title: Halide::Expr' is not contextually convertible to 'bool' -- Storing values of functions in variables
body: <p>I am new to using Halide and I am playing around with implementing algorithms first. I am trying to write a function which, depending on the value of the 8 pixels around it, either skips to the next pixel or does some processing and then moves on to the next pixel. When trying to write this I get the following compiler error:</p>
  
  <p>84:5: error: value of type 'Halide::Expr' is not contextually convertible to 'bool'</p>
  
  <pre><code>    if(input(x,y) &gt; 0)
  </code></pre>
  
  <p>I have done all the tutorials and have seen that the select function is an option, but is there a way to either compare the values of a function or store them somewhere?</p>
  
  <p>I also may be thinking about this problem wrong or might not be implementing it with the right "Halide mindset", so any suggestions would be great. Thank you in advance for everything!</p>