how to do the following?

If the color of v(x,y) is black and the colors of q(x,y) and r(x,y) are one white and one black, the white pixel is selected and set to black with the probability of 1/2. What does probability of 1/2 mean?

Answer 1

[Guillaume]

This is really not complicated. So far you've only implemented one half of any statement. Maybe spend more time understanding the statement?

E.g., for the first one: If the color of v(x,y) is black and the colors of q(x,y) and r(x,y) are both white, one of q(x,y) and r(x,y) is selected with equal probability, and the selected pixel is set to black.

if V(x, y) == 0 && q(x, y) == 1 && r(x, y) == 1  %Implements: If the color of v(x,y) is black and the colors of q(x,y) and r(x,y)
   if rand >0.5      %implements: one of q(x,y) and r(x,y) is selected with equal probability and set to black
        q(x, y) = 0;
   else
        r(x, y) = 0;
   end
end

[Walter Roberson]

On the matter of rand <= 0.5 compared to rand > 0.5 see http://www.mathworks.com/matlabcentral/answers/246999-probability-1-2-with-matlab-random-number-generators

It turns out to be difficult to get true equal probability.

[Elysi Cochin]

So what about in the other case when color of v(x,y) is black and the colors of q(x,y) is white

[Walter Roberson]

The chance that rand() exactly equals 0.5 is one in (2^53-2)

You should be following the same structure that Guillaume and I gave: when you have the test against rand to decide which to set, then you should have an else that sets the other one.

Answer 2

[Walter Roberson]

if rand() <= 0.5    %probability 1/2
   set white pixel to black at this point
end

[Walter Roberson]

        if V(x,y) == 0 && q(x,y) == 1 && r(x,y) == 1
            if rand() <= 0.5
              r(x,y) = 0;
            else
              q(x,y) = 0;
            end
        end

Answer 3

[KSSV]

You have two blacks v(x,y) and r(x,y); you have 50 - 50 chances to pick either v or r and set it to white. So the probability is 1/2.

[Elysi Cochin]

but how will it come in implementation side

        if V(x,y) == 0 && q(x,y) == 1 && r(x,y) == 0
            q(x,y) = 0;
        end

i did like the above one, but what about the probability...