# To change values in a matrix between two elements

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Brandon Lalkaka on 2 Jul 2019
Answered: Shashwat Bajpai on 16 Jul 2019
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 on 2 Jul 2019
You forgot to ask a question. What is the problem?

Shashwat Bajpai on 16 Jul 2019
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: