Thanks to Steve Boddy for his help about how to build a phantogram, and his cup example.
What is a phantogram
A phantogram is a pure "come out" stereo. It should be put on the floor
or on a table and viewed from a point of view at 45° from the
horizontal. The object on the stereo scene then seems to be put on the
floor or on the table, and stand over it.
Top left : the dotted triangle is what you see when looking at the
anaglyph, taken with a horizontal camera, viewed on a vertical print.
D is the distance to take into account to evaluate the stereo base (with the common rule : base = 1/30 x D). Top right
: the dotted triangle is what you see when looking at the anaglyph,
taken with a 45°camera, viewed on a 45° print.
D is the distance
to take into account to evaluate the stereo base (with the common rule : base = 1/30 x D). Bottom : the phantogram builder of AnaBuilder (and the method
that is explained below) is calculating from one of the top anaglyphs
the anaglyph that should be printed to see exactly
the same dotted triangle when this new anaglyph is viewed with a 45°
angle.
How to build it with the phantogram builder tool
1) print a regular square grid on a paper.
2) put this paper on a table with an object on it.
3) take a stereo couple of this object from a point of view making an
angle of 45° with the horizontal. The stereo base should be
centered with the object. When doing this, the regular square grid
appears as perspective, having the nearest squares larger than the
furthest.
4) load the stereo view in AnaBuilder, and call the menu "tools/Experimental/Phantogram builder", it opens a
new window with the left/right views being distored to reduce the
perspective effect on the square grid. The goal now is to tune the
values to get back a well-regular square grid, in order to perfectly
reverse the perspective effect. After having changed the values, click
on "View" button to calculate the new distortion, and "Export" button
to view the result in AnaBuilder. This can be done several times, till
having the best square grid as possible. 5) when this is obtained, change the resolution of the image to get a
better quality, and export to final stereo in AnaBuilder. Save it.
6) print the stereo as anaglyph and put it on a table. Look at it with
the same point of view as the original one. The object should seem to
stand over the table as the original.
Example with a perfect calculated image from Steve Boddy :
Left image :
Right image :
Phantogram obtained with values :
r = 42.5
xLeft = xRight = 0
z = 1.5
fov = 50
y/x = 2.0
1024x1024 image, (cropped afterward in AnaBuilder)
Here is a natural shot example (sorry for the little curve of the chess
board at the top, but the table was too short : the pliable check board
is a little bulging) :
Crossed view of the original (without horizontal left/right adjustment) :
Phantogram obtained with values :
r = 52.5
xLeft = 3
xRight = 0
z = 1.5
fov = 40
y/x = 2
1024x1024 image, (cropped afterward in AnaBuilder)
The principle of the phantogram builder
The phantogram builder uses an OpenGL-like 3D perspective viewer to
take a perspective view of the stereo photograph in a -45° angle :
inverted from the original. Here are the values tuning effects :
r is the reversion angle. It is
by default set to 45°, so that the front squares of the grid become
smaller, and the back squares of the grid become larger. When r is decreased the reversion effect becomes smaller. When r is increased the reversion effect becomes larger.
xLeft is an horizontal displacement of the left view. It is set to zero by default. When xLeft > 0 the left view is tilt to the left. When xLeft < 0 the left view is tilt to the right.
xRight is an horizontal displacement of the right view. It is set to zero by default. When xRight > 0 the right view is tilt to the left. When xRight < 0 the right view is tilt to the right.
fov is the size of the field of view. It is set to 70° by default. When fov is increased, the views are zoomed out without distortion. When fov is decreased, the views are zoomed in without distortion.
y/x enables to tune the horizontal/vertical ratio to make a rectangle grid to become a square grid.
z is a distance to the view. Changing it is equivalent to combining a r changing and a fov changing together.