subtracting two matrices in matlab, the negative values in result are substituted by zero - matlab

Question

I have two matrices in matlab,
> IRwindow =
>
> **183** 171 150 125 137
138 167 184 173 152
105 114 141 167 185
148 113 105 115 141
186 183 147 112 105
>
> ILwindow =
>
> **201** 170 165 177 203
181 174 167 169 189
154 150 156 168 181
187 175 158 131 144
173 186 183 167 141
I want to subtract these two matrices element-wise and get the result; for example for first element (183 - 201= -18 ) BUT the output for this element gives zero. the outcome result will be as below:
> IRwindow - ILwindow
ans =
**0** 1 0 0 0
0 0 17 4 0
0 0 0 0 4
0 0 0 0 0
13 0 0 0 0
how could I keep the real results? without getting zero for negatives in my result-matrix

Answer 1

Run the following example code:
%# Create random matrices
X = randi(100, 5, 5);
Y = randi(100, 5, 5);
%# Convert to strictly non-negative format
X = uint8(X);
Y = uint8(Y);
%# Perform subtractions
A = X - Y;
%# Convert to double format
X = double(X);
Y = double(Y);
%# Perform subtraction
B = X - Y;
For a given sample run:
A =
0 15 36 0 0
0 0 0 0 3
0 0 0 25 0
13 0 15 0 0
0 49 0 0 14
while:
B =
-8 15 36 -4 -65
0 -47 -45 -11 3
-18 -17 -11 25 -52
13 -53 15 -15 -1
-35 49 -47 -8 14
You will notice that all the negative numbers in A have been replaced by 0, while the negative numbers in B are displayed correctly.
Stated simply: if you use a numerical format that is not able to store negative numbers, then Matlab truncates at 0. The solution is to convert to a format that is able to accomodate "real" numbers (or a close approximation thereof) such as double, or perhaps in your case one of the int formats may be more appropriate, such as int8, int16, int32 or int64.

Answer 2

Another option is to use single or double on the subtraction in one line as follows:
ans=double(IRwindow-ILwindow)

Answer 3

I dont get the same problem as you: I have this code:
IRwindow = [
183 171 150 125 137
138 167 184 173 152
105 114 141 167 185
148 113 105 115 141
186 183 147 112 105]
ILwindow = [
201 170 165 177 203
181 174 167 169 189
154 150 156 168 181
187 175 158 131 144
173 186 183 167 141]
IRwindow - ILwindow
and i get this output:
IRwindow =
183 171 150 125 137
138 167 184 173 152
105 114 141 167 185
148 113 105 115 141
186 183 147 112 105
ILwindow =
201 170 165 177 203
181 174 167 169 189
154 150 156 168 181
187 175 158 131 144
173 186 183 167 141
ans =
-18 1 -15 -52 -66
-43 -7 17 4 -37
-49 -36 -15 -1 4
-39 -62 -53 -16 -3
13 -3 -36 -55 -36
Check that you are creating your matrices are being created properly (as doubles and not as unsigned integers).