How to Find the Pixel location using the known RGB values ?

I know the RGB value of a certain pixel in a image , I want to know the location of that pixel. How to do it ?

Thank u.

Answer 1

[Image Analyst]

Make a mask then use find() to find out where the mask is true:

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);

mask = redChannel == redValue & ...
  greenChannel == greenValue & ...
  blueChannel == blueValue;
imshow(mask);
% Get (row, column) list
[rows, columns] = find(mask);

redValue, greenValue, and blueValue are your reference RGB values that you already know. mask is a binary image that says where that color occurs in the image. rows and columns will be the location of other pixels in the image that have that same color (exact R value, exact green value, and exact blue value).

[Yaser Moazzen]

So thanks

[Shannon Sumpter]

I can only get this to work for values of [1 0 0] [0 1 0] and [0 0 1]. Why do you think this is? Here's the code I'm working with:

clc 
clear
close all

% Set parameters (these could be arguments to a function)
rOuter = 500;    % outer radius of the color wheel
ncolors = 255;      % number of colour segments
% Get polar coordinates of each point in the domain
[x, y] = meshgrid(-rOuter:rOuter);
[theta, rho] = cart2pol(x, y);
% Set up colour wheel in hsv space
hue = (theta + pi) / (2 * pi);     % hue into range (0, 1]
hue = ceil(hue * ncolors) / ncolors;   % quantise hue 
saturation = ones(size(hue));      % full saturation
brightness = double(rho <= rOuter);  % black outside wheel
% Convert to rgb space for display
colorwheel = hsv2rgb(cat(3, hue, saturation, brightness));

%find RBG points 
redChannel = colorwheel(:, :, 1);
greenChannel = colorwheel(:, :, 2);
blueChannel = colorwheel(:, :, 3);
mask1 = redChannel == 0 & ...
  greenChannel == 1 & ...
  blueChannel == 0;

% Get (row, column) list
[rows1, columns1] = find(mask1);

% Colorwheel result 
figure
imshow(colorwheel)
axis on
hold on 

plot(columns1, rows1, 'w')

[Image Analyst]

@Shannon Sumpter, I don't understand. The code runs with no errors and produces this image:

What exactly does not work? What do you expect to see?

I have absolutely no idea what "I can only get this to work for values of [1 0 0] [0 1 0] and [0 0 1]" means. As you can see there are many, many more colors in that picture than the 3 you listed.

[Shannon Sumpter]

For example, if I run the code with:

mask1 = redChannel == 0 & ...

greenChannel == 1 & ...

blueChannel == 1;

the colorwheel has no slices taken out of it. I'm not sure why. I had guessed that it would take out a slice where cyan is when I ran the above.

[Shannon Sumpter]

@Image Analyst

[Image Analyst]

That exact color does not appear in the color wheel, though there are colors that are close. Set a range instead of exact values, like

mask1 = redChannel <= 0.1 & ...
  greenChannel >= 0.9 & ...
  blueChannel >= 0.9;

[Shannon Sumpter]

Perfect, thank you!

[Image Analyst]

You can now use imsplit():

% Extract the individual red, green, and blue color channels using imsplit():
[redChannel, greenChannel, blueChannel] = imsplit(rgbImage);
mask = redChannel   == redValue   & ...
       greenChannel == greenValue & ...
       blueChannel  == blueValue;
imshow(mask);
% Get (row, column) list
[rows, columns] = find(mask);

[Shannon Sumpter]

Would you know how to adjust this color wheel so that the center is white and the furthest parts from the center were black? Creating a gradient effect for all of the colors

[Image Analyst]

Shannon, you'd have to do it in HSV color space. For the circle in the S plane, make the saturation 0 in the middle so it will be neutral, then 1 halfway out so the color will be 100% vivid, pure color, and then 0 out at the outer edge, meaning neutral again.

Then for the circle in the V plane, you'd have to make it go from 1 in the middle (white) to 0 at the outer perimeter (so it will be black instead of white).

Not too hard so give it a try.

[Shannon Sumpter]

@Image Analyst I tried adding this to the code to do what you suggested, but now saturation is a 1002x334 double instead of a 1001x1001 double as it needs to be.

centerSaturation = zeros(round(size(hue)/3)); % neutral saturation

halfwaySaturation = ones(round(size(hue)/3)); %full saturation

outerSaturation = zeros(round(size(hue)/3)); % neutral saturation

saturation = cat(1,centerSaturation,halfwaySaturation,outerSaturation);

How would you go about doing this differently?