There are a couple of issues here
- You need to use size , not numel to get the size of the array in one dimension. Use size's second dimension option to specify which dimension you want to measure.
- You concatenate the NaN-padding using square brackets to join them horizontally [x,nan(..)]. Square brackets can concatenate only along the first or second dimension (i.e. [x;y] and [x,y] respectively), but you require the third dimension (because this is where your arrays have different sizes). For this you can use padarray.
As an alternative, you could use cat to concatenate them in the dimension that they are different (e.g. the third in your example). This does not require adding the NaN's, so you need to decide what is best for your situation.
cat(3,imgData{:})
