Hi Sam,
From my understanding of the question, the error is being produced because you are trying to set the properties when the component is being created (This works correctly only if the value to on RHS is constant (e.g. size_x = 2 will work but size_x = size(X) won't work). The component creation step happens before calling the startup function. At the component creation time, Img was not defined, and hence it is throwing error.
Instead of initializing the properties values at declaration time, you should set them when startup function is called.
Following is the code that you will find helpful.
properties (Access = private)
size_Img % Image Size
size_Sag % Image Saggital Size
size_Trans % Image Transverse Size
size_Front % Image Frontal Size
...
end
properties (Access = public)
Img % Input Image
end
function startupFcn(app)
app.size_Img = size(app.Img); % Image Size
app.size_Sag = app.size_Img(1); % Image Saggital Size
app.size_Trans = app.size_Img(2); % Image Transverse Size
app.size_Front = size(3); % Image Frontal Size
end
For understanding in detail, you can refer the following links
