For singular dots in space your argument would be valid, but real objects are often more complicated. If the eyes can’t reliably lock onto the same spot along the X-axis due to a repeating pattern or a complete lack of detail along said axis, tilting the head shifts the whole situation and allows the eyes to zero in on a fixed point to perceive depth. An extreme example: If you look at two horizontal featureless lines (offering no details along their length to lock onto, brushed metal railings for example) positioned one behind the other, running perpendicular to the field of view in the direction of the X-axis. The only way for depth perception to work here is to tilt the head to introduce a difference along the Y-axis.
Repeating patterns with the right spacing (e.g. grids, lattices) in that same plane can also confuse depth perception, in which case the head tilt often helps.
Another (marginal) benefit of head tilting is the fact that as the head rotates, the eyes physically move, possibly revealing additional detail that may have been obstructed from the previous vantage points. All this for a much lower energy expenditure than the whole animal moving itself.
Oh and one thing that popped into mind from personal experience as I am writing this: In darkness tilting the head helps discern between shapes that are just lingering on your retinas after looking at a brighter thing earlier (rotates along with the eyes) vs. dim things that might actually be there right now (stays in the same orientation relative to the surroundings).
Your vision is always a singular dot in space, unless you have amblyopia or something else causing your eyes to point in different directions. Even if you’re looking at a featureless line, your eyes are fixed on a single point in space. It’s not like one eye is looking somewhere different from the other eye. The triangle still exists.
There is no difference to stereo vision depth perception regardless of how the view points are oriented in the X and Y axis. The practical proof for this is in rangefinders; neither simple consumer stereo rangefinders nor complicated military stereo rangefinders, such as those found on battleships
or antiaircraft guns, are oriented in the Y axis to gain some advantage. There’s no need. The triangle is the same regardless of X-Y orientation.
I see what you’re saying about the example of two completely featureless lines oriented exactly along the X axis in a completely featureless space, as this wouldn’t work with a coincidence rangefinder, but this is an edge case and not something you’d encounter in real life. You’d also have other cues for depth perception beyond just stereopsis.
For singular dots in space your argument would be valid, but real objects are often more complicated. If the eyes can’t reliably lock onto the same spot along the X-axis due to a repeating pattern or a complete lack of detail along said axis, tilting the head shifts the whole situation and allows the eyes to zero in on a fixed point to perceive depth. An extreme example: If you look at two horizontal featureless lines (offering no details along their length to lock onto, brushed metal railings for example) positioned one behind the other, running perpendicular to the field of view in the direction of the X-axis. The only way for depth perception to work here is to tilt the head to introduce a difference along the Y-axis. Repeating patterns with the right spacing (e.g. grids, lattices) in that same plane can also confuse depth perception, in which case the head tilt often helps.
Another (marginal) benefit of head tilting is the fact that as the head rotates, the eyes physically move, possibly revealing additional detail that may have been obstructed from the previous vantage points. All this for a much lower energy expenditure than the whole animal moving itself.
Oh and one thing that popped into mind from personal experience as I am writing this: In darkness tilting the head helps discern between shapes that are just lingering on your retinas after looking at a brighter thing earlier (rotates along with the eyes) vs. dim things that might actually be there right now (stays in the same orientation relative to the surroundings).
Your vision is always a singular dot in space, unless you have amblyopia or something else causing your eyes to point in different directions. Even if you’re looking at a featureless line, your eyes are fixed on a single point in space. It’s not like one eye is looking somewhere different from the other eye. The triangle still exists.
There is no difference to stereo vision depth perception regardless of how the view points are oriented in the X and Y axis. The practical proof for this is in rangefinders; neither simple consumer stereo rangefinders nor complicated military stereo rangefinders, such as those found on battleships or antiaircraft guns, are oriented in the Y axis to gain some advantage. There’s no need. The triangle is the same regardless of X-Y orientation.
I see what you’re saying about the example of two completely featureless lines oriented exactly along the X axis in a completely featureless space, as this wouldn’t work with a coincidence rangefinder, but this is an edge case and not something you’d encounter in real life. You’d also have other cues for depth perception beyond just stereopsis.