How can I find RGB values of a matrix of coordinates of an image?

Question

Eric Martz 2019-7-22

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/472897-how-can-i-find-rgb-values-of-a-matrix-of-coordinates-of-an-image

评论： Walter Roberson 2019-7-23

Hi,

I'm trying to pull the RGB values from a list of (x,y) coordinates that are centers of circles. My plan was to get the matrix of each component (r, g, and b) this way:

P = imread('image'); ...
x = centers(:, 1);
y = centers(:, 2);
rcenters = P(round(x), round(y), 1);

x is a 668 x 1 matrix, and so is y, so I was hoping rcenters would be 668 x 2, but instead it's 668 x 668, which does not make sense to me. I had to round the x and y coordinates to the nearest integer so that it would have integer indices.

How could I get it to return a 668 x 2 matrix with the R value of the x coordinate in column 1 and the R value of the y coordinate in column 2?

Thanks in advance!

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Image Analyst 2019-7-22

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/472897-how-can-i-find-rgb-values-of-a-matrix-of-coordinates-of-an-image#answer_384266

在 MATLAB Online 中打开

(x,y) is NOT (row, column) -- it's (column, row). You've made a very common beginner mistake. Put y first -- P(y, x)

x = round(centers(:, 1));
y = round(centers(:, 2));
rgbValues = zeros(length(x), 3)
counter = 1;
for col = 1 : length(y)
        for row = 1 : length(x)
            rgbValues(counter, :) = P(y(row), x(col), :);
            counter = counter + 1;
        end
end

3 个评论
显示 1更早的评论隐藏 1更早的评论

Image Analyst 2019-7-23

在 MATLAB Online 中打开

Did you see this line in the code:

rgbValues(counter, :) = P(y(row), x(col), :);

That gives you the RGB values. Red is column1, green is col2 and blue is col3.

Eric Martz 2019-7-23

在 MATLAB Online 中打开

I figured it out. I edited the code to make this:

x = round(centers(:, 1));
y = round(centers(:, 2));
rgbValues = zeros(length(x), 3);
counter = 1;
 for row = 1 : length(x)
            rgbValues(counter, :) = P(round(centers(row, 2)), round(centers(row, 1)), :);
            counter = counter + 1; 
 end
pts = [x, y];
rgb = horzcat(rgbValues, pts);

The issue with the code you provided was that it gave the rgb value of every possible combination of every x and y coordinate instead of just the x and y coordinates that went together.

请先登录，再进行评论。

Answer 2

Walter Roberson 2019-7-23

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/472897-how-can-i-find-rgb-values-of-a-matrix-of-coordinates-of-an-image#answer_384293

在 MATLAB Online 中打开

rcenters = P( sub2ind(size(P), y, x, ones(size(y))) );

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

Answer 3

Akira Agata 2019-7-23

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/472897-how-can-i-find-rgb-values-of-a-matrix-of-coordinates-of-an-image#answer_384294

在 MATLAB Online 中打开

Another possible solution:

row = round(centers(:, 1));
col = round(centers(:, 2));
R = arrayfun(@(x,y) P(x,y,1),row,col);
G = arrayfun(@(x,y) P(x,y,2),row,col);
B = arrayfun(@(x,y) P(x,y,3),row,col);