UI for legibility and control of distance, zoom, and orientation.

[kpreid]

Is your feature request related to a problem? Please describe.
It's often hard to manage the zoom level and depth of the camera pivot point, or to get into a desired configuration of depth, zoom, and orientation. This is understandable because we're viewing a space which is featureless except insofar as the data has features, but the data is not necessarily enough to give good human perception of the 3D space (especially when an application is in early development, or has a data generation bug producing wildly out of range or degenerate values).

Describe the solution you'd like
Any or all of the following (this is intended as a brainstorming sort of list, not something to be implemented as-is). I hope that many of these could work together to make comprehension and control easier.

• Zoom/depth-related:
  • A temporary visual cue as to the depth of the pivot point relative to the data. For example, the data intersecting the plane of depth the pivot point is at could be shaded with a highlight, or data behind the pivot could be “foggy”. These would only appear when actively moving, to avoid cluttering the actual data visualization.
  • A scale bar, indicating the onscreen size of 1 unit at the pivot point depth, and perhaps also the camera-to-pivot distance at that scale.
  • A visual cue when the minimum and maximum zoom / camera-to-pivot distance has been reached (see Zooming in changes orbital camera origin #4402).
  • A way to rotate the camera by exactly 90 degrees, so that one can get a look at the depth axis, possibly move on it with drag or WASD, and then jump back to the previous view. This may also be useful for understanding the actual data.
  • A way to save and restore the current camera state, so one can explore away from a good position and then restore it.
  • A way to lock the pivot point or lock the zoom, so as to manipulate one without accidentally affecting the other by pressing the wrong button.
  • Configurable (blueprint) limits for the position of the pivot within the space, so that the application or user can add useful constraints from knowing the character of the data.
    • Or just optionally lock it to within the "accumulated bounding box".
    • Maybe also limits on the camera-pivot distance.
• Orientation-related:
  • A UI element in the corner of the Space View which displays the current coordinate axes, which can be clicked to rotate an axis either "up" or "backward". For context and debugging, it should also display both XYZ and front/right/up axis labels.
    (This is like the existing “show origin axes” option but it would have a constant screen-space position and size.)
  • An infinite gridded plane perpendicular to the "up" axis. (This also assists zoom/depth by presenting a grid of specified units.) (The application could provide a finite grid but it would interfere with the double-click-to-fit action unless it could be excluded somehow.)