Starting at square one with image processing

[James Hurley]

I'm trying to understand the rudiments of image processing in Run Rev.

I used the RR pencil (image not graphic) to draw a horizontal line  
and used the following script to see how the data was stored:

on mouseUp
   put the imagedata of img id 1003 into tData
   repeat for each char tChar in tData
     put chartonum(tChar) after results
   end repeat
   put the number of characters in results into  msg box
   put results into field 1
end mouseUp

All I got for my troubles was  string of zeros.

So I made a small png image in Photoshop (a short horizontal black  
line) and the same script produced the following string:

025525525502552542540254254253025425325202542532520254253252

Plus a long string of similar 10 element units.

How does this represent a black line? Am I just observing the png  
compression code? How do I get a look at data that tells me the image  
is a black line?

Jim