Selecting dark areas of an image

I have a question on how to upload an image, and then get matlab to select areas of the image that are darker in color. The image has a colors from yellow to black. I would like to get the program to select the black areas, and draw an elipse around it.

[Mark Sherstan]

Can you please post an example of the image?

I would reccomend using the Color Thresholder App which is part of the Image Processing Toolbox. You can load in an image either from a GUI or from your workspace (use imread).

Use HSV color space and adjust your V value as you want the darker region.

An example that might push you in the correct direction can be found here.

[KSSV]

Upload your image..so that we can suggest you on the task.

[justin kain]

This is not the image, but a similar image to this one, Selecting dark regions like the one in the middle towards the bottom.

[Image Analyst]

Not exactly sure which region you're referring to. Can you attach another image where you've outlined the region of interest in red?

Answer 1

[Mark Sherstan]

Try the following. I just put it together quickly (results on the bottom). You will need to adjust the tolerances of channel 3 to meet the requirments of your data. From there you can use bwareafilt, regionprops and insertObjectAnnotation to draw your circles. I draw a circle around the largest blob (according to the random channel 3 - value settings).

I = imread('image.png');
[BW,maskedRGBImage] = createMask(I);
BW2 = bwareafilt(BW,1);
stats = regionprops(BW2,'Centroid');
RGB = insertObjectAnnotation(I,'circle',[stats.Centroid 20],'','Color','cyan');
imshow(RGB)

function [BW,maskedRGBImage] = createMask(RGB)
%createMask  Threshold RGB image using auto-generated code from colorThresholder app.
%  [BW,MASKEDRGBIMAGE] = createMask(RGB) thresholds image RGB using
%  auto-generated code from the colorThresholder app. The colorspace and
%  range for each channel of the colorspace were set within the app. The
%  segmentation mask is returned in BW, and a composite of the mask and
%  original RGB images is returned in maskedRGBImage.

% Auto-generated by colorThresholder app on 12-Dec-2018
%------------------------------------------------------


% Convert RGB image to chosen color space
I = rgb2hsv(RGB);

% Define thresholds for channel 1 based on histogram settings
channel1Min = 0.000;
channel1Max = 1.000;

% Define thresholds for channel 2 based on histogram settings
channel2Min = 0.000;
channel2Max = 1.000;

% Define thresholds for channel 3 based on histogram settings
channel3Min = 0.000;
channel3Max = 0.275;

% Create mask based on chosen histogram thresholds
sliderBW = (I(:,:,1) >= channel1Min ) & (I(:,:,1) <= channel1Max) & ...
    (I(:,:,2) >= channel2Min ) & (I(:,:,2) <= channel2Max) & ...
    (I(:,:,3) >= channel3Min ) & (I(:,:,3) <= channel3Max);
BW = sliderBW;

% Initialize output masked image based on input image.
maskedRGBImage = RGB;

% Set background pixels where BW is false to zero.
maskedRGBImage(repmat(~BW,[1 1 3])) = 0;

end