If some modes of 
representation can plausibly be 
explained as conventions, can 
the use of lines in pictures be 
explained in the same way? 
Perhaps a sharp outline of a 
cloud is accepted, by 
convention, as representing a 
cloud, even though the 
boundaries of real clouds are 
typically much "softer" and 
gradual. Perhaps the circles on 
a giraffe are partially based on 
convention.  Although these 
particular explanations may be 
correct, in general the 
convention theory can be 
faulted both on logical and on 
empirical grounds, as Kennedy 
has noted. On logical grounds, 
the fact is that, despite solidly 
colored and textured surfaces 
between edges in real scenes, it 
is the contours at such edges 
that are of primary importance 
in conveying information about 
object and depth relations in 
the scene. Contours at edges are 
very similar to lines as used in 
pictures. Thus the drawing of 
the hand contains much the 
same information as does the 
eye’s image of the hand itself.