To change values in a matrix between two elements

I have a matrix of zeros 768*1024, considering 4 points (50,100) (450,100) (200,600) and (300,600) i change the values between them and create two lines when i show the image.

i want to change all the points between these two lines to 1's so that i achieve what looks like a white trapezoid in the black background image. Also, this should overwite the previous 0's to 1's in the matrix

code is as follows:

M=zeros(768,1024);

p1=[50,100];
p2=[200,600];

x_value=[50, 200];
y_value=[100, 600];

X=[x_value(1):x_value(2)];
Y=round(interp1(x_value,y_value,X));

i_max=size(Y);

for i=1:i_max(2)
    M(Y(i),X(i))=255;
end


p4=[300,600];
p3=[450,100];

a_value=[450,300];
b_value=[100,600];

A = [a_value(2):a_value(1)];
B = round(spline(a_value,b_value,A));

j_max=size(B);

for i=1:j_max(2)

    M(B(i),A(i))=255;

end


imshow(M);

[Jan]

You forgot to ask a question. What is the problem?

Answer 1

[Shashwat Bajpai]

Hi,

I understand that you are trying to fill the trapezoid created by the points (50,100) (450,100) (200,600) and (300,600).

Please refer to the following code to see if this is what you desire:

xf=[X' A']; 
num_el=numel(xf); 
yf=[Y' B']; 
for i=1:num_el/2 
    M(yf(i):yf(end-i),xf(i):xf(end-i))=255; 
end 
imshow(M) 

The following link provides a good understanding of how to index into matrices and modify their values:

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html