With Apple claiming the Tap to Zoom and Pinch to Zoom patents and possibly enforcing them, what is an alternative that is intuitive and still distinct from tap/pinch?
Let's assume for a moment that we're talking about a touch interface without any other controls.
As noted in the comments above, there are a limited number of gestures for directly manipulating content. From gestureworks:
This leaves for direct manipulation either a rotation gesture (screw-to-zoom?) or a difficult to control hold gesture. Neither are ideal.
There are three primary methods for zooming using controls. These are:
Overall, methods using controls are likely to be the best bet when using a touch interface for zooming, assuming that we're unable to use pinching or tapping (which, in my opinion, are the most obvious and intuitive of the gestures and would otherwise be the preferred solution).
This won't work with just thumb, but neither does pinch.
Have the user put 4 fingers spread on the screen, the map then zooms in so that the polygon whose vertices are your fingers fills the whole screen. To me this seems much more intuitive than pinch zoom, especially for pictures and maps. For instance, if I have a picture with a face in it, I just put my fingers in a little box shape around the face and boom it zooms in to exactly what I want.
This is like the boundary boxes idea, except instead of drawing a boundary box, you just place your fingers to be the vertices. It will also work with just 3 fingers (boundary triangle) but might be less intuitive.
Put fingers on screen:
Figure out bounding box:
Zoom to have bounding box fill screen:
The thing with Touch is despite all the talk of "gestures" there are really only a few basic gestures humanly possible:
As you'll note even these gestures you're partially repeating yourself; tap and hold is just a longer tap, swiping is just dragging but letting go quickly.
Tapping is the basic action. Pinch is the obvious choice. But for he context of this question we have to ignore those; I am not considering those "illegal" nor would I encourage you to avoid them, I'm just playing the thinking game.
Basically what we have left are swipe/drag and tap and hold left. Tap and hold doesn't make sense, and it's a one way action. It's also not very discoverable; I'll consider it excluded.
Swiping or dragging up/down for zoom in out is a legitimate possibility, but it complicates the potential for a drag gesture to pan. I wouldn't recommend swipe/drag if any form of panning/scrolling is involved.
So that's all direct manipulation options excluded. What's left? Buttons. A viable option for large screens is the slider like Google Maps desktop uses:
Alternately, plain old plus minus buttons (maybe in a magnifying glass):
Basically if you really want something that clearly and intuitively indicates zoom, you have to go back to visual controls if you're not allowed pinch to zoom.
What about 2-finger swipes up & down? Granted, it may not be immediately intuitive since pinch is the accepted "norm" but it seems like it would be easy to catch on, and is different enough from a 1-finger swipe used for panning or sliding.
I agree that if a touch method isn't necessary, control buttons would be the easiest for a user to learn.
The browser that shipped with the Nokia N900 used a 'circle-to-zoom' - you made a circle shape with your finger or thumb to zoom, clockwise to zoom in and counterclockwise to zoom out.
There is also the old Android method pressing with both thumbs at the same time and tilting the device. Very janky and awkward.