid(x,y) ;
hold on
plot3(X,Y,indx,'*b') ;

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KSSV 2016-11-16

You try it out...this is the way the matrix size of x,y making equal to indx. Do consider the Guillaume comments.

Prachi Sharma 2016-11-16

I tried it out,switching places of r and c and it didn't work that way.

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Answer 2

Guillaume 2016-11-16

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编辑：Guillaume 2016-11-16

在 MATLAB Online 中打开

find will return the locations of all the values above the threshold, there's no need for a loop

img = imread('test.jpg');
redchannel = img(:, :, 1);
[row, column] = find(redchannel > 180);  %no need for loop
redimage = img; redimage(:, :, [2 3]) = 0;
figure;
imshow(redimage);
hold on;
plot(column, row, 'b*');  %column, row <=> x, y. Stupid matlab can't decide on a single coordinate system!

and do the same for green and blue.

edit: also see my comment to the question about using zeros with no class specified.

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Guillaume 2016-11-16

在 MATLAB Online 中打开

redimage(:, :, [2 3]) selects all the rows, all the columns and only index 2 and 3 of the third dimension. index 2 is the green channel, and index 3 is the blue channel. In effect, this set all the green and blue value to 0. If you wanted to keep just the green channel, then:

img(:, :, [1 3]) = 0; %1and 3 is red and blue respectively

find use the normal matlab convention where the first value it returns is the row of the matrix, and the second value is the column of the matrix. plot and a few other functions (most of of the image processing toolbox, as well as gradient, meshgrid and maybe more) use a different coordinate system (x, y) where x corresponds to the columns, and y to the rows. This is matches the way you normally references coordinates outside matlab but as a result you end up with two different coordinates system where one is swapped compared to the other.

Personally, I think it's stupid (hence my comment in the code). In my opinion it would be much better to be internally consistent but that's what it is: plot uses (x, y) coordinates which is (column, row), find uses (row, column) which is (y, x).

KSSV 2016-11-17

Dear friend then type it..what is stopping you?

Prachi Sharma 2016-11-17

编辑：Prachi Sharma 2016-11-17

This will also work right?

%for redchannel

mask_r1=(redchannel<x);

newm1=uint8(mask_r1);

mult1=newm1.*redchannel;

%for greenchannel

mask_r2=(greenchannel<y);

newm2=uint8(mask_r2);

mult2=newm2.*greenchannel;

%for blue channel

mask_r3=(bluechannel<z);

newm3=uint8(mask_r3);

mult3=newm3.*bluechannel;

%or ing the result

ora=(newm1|newm1)|newm3;

uora=uint8(ora);

figure

imagesc(uora)

title('Ored result of all the threshold values');

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Answer 3

Image Analyst 2016-11-17

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https://ww2.mathworks.cn/matlabcentral/answers/312459-problem-in-plotting-the-matrix-over-an-image#answer_243729

在 MATLAB Online 中打开

Try something like this:

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Make mask where ANY of the color channels is saturated.
saturatedPixels = redChannel  == 255 | greenChannel  == 255 | blueChannel == 255;
% Make red channel 255, and other color channels 0 where there is saturation
redChannel(saturatedPixels) = 255;
greenChannel(saturatedPixels) = 0;
blueChannel(saturatedPixels) = 0;
% Recombine separate color channels into a single, true color RGB image.
rgbImage = cat(3, redChannel, greenChannel, blueChannel);
imshow(rgbImage);

Now your RGB image will be pure red wherever ANY pixel of ANY color channel is saturated (value of 255). For an image in the range 0-1, use 1 instead of 255. For a uint16 image, use 65535 instead of 255.

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Problem in plotting the matrix over an image.

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