This article's definitions of "motion parallax" and "shape from shading" are quite different from my understanding. Can anyone shed any light on this? Specifically:
"Motion parallax has to do with the apparent size of an object. If you put a soda can in front of you and then move it closer, it will get bigger in your visual field. Your brain assumes that the can didn’t suddenly grow and concludes that it’s just got closer to you."
Whereas I believed "motion parallax" to be moving one's head so as to compare an object's displacement against a more distant background. Size is irrelevant.
"Shape-from-shading is a bit trickier. If you stare at a point on an object in front of you and then move your head around, you’ll notice that the shading of that point changes ever so slightly depending on the lighting around you. The funny thing is that your eyes actually flicker constantly, recalculating the tiny differences in shading, and your brain uses that information to judge how far away the object is."
"Shape from shading" I believed to be simply recreating a 3d structure from the way light falls on it, a depth cue that occurs even without motion. The quoted description seems like it is referring to specularity, which does play a role in shape from shading, but also seems well handled by (many) rendering engines.
My understanding of both of those terms agrees with yours.
I think in the case of "motion parallax" it's nearly the same, though.... whether you're "moving your head" or "moving the object", in both cases the spatial relation of three things is changing: your Center of Projection, and two other things (e.g. the two sides of a soda can, or e.g. a cloud and the moon).
Now that I found someone to discuss the nerditude of detailed depth cues... why on earth would eye-flickering change the shading? Am I supposed to believe that it's because the CoP moves (because my pupil moves)? I find it implausible that moving the CoP one millimeter, on an object that's 100mm to 10000mm from the CoP, while also changing which center-surround is processing the photons from a point, can detect a difference in shading, even on highly highly specular surfaces. Can anyone explain this to me?
Actually, as far as I've read, the minor eye movements happen exactly to enable motion parallax (on the very edges of objects) even while the object is static relating to you - so it's kind of completely opposite what the original article was proposing.
That seems right. It's conceivable that microsaccades could be affected by diffraction effects in unusual situations. But it seems like shading (even specular highlights) couldn't be perceptibly affected.
1. Shading.
2. Motion parallax. When you move your head, objects in the foreground move more.
3. Microsaccades. If your eye points perfectly steady, your retina adapts to the constant stimulus and the image disappears. So your eye moves slightly, all the time.
"Motion parallax has to do with the apparent size of an object. If you put a soda can in front of you and then move it closer, it will get bigger in your visual field. Your brain assumes that the can didn’t suddenly grow and concludes that it’s just got closer to you."
Whereas I believed "motion parallax" to be moving one's head so as to compare an object's displacement against a more distant background. Size is irrelevant.
"Shape-from-shading is a bit trickier. If you stare at a point on an object in front of you and then move your head around, you’ll notice that the shading of that point changes ever so slightly depending on the lighting around you. The funny thing is that your eyes actually flicker constantly, recalculating the tiny differences in shading, and your brain uses that information to judge how far away the object is."
"Shape from shading" I believed to be simply recreating a 3d structure from the way light falls on it, a depth cue that occurs even without motion. The quoted description seems like it is referring to specularity, which does play a role in shape from shading, but also seems well handled by (many) rendering engines.