To convert only black color to white in Matlab

Question

I know this thread about converting black color to white and white to black simultaneously. I would like to convert only black to white. I know this thread about doing thi

Answer 1

Here is a variation on @rayryeng's solution to extract the blue signal:

%// retrieve picture
imgRGB = imread('http://i.stack.imgur.com/cFOSp.png');

%// detect axis lines and labels
imgHSV = rgb2hsv(imgRGB);
BW = (imgHSV(:,:,3) < 1);
BW = imclose(imclose(BW, strel('line',40,0)), strel('line',10,90));

%// clear those masked pixels by setting them to background white color
imgRGB2 = imgRGB;
imgRGB2(repmat(BW,[1 1 3])) = 255;

%// show extracted signal
imshow(imgRGB2)

To get a better view, here is the detected mask overlayed on top of the original image (I'm using imoverlay function from the File Exchange):

figure
imshow(imoverlay(imgRGB, BW, uint8([255,0,0])))