Overlapping grayscale and RGB Images

Question

I would like to overlap two images, one grayscale and one RGB image. I would like to impose the RGB image on top of the grayscale image, but ONLY for pixels greater than a certain value. I tried using the double function in MATLAB, but this seems to change the color scheme and I cannot recover the original RGB colors. What should I do in order to retain the original RGB image instead of mapping it to one of the MATLAB colormaps? Below is my attempt at superimposing:

pixelvalues = double(imread('hello.png'));
PixelInt = mean(pixelvalues,3);
I1 = ind2rgb(Brightfield(:,:,1), gray(256)); %Brightfield
I2 = ind2rgb(PixelInt, jet(256)); %RGB Image
imshow(I2,[])
[r,c,d] = size(I2);
I1 = I1(1:r,1:c,1:d);
% Replacing those pixels below threshold with Brightfield Image
threshold = 70;

I2R = I2(:,:,1); I2G = I2(:,:,2); I2B = I2(:,:,3);
I1R = I1(:,:,1); I1G = I1(:,:,2); I1B = I1(:,:,3);

I2R(PixelInt<threshold) = I1R(PixelInt<threshold);
I2G(PixelInt<threshold) = I1G(PixelInt<threshold);
I2B(PixelInt<threshold) = I1B(PixelInt<threshold);
I2(:,:,1) = I2R; I2(:,:,2) = I2G; I2(:,:,3) = I2B;

h = figure;
imshow(I2,[])

Original RGB Image:

Brightfield:

Overlay:

Answer 1

Is the content of pixelvalues what you show in your first image? If so, that image does not use a jet colormap. It has pink and white values above the red values, whereas jet stops at dark red at the upper limits. When you take the mean of those values and then generate a new RGB image with ind2rgb using the jet colormap, you're creating an inherently different image. You probably want to use pixelvalues directly in generating your overlay, like so:

% Load/create your starting images:
pixelvalues = imread('hello.png');           % Color overlay
I1 = repmat(Brightfield(:, :, 1), [1 1 3]);  % Grayscale underlay
[r, c, d] = size(pixelvalues);
I1 = I1(1:r, 1:c, 1:d);

% Create image mask:
PixelInt = mean(double(pixelvalues), 3);
threshold = 70;
mask = repmat((PixelInt > threshold), [1 1 3]);

% Combine images:
I1(mask) = pixelvalues(mask);
imshow(I1);

Note that you may need to do some type conversions when loading/creating the starting images. I'm assuming 'hello.png' is a uint8 RGB image and Brightfield is of type uint8. If I load your first image as pixelvalues and your second image as I1, I get the following when running the above code:

Answer 2

Create a mask and use it to combine the images:

onionOrig = imread('onion.png');
onionGray = rgb2gray(onionOrig);
onionMask = ~(onionOrig(:,:,1)<100 & onionOrig(:,:,2)<100 & onionOrig(:,:,3)<100);

onionMasked(:,:,1) = double(onionOrig(:,:,1)) .* onionMask + double(onionGray) .* ~onionMask;
onionMasked(:,:,2) = double(onionOrig(:,:,2)) .* onionMask + double(onionGray) .* ~onionMask;
onionMasked(:,:,3) = double(onionOrig(:,:,3)) .* onionMask + double(onionGray) .* ~onionMask;

onionFinal = uint8(onionMasked);
imshow(onionFinal)