Color-pass filter by Hue

Question

Why doesn't color filter below find green peppers?

The code:

function [ outhsv ] = ColorFilter( hsv, h, s )
%COLORFILTER Summary of this function goes here
%   Detailed explanation goes here

    if nargin < 2
        h = [];
    end
    if nargin < 3
        s = [];
    end

    if size(h,2)==1
        h = padarray(h, [0 1], 1/100, 'post');
    end

    if size(s,2)==1
        s = padarray(s, [0 1], 1/100, 'post');
    end

    if isempty(h)
        v_of_h = ones(size(hsv,1), size(hsv,2));
    else
        v_of_h = WeightFunction( hsv(:,:,1), h(:,1), h(:,2));
    end

    if isempty(s)
        v_of_s = ones(size(hsv,1), size(hsv,2));
    else
        v_of_s = WeightFunctionOnce( hsv(:,:,2), s(:,1), s(:,2));
    end

    outhsv = hsv;
    outhsv(:,:,3) = hsv(:,:,3) .* v_of_h .* v_of_s;



function y = WeightFunction( x, mu, sigma ) 

    %y = WeightFunctionOnce(x,mu,sigma) + WeightFunctionOnce(x-1,mu,sigma);
    y = 1 - (1-WeightFunctionOnce(x,mu,sigma)) .* (1-WeightFunctionOnce(x-1,mu,sigma));

function y = WeightFunctionOnce( x, mu, sigma ) 

    if nargin<2
        mu=0;
    elseif nargin<3
        sigma=1./100.;
    end


    if any(size(mu) ~= size(sigma))
        error('mu and sigma should be of the same size');
    end

    y = zeros([size(x) numel(mu)]);

    for i=1:numel(mu)
        y(:,:,i) = exp(-((x - mu(i)) .^ 2 ./ (2 .* sigma(i) .^ 2)));
    end

    %y = sum(y,3)/size(y,3);
    y = 1-prod(1-y,3);

Display code:

hue = 120;
h = [hue/360 0.05];
s = [];

rgb1 = imread('huescale.png');
%rgb1 = imread('peppers.png');
hsv1 = rgb2hsv(rgb1);
hsv2 = ColorFilter(hsv1, h, s);
rgb2 = hsv2rgb(hsv2);
bitmask = hsv1(:,:,1)>(h(1)-h(2)) & hsv1(:,:,1)<(h(1)+h(2));


figure; 
subplot(3,1,1); imshow(rgb1);
subplot(3,1,2); imshow(rgb2);
subplot(3,1,3); imshow(bitmask);

result on scale

(works)

result on peppers:

(does not)

Why?

Answer 1

If you looked closer at the H values, those green peppers are kind of yellowish, so you might want to widen the rule a bit.

I would suggest something in between 0.15 and 0.5. You can also combine with saturation channel, say only consider portions of images that are vibrant, i.e., we want to get rid of the onions. Try the following codes to get a preview.

hsv_dat = rgb2hsv(imread('peppers.png'));
imagesc(hsv_dat(:,:,1) > 0.15 & hsv_dat(:,:,1) < 0.5 & hsv_dat(:,:,2) > 0.3)
colormap(gray)

You should get