Edge detection doesn’t explain line drawing
&gt;<i>A classic answer to this question is what I will call the Lines-As-Edges hypothesis. It says that drawings simulate natural images because line features activate edge receptors in the human visual system.</i><p>Why does the explanation need to go to that direction?<p>Line drawings have a liking to the thing being depicted.<p>They&#x27;re a crude representation of it (compared to a photograph or a photo-realistic oil painting) but are nonetheless a represenation.<p>That&#x27;s why they work, in the sense of people understanding what they show: they share similar patterns with the things being show. And we are pattern matches.<p>In this case the patterns are edge patterns, but it could just as well be non-edge patterns. Imagine a color drawing of a human face, where the ink drawn edges have been removed, and it&#x27;s just blocks of color for the head, the eyes, the pupils, the mouth, the nostrils, ears, etc. We could still tell it&#x27;s a face, even if we applied some gaussian blur to those blocks.<p>&gt;<i>The most basic statement of the problem with Lines-As-Edges is that the human visual system isn’t just an edge detector. You can see colors, you can see absolute intensities. You can tell the difference between a thin black line and the silhouette of a dark object against a light background; we have both kinds of receptors in the primary visual cortex, as well as others. Yet Lines-As-Edge supposes that the vision system discards all of this other information present in an image, for just this one special case. Why?</i><p>Isn&#x27;t this taking things backwards?<p>It&#x27;s not the vision system which &quot;discards all of this other information present in an image&quot; in our regular operation.<p>Rather, it&#x27;s the line drawing with does away with (discards) all of this other information and only focuses on a thing&#x27;s edges.<p>In other words, our visual system has capabilities A, B, C (say edge detection, color, 3d placement, etc). And, a line drawing gives it only A - which is still enough.<p>When our visual system also gets B and C, it can perceive  objects even better. But for merely identifying something, A is apparently enough.<p>What I described here is totally compatible with Lines-As-Edges hypothesis, and makes sense too, so I don&#x27;t see where the author&#x27;s issue is, and why he thinks the fact that &quot;the human visual system isn’t just an edge detector&quot; invalidates the lines-as-edges hypothesis.<p>&gt;<i>Now you get a sense of the color of the object, and not just its outlines. How would one generalize Lines-As-Edges to account for these different types of depiction? The visual system is no longer ignoring everything aside some gradients; it’s now paying attention to some colors (and not others).</i><p>Yeah, so? It just means that the visual system can work with less or with more (and multi-type) information.<p>Lines-as-edges is a hypothesis for why line drawings &quot;work&quot; (are recognizable as the thing). The hypothesis doesn&#x27;t say however that edges are the only thing the visual system can understand.<p>So, there&#x27;s no need to &quot;generalize Lines-As-Edges to account for these different types of depiction&quot;.<p>Lines-as-edges explains how line drawings are understood, period.<p>Line drawings with shading and color, add additional information, aside from the edges.<p>That&#x27;s fine: no proponent of Lines-As-Edge ever said that the visual system only works with edges. Just that it works with edges when interpreting a line drawing which only offers edges.