Stereo window placement
Posted: Sun Jun 16, 2019 1:59 pm
I just want to introduce important notion in stereo photography (it applies to video too) for enthusiasts who come to stereo through stereopi:
the stereo window.
In traditional photography, there is a frame in which all elements are included.
While framing, among other things, it is important to pay attention to what is cut by the edges.
The 3D equivalent is the stereo window though which we see all the elements.
Being aware of the edges is tremendously more important in this context (we will see why thereafter).
The edges of your picture create a delimitation beyond which no pixels are visible. Typically, this is a rectangular shape. But because your content has depth, this rectangle get a position in the third dimension too (or more precisely, the content has a depth relative to this rectangle). And because man cannot see on the sides (no pixels) it acts like a window we see the scene through. That's why it is named stereo window.
As stereo(photo/video)graphers, we have some freedom to choose were the window is placed in our scene, and we can also make some elements go through the window (out of the screen). There are essentially two limits:
1) There shall not be too much disparity
This means that no element in the photograph shall be more than a certain distance apart (i.e. not too far from where the same element would be if the two photographs were superposed). Actually, this distance depends a lot of geometric factors such as the size of the screen/print and the distance you watch it from. For the spectator, it shall not exceed 6cm apparent (perspective into account) size. Note that this distance is far smaller for objects which pop out of screen.
This constraint is easily achieved as soon as the closer element is not too close, say about 1.5m-2m with the base (distance between cameras) of 65mm of the stereopi. For closer objects, you might have to reduce the distance between the cameras (for this reason and some other factors too).
2) There shall not be front element cut by the edge
This one is really important. The brain does not like it at all, although people react differently, because it does not correspond to a real-world situation: when an object is in front of the window, it cannot be masked by it. This is called window violation.
So beware to not cut anything close with your frame (note that you can choose window placement in depth, which changes what elements are close).
This stereo picture from Randy Hester http://phereo.com/image/56b49b02e7e564162d0000df shows it well: black and yellow cuts are okay, white is bad. Also, the yellow one creates a pop-out effect.
To place the stereo window, we just have to [beware of the resulting edges]:
1) choose a point in the 3d scene which will be exactly at the screen plane
2) align the two images on top of each other so that this point matches
3) cut every parts of the images that do not overlap
And of course, merge the two resulting images with the format of your choice.
To avoid windows violations, the easiest way is to choose the closer point of the photograph; or at least the closer which is next to the edges (because closer point in the middle can pop out often safely).
Note that because of the way the cameras are placed and their own field of view, you always have to cut the edges of raw images from the sensors anyway, at least if you do not want quite bad stereo images to be shown.
A last note about pop-out effect (a.k.a. through-the-window/TTW effect)...
The elements that pop out cannot be placed anywhere. Actually, the 3D space of a stereo photography is like a truncated pyramid (we can see larger background than the window), so the popping elements shall not be too close to the edges or you might get a window violation.
Edit: fix one typo that changed meaning + add precision about closer point choice.
the stereo window.
In traditional photography, there is a frame in which all elements are included.
While framing, among other things, it is important to pay attention to what is cut by the edges.
The 3D equivalent is the stereo window though which we see all the elements.
Being aware of the edges is tremendously more important in this context (we will see why thereafter).
The edges of your picture create a delimitation beyond which no pixels are visible. Typically, this is a rectangular shape. But because your content has depth, this rectangle get a position in the third dimension too (or more precisely, the content has a depth relative to this rectangle). And because man cannot see on the sides (no pixels) it acts like a window we see the scene through. That's why it is named stereo window.
As stereo(photo/video)graphers, we have some freedom to choose were the window is placed in our scene, and we can also make some elements go through the window (out of the screen). There are essentially two limits:
1) There shall not be too much disparity
This means that no element in the photograph shall be more than a certain distance apart (i.e. not too far from where the same element would be if the two photographs were superposed). Actually, this distance depends a lot of geometric factors such as the size of the screen/print and the distance you watch it from. For the spectator, it shall not exceed 6cm apparent (perspective into account) size. Note that this distance is far smaller for objects which pop out of screen.
This constraint is easily achieved as soon as the closer element is not too close, say about 1.5m-2m with the base (distance between cameras) of 65mm of the stereopi. For closer objects, you might have to reduce the distance between the cameras (for this reason and some other factors too).
2) There shall not be front element cut by the edge
This one is really important. The brain does not like it at all, although people react differently, because it does not correspond to a real-world situation: when an object is in front of the window, it cannot be masked by it. This is called window violation.
So beware to not cut anything close with your frame (note that you can choose window placement in depth, which changes what elements are close).
This stereo picture from Randy Hester http://phereo.com/image/56b49b02e7e564162d0000df shows it well: black and yellow cuts are okay, white is bad. Also, the yellow one creates a pop-out effect.
To place the stereo window, we just have to [beware of the resulting edges]:
1) choose a point in the 3d scene which will be exactly at the screen plane
2) align the two images on top of each other so that this point matches
3) cut every parts of the images that do not overlap
And of course, merge the two resulting images with the format of your choice.
To avoid windows violations, the easiest way is to choose the closer point of the photograph; or at least the closer which is next to the edges (because closer point in the middle can pop out often safely).
Note that because of the way the cameras are placed and their own field of view, you always have to cut the edges of raw images from the sensors anyway, at least if you do not want quite bad stereo images to be shown.
A last note about pop-out effect (a.k.a. through-the-window/TTW effect)...
The elements that pop out cannot be placed anywhere. Actually, the 3D space of a stereo photography is like a truncated pyramid (we can see larger background than the window), so the popping elements shall not be too close to the edges or you might get a window violation.
Edit: fix one typo that changed meaning + add precision about closer point choice.