Stereo Window basics

written for the stereosite by David Kuntz (Rancho Palos Verdes, California, USA)

Stereo pho­tog­ra­phy is a bit more demand­ing than tra­di­tion­al flat pho­tog­ra­phy, because a poor­ly ren­dered 3D image can be dif­fi­cult or unpleas­ant to view.  So, the stereo pho­tog­ra­ph­er has to take addi­tion­al steps, not required with flat pho­tos, to avoid the prob­lems that lead to view­er dis­com­fort.  The good news is that under­stand­ing a cou­ple of rel­a­tive­ly sim­ple con­cepts will allow you to con­sis­tent­ly pro­duce 3D images that are easy to look at, and have good 3D impact.  And, once you’ve absorbed these ideas, it won’t take you any time at all to rou­tine­ly per­form the actions nec­es­sary to accom­plish this.

What is the Stereo Window?

The series of three pho­tos shown here illus­trate a key 3D pho­tog­ra­phy con­cept that affects view­ing com­fort, name­ly, the stereo win­dow.  Imag­ine that your screen has a rec­tan­gu­lar hole in it, and you’re view­ing the 3D con­tent in this image through it.  That is, you’re see­ing the boy point­ing his fin­ger at you through a win­dow in your screen.  The lat­tice of green lines in each image lies in the plane of your screen; in oth­er words, your eyes see the green lines as being the same dis­tance from you as your screen itself.

In the first 3D image pair (on top), the boy’s hand (which is the clos­est object in the com­po­si­tion to you), appears behind the green lines, or behind this win­dow.  In the next stereo pair (the mid­dle), his fin­ger has been moved clos­er, so that his fin­ger­tip is right at the plane of the win­dow; the exact same dis­tance from you as the green lines.  In the last 3D image (bot­tom), his fin­ger­tip has now come through the win­dow, and is clos­er to you than the green lat­tice of lines.

These three 3D images are all the same pho­to.  So, how did this trans­for­ma­tion in terms of their depth rela­tion­ship to the stereo win­dow (or plane of the screen) between them occur?  The lit­tle red arrows between the pho­tos indi­cate how it works.  If you com­pare the posi­tion of the boy’s eyes in each of the left side images with the left­most green ver­ti­cal line, you can see that the entire image has been moved slight­ly to the right in each suc­ces­sive pair.  The right side image doesn’t move at all.  This is pos­si­ble because the orig­i­nal pho­to has lots of addi­tion­al mate­r­i­al off the left and right edges that is being cropped off here.  

This crop­ping, and how it relates to the win­dow is shown again in the next two pho­tos.  Here, you can see this image is cropped down quite a bit (in oth­er words, there’s lots more in the orig­i­nal image that’s not being shown out­side the frame of the crop).  But, by slid­ing the image left/right rel­a­tive to crop­ping rec­tan­gle (or stereo win­dow), the depth rela­tion­ship of the sub­ject mat­ter with respect to the win­dow is changed.  What is not changed, how­ev­er, is the depth rela­tion­ship of ele­ments with­in the com­po­si­tion itself (such as the per­ceived depth dis­tance from the tip of the boy’s fin­ger to the back­ground).  Those were set when the image was orig­i­nal­ly tak­en, and can­not be sub­se­quent­ly changed.  All, that’s hap­pen­ing here is an adjust­ment in the rela­tion­ship of the image con­tent with respect to the frame it’s con­tained in.  

Why worry about the Stereo Window?

The next series of pho­tos is intend­ed to demon­strate why you should care about the posi­tion­ing of the stereo win­dow rel­a­tive to your pho­to.  In the first one, the flower petals at the edge of the frame are in front of the stereo win­dow.  This caus­es a bit of visu­al con­fu­sion, espe­cial­ly at the left edge of the frame.  There’s also some “shim­mer­ing” at the frame edges, because there’s so much image mate­r­i­al that shows up at the edge of one image that doesn’t appear at the cor­re­spond­ing edge of the oth­er image.  

Too far in front of the win­dow.

In the next pho­to, every­thing is far behind the stereo win­dow.  This again caus­es a bit of “shim­mer­ing” on the left and right sides of the image because so much of what appears at the edges doesn’t match up between the left and right.  How­ev­er, this ver­sion isn’t that unpleas­ant to view.  

Too far behind the win­dow.

The third image has every­thing in the image set just behind the win­dow.  It’s com­fort­able to view, doesn’t have a lot of shim­mer­ing at the edges.

Good rela­tion­ship between sub­ject and the win­dow.

If the dif­fer­ences point­ed out here aren’t obvi­ous to you, don’t wor­ry.  Your sen­si­tiv­i­ty to how the win­dow is adjust­ed depends on a few fac­tors.  First there are indi­vid­ual vari­a­tions in how we each per­ceive stereo pho­tos, and our tol­er­ance for these adjust­ment fac­tors.  Next, the size of your dis­play, and your dis­tance from it, are also impor­tant fac­tors.  If you’re view­ing this tuto­r­i­al on a small phone screen, it’s quite pos­si­ble that you either won’t see the dif­fer­ences between the last three pho­tos, or won’t be both­ered by them if you do.  But, as dis­play size goes up, these dif­fer­ences become much more notice­able and dis­turb­ing.  When viewed on a large screen 3D TV, or if pro­ject­ed on a large screen, even much small­er dif­fer­ences than those illus­trat­ed here can pro­duce a sig­nif­i­cant amount of view­ing dis­com­fort.  The take­away from this is that if you’re only show­ing your images on Insta­gram, you don’t need to wor­ry about all this near­ly as much as if you’re pro­ject­ing them for a crowd. 

How Do I Control the Stereo Window?

So, if you now under­stand the need to set the posi­tion of your sub­ject mat­ter with respect to the stereo win­dow, the next ques­tion is how to accom­plish that.  That turns out to be the easy part.

Two of the appli­ca­tions that can be used to read­i­ly per­form this adjust­ment are the Win­dows-based pro­gram StereoPho­toMak­er, and the cor­re­spond­ing iPhone/Android app, 3DStereoid (both pro­duced by Masu­ji Suto).  No mat­ter which pro­gram you use, the very first step you should take, before per­form­ing this win­dow adjust­ment, is to “auto-align” your image.  This gets rid of any ver­ti­cal, rota­tion­al, per­spec­tive or oth­er mis­align­ments that can make a 3D image uncom­fort­able to view. 

With auto-align­ment com­plete, set the win­dow in StereoPho­to Mak­er by going to the “Adjust” menu, and then select­ing “Easy Adjust­ment.”  This brings up a large dia­log box with lots of options, along with your image dis­played in anaglyph (red/cyan) for­mat.  To adjust the win­dow, just use the slid­er at the top of the win­dow (indi­cat­ed in red).  This allows you to move the two images hor­i­zon­tal­ly rel­a­tive to each oth­er.  As explained pre­vi­ous­ly, this will change the rela­tion­ship of your sub­ject mat­ter to the stereo win­dow.  

Anoth­er option in StereoPho­to Mak­er is to stay in the main pro­gram win­dow, and then just use the left and right arrow keys to make this adjust­ment.  Each press of the arrow key shifts the image a small amount, so it’s eas­i­er to use the Easy Adjust­ment menu if a large shift is need­ed in order to get the right win­dow set­ting.  This adjust­ment can be per­formed with your image dis­played in vir­tu­al­ly any for­mat; anaglyph, par­al­lel pair, cross-eye pair, and so on.  So, you should prob­a­bly choose a dis­play method which you are actu­al­ly able to view in 3D.  

In the 3DStereoid app, this same func­tion­al­i­ty is avail­able by select­ing “Edit” from the main menu.  Then, the left and right arrow but­tons (indi­cat­ed in yel­low in the graph­ic) will shift the images rel­a­tive to one anoth­er.

How do you know when you’ve got it right?  Any part of your pho­to that doesn’t have a red or blue fringe (that is, the red and blue images are per­fect­ly over­lapped there) when viewed in anaglyph is right at the stereo win­dow.  This sit­u­a­tion is illus­trat­ed in the next graph­ic, which shows that the right side of the right­most flower in the image has no col­or fring­ing, and will there­fore be exact­ly at the stereo win­dow in terms of depth.  The petal next to that does show a fringe, and won’t be at the win­dow.  It can be help­ful to view your image through anaglyph (red/cyan) glass­es dur­ing this process to visu­al­ize the rela­tion­ship of sub­ject mate­r­i­al to stereo win­dow, or alter­nate­ly, in anoth­er for­mat that you can actu­al­ly view in stereo. Often, a final step of crop­ping your 3D image may be help­ful after set­ting the win­dow.  This can be help­ful to elim­i­nate parts of your pho­to that don’t match up well or are visu­al­ly dis­tract­ing after win­dow­ing has been per­formed.

Conclusion

Hope­ful­ly, this tuto­r­i­al has shown you the need for stereo win­dow adjust­ment and it’s ben­e­fits, and giv­en you a sim­ple, easy to use method for per­form­ing that adjust­ment. The key take­aways to remem­ber are:

  • “Shoot loose.”  That is, be aware when you’re tak­ing your pho­to that you’ll prob­a­bly end up los­ing some of it on top and bot­tom dur­ing auto-align­ment, and on the left and/or right dur­ing win­dow­ing.  So, pur­pose­ful­ly include a bit more on all sides to accom­mo­date this even­tu­al­i­ty. 
  • The depth con­tained with­in your image is deter­mined when you take it, and can nev­er be changed. What you can change dur­ing sub­se­quent adjust­ment is the rela­tion­ship between you sub­ject mat­ter and the stereo win­dow (the frame of the pho­to).   
  • Always per­form an auto-align­ment on your 3D image(s) first, before com­menc­ing any oth­er pro­cess­ing oper­a­tions.
  • You’ll often want to per­form a final crop­ping step after win­dow­ing an image to “clean it up.”  That is, crop out any­thing that some­how got cut off by the process or doesn’t look right. 
  • Win­dow­ing can help make your pho­tos eas­i­er to view, and should be done on every sin­gle 3D image you make.  But, it becomes increas­ing­ly impor­tant as you go to larg­er dis­plays.  So, it’s not near­ly as impor­tant for images intend­ed for view­ing on a small cell­phone screen, but it’s absolute­ly crit­i­cal for large screen pro­jec­tion.  But, it’s easy to do, so why not get in the habit right now?