Evaluating expressions

The expression evaluator can be accessed directly with the evaluate command. This has several uses:

1. Checking the current value of variables, for example:

   evaluate(abc);or

   evaluate(aaa==12);

2. Performing calculations and checking syntax: evaluate(sin(erf(2.34)));
3. Setting variables: evaluate(a=4); (set() is the preferred way of setting variables.)

The evaluate command prints the answer in a small information box.

The command whatis is synonymous with evaluate. Another use of whatis is in conjunction with print. Normally, print would iterate over all the pixels, creating a long printout. However, if you were to enter

          
        whatis(print("The pixel is ", image[0][2][200][200]));

the value of a single pixel would be printed.

Evaluate and whatis should not be used in loops.

Comments and multiline equations

Any line that starts with the # character is a comment and is ignored. Similarly, blank lines and carriage returns in equations are ignored. Thus you could have:

          
       i = image[0][0][x][y] +
           image[2][0][x+4][y+4];

However, comments cannot appear in the middle of an expression.

Relationship between image math and macros

Although pixel math equations and macros use a `C'-like syntax, the main difference is that, by default, all math commands iterate over all the pixels in the currently-selected image or selected area. This makes it consistent with the fact that macro commands such as `filter' and `fft' also work on entire images, even though they are only executed once. Some other commands, such as fopen and fclose are also only executed once. Thus if you have:

          
       fopen("test");
       write("test","the answers are i=",i," x=",x," y=",y,"\n");
       fclose("test");

the write statement is iterated over all the pixels, while the fopen and fclose are only executed once.

A number of sample macros (*.macro) are included with imal.

Mixing image math and macros

The fact that imal commands and image math can appear in the same macro is a tremendous convenience in writing macros. For example, here is a sample macro:

          
   # 1.macro - Simple macro
   open(gray);
   if(x>50) i+=27;
   if(y>x) i+=66;

This macro opens the image file gray, and applies two equations to it. The variable ``i'' is used instead of r,g, and b because gray happens to be a grayscale image. However, imal has to check each statement to determine whether it is a macro or a math equation. Slightly greater speed can be obtained by enclosing the math equations in a ``loop'':

          
   # 1.macro - Simple macro
   open(gray);
   loop
   {
      if(x>0) i+=27;
      if(y>x) i+=66;  
   }

In this macro, the ``loop'' keyword tells imal that the statements enclosed in braces are mathematical equations and not macro statements, so no checking is done against macro names. This has several advantages:

1. It is slightly faster.
2. Sometimes it is confusing as to whether a statement is looped or not. The `loop' notation makes it clearer that these equations will be looped over all the pixels.
3. Variable names that are the same as macro commands can be used.