My time has come!
The above stereographic image is for cross-eyed viewing (most stereograms are wall-eyed, so you may need to put your finger in front of your screen until this one comes into focus)
This is an image of Honolulu, Hawaii, published by NASA. Note Diamond Head (the volcanic crater) in the south.
Here are some other stereopairs published by JPL:
Wheeler Ridge, California
Mount Saint Helens
Salt Lake Valley, Utah
Wellington, New Zealand
I can still view these, but it’s much much harder for me.
I don’t know why parallel isn’t the default.
It varies per person. I for one can't view wall-eyed, only cross-eyed.
Same here basically, cross-eyed viewing is super easy for me but I have to work for minutes to perform wall-eyed viewing. I was really excited to see a post with cross-eyed stereograms.
Lots of people can really easily go cross-eyed and look at these with no practice whatsoever. Fewer people can do the parallel kind with no practice or with the amount of practice they've already done.
What is parallel ?
Parallel are the ones where you put the image between you and your point of focus, instead of your point of focus being between you and the image.
That seems hard to do
I've tried it (to get reverse-depth) and didn't manage to...
Cross eyed is so much more uncomfortable. It also looks smaller than parallel to me.