MATLAB: How to fix weird variable behavior

loopsvariablesweird

I am trying to calculate the running sum of total values for all pixels in an image in order to get the average. The code should be relatively easy:

% Find total and divide by number of pixels totalpixels = 0; runningsum = 0;

for y = 1:image1Y for x = 1:image1X totalpixels= totalpixels+1; % in an incredibly frustrating bit of programming confusion % MATLAB won't allow the value of runningsum to go above 255 temp1 = TestImage1(y, x) temp2 = runningsum runningsum = temp1 + temp2 end end

I started with: runningsum = runningsum + TestImage1(y, x);

No matter what I do, the value of running sum won't go above 255. How do I fix this?

I understand I can use a built-in function. At this point, I want to understand how MATLAB handles variables.

Best Answer

TestImage1(y, x) is a uint8, and so the sum is also. Uint8 varaibels are not allowed to go above 255 and will get clipped if that were to be the case. Try casting to double before summing:

temp1 = double(TestImage1(y, x));

Related Solutions

MATLAB: Byte ordering in calls from C/C++ to a Matlab-compiled DLL

I don't think you understood what Walter was getting at. These lines will not work if myBuffer is supposed to contain float data:

memcpy(mxGetPr(x_ptr), (double*)myBuffer, numSamples*sizeof(double));
memcpy(myBuffer, (float*)mxGetPr(y_ptr), numSamples*sizeof(float));

Simply casting the result of mxGetPr, a double * , to a float * , does not change the fact that the underlying data that it is pointing to is still double. And vice-versa for the double * cast you have on myBuffer. You will need to physically write a for-loop and do the conversion from single-to-double (or the reverse) manually one element at a time. You cannot use memcpy for this.

As a side note, it is pointless to put any cast at all for the 2nd argument to memcpy since the compiler will immediately turn around and cast it to a void * per the prototype. (You did include the prototype, right?).

MATLAB: How to use the functions

temp1 = first_function(YourInput);
temp2 = second_function(temp1);
temp3 = third_function(temp2);
output = fourth_function(temp3);

Best Answer

Related Solutions

MATLAB: Byte ordering in calls from C/C++ to a Matlab-compiled DLL

MATLAB: How to use the functions

Related Question