# saush

## Create 3D anaglyph images with 3 lines of Ruby code

Posted in Ruby by sausheong on July 19, 2011

3D has always fascinated me. When I was young my brother and I had a ViewMaster and a Pan-Pet Panorama Stereo Viewer, both of which totally bowled us over when we first saw it. My brother as usual totally took it apart and fixed it up again, multiple times while I simply spent hours goggling at the thing. I have no idea where they are now but thinking back they were my first recollection of understanding stereoscopy is.

ViewMaster

Pan-Pet Panorama Stereo Viewer

# A bit of history

It’s probably surprising to most people (at least it was to me) that the modern techniques of 3D imaging and stereoscopy dated way back before even photography. In fact the first few stereoscopic images were drawings. The picture below shows one of the earliest stereoscopic drawings by Jacopo Chimenti, a painter from Florence, Italy.

Jacopo Chimenti's first stereoscopic image

In 1838, Charles Wheatstone, a British inventor, published a paper that provided the scientific basis for stereography. He showed that the brain unifies the slightly different two-dimensional images from each eye into a single object of three dimensions. Wheatstone’s early stereographs were also drawings rather than photographs.

Wheatstone's stereoscope

Photographic stereographs were first produced in 1849 by the English physicist David Brewster who improved the stereoscope and in 1849 the first true stereo camera with two lenses.

# 3D/stereographic imaging techniques

The principles of stereoscopy are quite simple. We see things in 3 dimensions (i.e. being able to see 3-dimensional depth) because each of our 2 eyes actually see a slightly different image. This is because our eyes positioned are apart from each other which generates what is called binocular disparity. Recreating this effect with a 2-dimensional image then allows us to ‘see’ the image in 3D.

There are a number of ways to do this but generally the principle revolves around creating a set of 2 images, one for each eye and ‘forcing’ the left eye to view the left image and the right eye to view the right image.

## Freeviewing

This method places the left image on the right side and the right image on the left side. To view the image in stereo, force your eyes to go cross-eyed, which will produce 3 images. Then slowly ease the eyes to view the middle image in 3D. This is not as silly as it sounds, and actually works though it can be a strain on the eyes (obviously).

Stereogram with cross-eyed method

## Wiggle method

Stereogram with wiggle method

The wiggle method surprises a lot of people (including me when I first read about it) but it can sometimes be pretty effective. Basically you use the 2 images and create a GIF that alternates between each other.

## Viewers

This method uses various kinds of viewers, from the 18th century Wheatstone stereoscope to the popular Holmes American stereoscope and the transparency viewers like the ViewMaster and the Pan-Pet that I grew up with. It also includes high tech head-mounted displays.

Holmes' American Stereoscope (reproduction)

## Parallax barrier and lenticular printing

These 2 methods are similar though parallax barrier is pretty high-tech while lenticular prints is as low-tech as it can be. Parallax barrier essentially places a barrier in front of an image source, usually a LCD display, with a series of precision slits, allowing each eye to see a different set of pixels. This is famously used in the Nintendo 3DS.

Nintendo 3DS

Lenticular printing uses a similar technique but with lenticular lenses. Lenticular prints are popular as novelty items and you’d probably encountered them in many places without knowing what it was called.

These 2 methods are often also classified as ‘autostereoscopy’ or glasses-free 3D.

Difference between parallax barrier and lenticular printing

Lenticular print of a promotion item

## 3D glasses

This is probably the method you’re most likely to encounter nowadays in movies and in 3D TVs. I classify both passive and active glasses in this category though the actual technologies can be vastly different such as alternating different frames with special projectors and using polarized light.

Which brings us to the type of 3D image we’ll be trying out today — anaglyphs.

The idea of anaglyphs is simple. We start with the 2 left and right images again. This time they are superimposed on each other, but the left would be corrected to show only red color while the right would be corrected to show cyan color. Actually we can use other colors besides red and cyan but these 2 colors are the most popular (and patent-free).

The image is then viewed with a pair of glasses that filter soff red on the left lens and cyan on the right lens. The results is that the left eye would only see the left image and the right eye the right image, therefore generating the perception of depth.

Red-cyan anaglyph glasses

The main problem with this technique (besides the necessity of wearing glasses) is that the colors are a bit wonky. Also if some color from the left image gets into the right eye (and vice versa) a faintly colored “ghost” will be seen. And if the filter from each lens filters off different amount of light resulting in luminance imbalance, it can easily cause headaches (happened to me lots of times during the experiments I did below).

However there are plenty of advantages of anaglyphs. Firstly there isn’t a need for fancy high-tech equipment. Anaglyph red-cyan glasses can be easily created at home or bought cheaply and as you will see below, creating anaglyphs is child’s play.

# Creating anaglyphs with Ruby

Creating anaglyphs is ridiculously easy with RMagick. This is the whole script I used.

```#!/usr/bin/env ruby

require ‘rubygems’
require ‘rmagick’
include Magick

left = ImageList.new(ARGV[0]).gamma_correct(1,0,0)
right = ImageList.new(ARGV[1]).gamma_correct(0,1,1)
anaglyph = left.composite right, CenterGravity, ScreenCompositeOp

anaglyph.write(‘anagylph.jpg’)
```

As you can see, the real work is done in only 3 lines of code. Firstly I create an ImageList object (assuming the first parameter is the file name of the first image). Then I use #gamma_correct and filter off the greens and blues of the left image while keeping the reds. The for the right image, I do the same thing, except this time I filter off the reds while keeping the greens and blues. Finally I use #composite and blend the 2 images together using the screen blending mode (which lightens the image after blending). I used CenterGravity to place the right image at the center of the left image here but it really doesn’t matter since both images are supposed to be the same size anyway. And what remains is just to write the anaglyph back into a file.

Of course, all of these means nothing if we can’t capture the left and right images. For this there are the stereo cameras, ranging from the amazing to the weird and the totally slap-together.

3D World 120 Tr-Lens: Stereoscopic Three Lenses 3D Camera

Fujifilm FinePix REAL 3D W3

2 instant cameras as conjoined stereo camera

Alternatively you can do the same thing with a single camera, although I wouldn’t recommend it for anything else except still shots. To do this, some recommend to use what is known as the ‘cha-cha’ technique. This requires the photographer to snap a picture then shifting weight slightly to the left or right therefore moving a few centimeters to take a reasonably good second image.

Me? I didn’t want to buy a fancy 3D camera and wasn’t inclined do the cha-cha so I applied a bit of MacGyverism on my primary camera.

MacGyver'ed dual iPhones with rubber-bands

It’s not perfect but it does take a reasonably good picture.

Left image of Kai Wen

Right image of Kai Wen

Anaglyphic image of Kai Wen

### 8 Responses

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5. Stereogranimator | 106 drop in said, on May 21, 2012 at 2:04 am

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6. yushuohuanxiu said, on June 19, 2012 at 12:27 am

It sometimes confused me. Why do we use red-cyan, not red-blue, red-green or green-blue?

7. [...] Very nice summary of 3D still image concepts: Create 3D anaglyph images with 3 lines of Ruby code « saush. [...]

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