The Magic of Hyper Stereos

written for the stereosite by Pascal Martiné, Germany

Introduction

When I start­ed to take my first own stereo pho­tos I soon real­ized that I can kind of adjust the amount of depth by shift­ing the cam­era more or less between the two shots. Like most of us it took me quite a while to devel­op the right feel­ing and reduce the amount of stereo pairs that were not real­ly sat­is­fy­ing. But cap­tur­ing land­scapes was still a chal­lenge when I first heard of so called hyper stereo pho­tos. After my per­son­al dis­cov­ery of stere­oscopy this opened a whole new world once more. Dur­ing the sum­mer of 2020 I had the long await­ed oppor­tu­ni­ty to take stereo pho­tos with a drone. But to tell you all about the mag­ic of hyper stere­os it’s best to start soon after the birth of stereo­scop­ic photography.

While stereo­scop­ic pho­tog­ra­phy always had more tech­ni­cal require­ments, includ­ing the cam­era as well as view­ing devices, the view­ing expe­ri­ence sur­passed that of mono pho­tog­ra­phy. This may not apply on por­traits but does cer­tain­ly on trav­el pho­tog­ra­phy, where you could step right into the scene depict­ed in a stereo view.

But when it comes to wide and dis­tant land­scapes their flat­ness is an unde­ni­able draw­back for the stereo­scop­ic effect when a stereo cam­era with lens­es spaced at the same dis­tance as human eyes is used. Watch­ing the fol­low­ing slides through a Brew­ster stere­o­scope would offer a lit­tle more depth than free view­ing them. Nev­er­the­less, one can see that the lack of 3D is already quite bor­ing com­pared to the two stereo slides shown above.

If you ever took your own stereo pho­tos and referred to the dis­tance of your eyes when shift­ing the cam­era between the two shots you may have encoun­tered that all dis­tant objects appear as one sin­gle flat back­ground. The same effect explains why we can­not esti­mate the dif­fer­ent dis­tances of clouds when we look to the sky.

Stereopsis

To under­stand why this is not pos­si­ble we need to con­sult some the­o­ry. The abil­i­ty of extract­ing depth infor­ma­tion from our binoc­u­lar vision is called Stere­op­sis. One of its con­di­tions is relat­ed to the fact that our eyes have a cer­tain dis­tance to each oth­er. Now, if we look at an object (F) both our eyes will imme­di­ate­ly turn towards the object, lead­ing to a vision of the object right in the cen­ter of both reti­nas, result­ing in one sin­gle vision for both eyes (Fig. 1).

The ver­ti­cal orange line rep­re­sents the dis­tance between us and the object. The hor­i­zon­tal orange line is called base­line in the con­text of stere­oscopy, i.e. the dis­tance between our eyes, or the dis­tance between the two cam­era lenses.

One could assume that every object which is as far away from us as object F would cause such a sin­gle vision. But this is not true. If you would draw two rays from equal points on both reti­nas through the two lens­es you would find out that their inter­sec­tions rather cre­ate a cir­cle. This cir­cle is called Horopter (Fig. 2).

Note that this is only the the­o­ret­i­cal horopter. There also exists an empir­i­cal horopter and a cer­tain neu­ronal tol­er­ance, sum­ma­rized in the so called Panum’s fusion­al area. But we will now focus on stere­op­sis again.

If an object O (red) is clos­er than the horopter its vision will have dif­fer­ent posi­tions on each reti­na (Fig. 3). Those dif­fer­ent posi­tions on the reti­nas cause a dou­ble vision which enables our brain to sense the dis­tances of objects. We also use addi­tion­al tech­niques such as com­par­i­son of size, move­ment, etc. to enhance that sense, but we can ignore that for the dis­cus­sion of stereopsis.

Unfor­tu­nate­ly, the dou­ble vision method works only for close objects. Here’s why:

Fig. 4 shows what hap­pens if you increase the dis­tance between us or the cam­era and the object we are look­ing at (ver­ti­cal orange line). The base­line and the dis­tance between the horopter and the clos­er object are the same as in the pre­vi­ous fig­ure. It’s just like you were step­ping back to take a look from fur­ther away. As you can see, the left vision of the red object moves clos­er to the vision of point 3. Ulti­mate­ly, this is what happens:

The for­mer dou­ble vision of the red object trans­forms into a sin­gle vision. This means stere­op­sis is not pos­si­ble any­more and we are thus not able to sense the dif­fer­ent dis­tances of the two objects — we are just too far away now.

This prob­lem affects stereo­scop­ic pho­tog­ra­phy even more. If you want to take a pho­to of some­thing that is just too large to fit entire­ly on your lens — like a build­ing, a moun­tain, a land­scape or a city panora­ma — the only way is to get far­ther away from the sub­ject and loose the stereo effect. Fur­ther­more, we some­times wish to get clos­er to par­tic­u­lar object but we can’t — like a ship on the sea, an ani­mal or the clouds men­tioned above.

Wait! Didn’t we con­clude that it’s just impos­si­ble to sense depth in dis­tant clouds? Yes, that’s true on one hand, but obvi­ous­ly clouds are also as three dimen­sion­al as a moun­tain. Luck­i­ly, we are not only able to bring back the depth, we are also able to make it vis­i­ble in a way that we have nev­er seen it before. That’s why the title of this arti­cle speaks of magic.

While magi­cians work with illu­sions or dis­trac­tions we will actu­al­ly not do any­thing more than reveal­ing real­i­ty. That means mak­ing stere­op­sis pos­si­ble for dis­tant objects. In the­o­ry it’s quite sim­ple to bring the dou­ble vision back. All you have to do is increase the base­line (Fig. 6).

If you want to exam­ine the effects between dis­tance and base­line on your own you can access this fig­ure as an inter­ac­tive GeoGe­bra file online here.

The effect might seem poor in the exam­ple above because the dou­ble vision on the last fig­ure is far less than on the first. But as I said you will use that tech­nique for large build­ings rather than for a still life on a table. So, if you increase the dis­tance to your sub­jects, their inner dis­tances will grow like­wise (while the dis­tance between the red object and the horopter remained the same through all figures).

Historical hyper stereos

It’s time to leave the­o­ry behind to prove that the tech­nique works. And how it works! When I looked through my col­lec­tion of glass slides I can eas­i­ly con­clude that hyper stereo­scop­ic pho­tog­ra­phy is no new dis­cov­ery, but was used for the same pur­pose as today as it was in the 19th century:

Typical settings

There are a few require­ments to take sat­is­fy­ing hyper stere­os such as an emp­ty fore­ground, equal ground, and space to move side­ways. This results in typ­i­cal sit­u­a­tions suit­able to take hyper stereos:

Walk along the riverside

Walk over bridges

Look down from large buildings

Take photos while you’re in a moving vehicle, …

… a plane or watching a movie.

Don’t move at all, but let the scenery move.

In my expe­ri­ence, it does not mat­ter if your base­line is a lit­tle too big — at least in most cas­es I don’t have time to cal­cu­late, or I just don’t know the dis­tance between the cam­era and the sub­ject. That’s why I always shoot a hor­i­zon­tal sequence of 4 to 7 pho­tos, and choose the final stereo pair after­wards. If I take simul­ta­ne­ous stereo pho­tos I leave one cam­era where it is and increase the dis­tance to the oth­er cam­era mul­ti­ple times. That way I can choose the best pair after­wards as well. For more infor­ma­tion about how to choose the base­line I rec­om­mend David Kuntz’s arti­cle Get­ting the Right Depth in 3D Pho­tog­ra­phy.

Hyper stereos taken by a drone

A few years ago, a great pos­si­bil­i­ty for stereo­scop­ic pic­tures went rather unno­ticed when drones became avail­able for every­one at a mod­er­ate price lev­el. Here are a few exam­ples that I took togeth­er with Ihab Zaidan who flew the drone:

Castle Waldthausen, Mainz, Germany

Russian Orthodox Church, Wiesbaden, Germany

Fly­ing high obvi­ous­ly enables you to have an emp­ty fore­ground wher­ev­er you are, and lets you choose the per­fect per­spec­tive. But one of the most impor­tant ben­e­fits is that the remote con­trol allows you to move exact­ly side­ways — no slope of a road, no acci­den­tal cir­cu­lar movement.

Of course, there are also draw­backs and lim­i­ta­tions such as strong wind, flight restrict­ed areas, and the bat­tery of the drone. The con­di­tions of sequen­tial stereo pho­tos also affect drone stere­os – but since you are quite far away and can move rather quick­ly this is not a big prob­lem. I would say it has nev­er been eas­i­er to take sat­is­fy­ing hyper stere­os than with a drone.

A series of more drone stereo pho­tos can be found in the cor­re­spond­ing gallery Cas­tles along the Rhine.