From: is94drj_brunel_ac_uk (DR J)
Newsgroups: rec.arts.ascii,alt.ascii-art
Subject: Info-3D: Stereogram Guide
Date: 10 Oct 1994 11:45:53 -0500
Lines: 948
__________________________________________________________________________
____ ____ ___
/ __ \/ __ \ / ( )____
/ / / / /_/ / __ / /|/ ___/
/ /_/ / __ / / /_/ / (__ )
/_____/_/ |_| \____/ /____/
_____ _ __ ______ ____ _
/ ___/_ __(_)__/ /__ /_ __/__ / __ \_______ __ __(_)__ ___ _
/ (_ / // / / _ / -_) / / / _ \ / /_/ / __/ _ `/ |/|/ / / _ \/ _ `/
\___/\_._/_/\___/\__/ /_/ \___/ /_____/_/ \_._/|__.__/_/_//_/\_ /
____ ____ ___ _________________ ___ _ __ /___/
|_ // __ \ / _ \/ __/ ___/ _/ _/ / _ \(_)___/ /___ _________ ___
_/_ </ /_/ / / _ /\ \/ /___/ /_/ / / ___/ / __/ __/ // / __/ -_|_-<
/____/_____/ /_//_/___/\___/___/___/ /_/ /_/\__/\__/\_._/_/ \__/___/
__________________________________________________________________________
Copyright (c) 1994 DR J, Leaping Off The Screen Enterprises.
__________________________________________________________________________
Release v1.0
__________________________________________________________________________
Acknowledgements
Thanks to Bob Allison for writing the Ascii-Art FAQ and Rowan Crawford
for writing Ascii Graphical Techniques, both of which were invaluable when
setting out this guide.
Thanks to Stuart Inglis and Todd Hale for writing the Stereogram FAQ,
available from alt.3d or the ftp site katz.anu.edu.au in the directory
pub/stereograms.
Thanks to Mike Jittlov for the original idea.
Big shout to the following for proofreading this document and making
their own suggestions:
The Bionic Babe, Rowan Crawford, PH, Mike Hertz, Edwin Khachatourian,
James Middleton.
__________________________________________________________________________
Contents
1. Introduction ____ _____ ____ _____
2. The basics |___ \| __ \ |___ \| __ \
3. Spacing _) | | | | _) | | | |
4. Changing depth ASCII_ ASCII| |ASCII|_ ASCII | |ASCII
5. Multiple layers ___) | |__| | ___) | |__| |
6. To mask or mix |____/|_____/ |____/|_____/
7. Possible traps and pitfalls
8. Conclusion
A. Viewing 3D pictures
__________________________________________________________________________
1. Introduction
This guide doesn't offer any advice on drawing Ascii pictures, there
are plenty of artists in a better position to offer guidance on Ascii art
techniques. It is instead a way of listing the techniques peculiar to
drawing pictures that can be made to appear three dimensional (3D) and the
methods I use both to create and improve them.
I'm assuming that you already know how to view stereograms. For those
of you who don't, I've included an appendix with information about how to
view stereograms. If this is no help, try reading the stereogram FAQ (see
Acknowledgements) as it contains far more helpful information on the
various techniques that can be used.
This guide is about drawing stereograms that still look like pictures
when viewed normally (non-random or picture-type) rather than the random-
letter type that look like PGP key blocks. This is because several people
have written software that can automatically turn a simple text file into
a random-letter stereogram. They are also a right pain to draw by hand!
Parts of the guide concern the appearance of 3D pictures and this is
mostly personal opinion. I have however, tried to justify everything I say
on the topic.
. It helps a lot if you have good eyesight.
/ \ I prefer relaxing my eyes to look behind
/ \ stereograms (wide-eye) rather than crossing
___/_____\___ my eyes, as it hurts less and allows me to
| | stay sitting. If you view them cross-eyed,
| H R D Z A S | all my drawings will appear inside-out. If I
| x r u h i | mention an image appearing nearer, cross-eyed
| * * * * | viewers will see it being further away and
| '.'.' | vice-versa. Of course, if you spend all day
|_____________| drawing 3D pictures you end up pretty cross-
eyed no matter how you view them!
__________________________________________________________________________
2. The basics
'Let's start at the very beginning...'. Easier said than done. The main
principle of any stereogram is that the brain can be fooled by identical
images spaced horizontally. Instead of looking at one of the images, each
eye looks at a different image, but the brain can't tell the difference
between the two separate images and is convinced that there is only one.
To solve the paradox, the brain pretends that there really is only one
image, but at a different depth.
By using a non-proportional text font (where all the letters are of the
same width) we have two immediate advantages when trying to create our own
stereograms, the ability to create countless identical copies of an image
and control of the spacing between them.
First step is to get two images that look the same, like this -
@__@ @__@
|| ||
|| ||
|| ||
-~~- -~~-
When you look at the picture, the brain has two images, one from each
eye and it wants to overlap them to form a single image. The brain looks
for visual cues (any bits of the two images that are obviously from the
same object) to decide how to overlap them. Normally it would overlap them
so this part from the left eye sits ontop of this part from the right.
\__ ______/
| |
___|____________________ ___|____________________
L| | | | | |R
e| | | |i
f| @__@ @__@ | | @__@ @__@ |g
t| || || | | || || |h
| || || | | || || |t
e| || || | | || || |
y| -~~- -~~- | | -~~- -~~- |e
e| | | | | |y
|___|____________________| |_________________|______|e
| |
|_________________________ ____|
\ /
However, unfocussing matches this part with this part (Cross-eyed
viewing matches the inner two). This should result in you seeing three
columns, a solid one in the centre of your vision and two translucent ones
on either side. It's the one in the middle that we're interested in
because it appears to be closer than the rest of the screen.
To show the importance of having two identical images, look what
happens when one of the columns is altered -
__ __
@__@ @__@
|| ==
|| ==
|| ==
-~~- -~~-
The sections that are identical remain three dimensional, but where
the overlapping images are different, the effect is gone and the image is
a flat mess of characters.
If we wanted to have more than one column in three dimensions, we just
add another next to the first two -
@==@ @==@ @==@
|| || ||
|| || ||
|| || ||
=~~= =~~= =~~=
Now we have two columns in three dimensions. Notice we still have the
two translucent columns at either side. The 3D effect works on real
columns too, try it on your next holiday to Athens!
Some stereograms are simply two images that combine into a single 3D
image, like the two columns above. Alternatively we can lay down a whole
screen full of images as below -
___ ___ ___ ___ ___
.' _ '. .' _ '. .' _ '. .' _ '. .' _ '.
|R: '.' |R: '.' |R: '.' |R: '.' |R: '.'
|o| |o| |o| |o| |o|
|c| |c| |c| |c| |c|
|k| |k| |k| |k| |k|
|_| |_| |_| |_| |_|
If we now link the images together we can give the appearance of one
continuous layer that hovers infront of the screen -
--.____.--~~~~~~--.____.--~~~~~~--.____.--~~~~~~--.____.--~~~~~~--.____.--
____ ____ ____ ____
'. .'~ ~'. .'~ ~'. .'~ ~'. .'~ ~'. .'
\ / \ / \ / \ / \ /
Y Y Y Y Y Y Y Y Y Y
| | | | | | | | | |
| | | | | | | | | |
| | | | | | | | | |
|__| |__| |__| |__| |__|
The double images work best for simple pictures or where you want to
draw attention to one particular image. The multiple images work better if
you're trying to create a larger picture.
__________________________________________________________________________
3. Spacing
Next we must consider the spacing of the images. The 3D effect relies
heavily on geometry, so we have to make some assumptions about how the
images will be viewed.
The majority of people who see 3D Ascii artwork will be using an 80
column display, on a screen about 12 - 14 inches wide and sitting about 3
or 4 feet away from it. Ignoring any difficult maths, the effect of this
arrangement is that most people won't be able to overlap images that are
separated by more than 23 - 25 columns.
This means that the repeating part of the picture is limited to this
width (see if you can make the two vertical lines overlap) -
| |
_____\| |/_____
/| |\
| |
Even though the overall picture can be wider than this, it will have to
consist of segments that are this wide. Needless to say this makes it more
restrictive to draw 3D images than normal Ascii pictures as you have less
space in which to work.
Remember, the space is not the distance between the two images (A on
the diagram below), but the distance between the repeating parts of the
stereogram (distance B) -
____ ____
.' '. .' '.
'.____.' '.____.'
'.____.' '.____.'
'.____.' '.____.'
'.____.' '.____.'
'.____.' '.____.'
'.____.' '.____.'
| |<--- A -->|
| |
|<------ B ------>|
If you're new to viewing 3D art, you may find that images over 15 - 18
columns are too wide. This is because your eye muscles will need time to
strengthen and your brain will need time to learn to relax the eyes to
very wide angles before you can unfocus by a large amount. Persevere, it's
worth it and some people reckon it improves their eyesight!
__________________________________________________________________________
4. Changing depth
When the brain calculates how far away an object is, it measures the
angle between the two eyeballs. The closer the eyes are to being parallel,
the further away the object. As we have seen, getting the brain to overlap
two separate images gives the impression of there being one image at a
different depth. Therefore it follows that if we adjust the gap between
the repeated images, we can alter the depth at which the object appears.
If we take two objects again, but this time draw them with different
spacings -
. .
/|\ /|\
/ |:\ / |:\
/ |:/ / |:/
/___|/ /___|/
. .
/|\ /|\
/ |:\ / |:\
/ |:/ / |:/
/___|/ /___|/
. .
/|\ /|\
/ |:\ / |:\
/ |:/ / |:/
/___|/ /___|/
Start at the top two and get the single 3D image. Now if you look down
you should see that the next pyramid is closer to you. The bottom pyramid
is closer still.
To make the pictures symmetrical, I've decreased the spacing by two
characters each time. Changing the spacing by a single column gives you
more control over the depth of the image.
The general rule for viewers who relax their eyes (wide-eye) is that
bringing images closer together makes them appear nearer to the viewer,
spacing them out makes them appear deeper into the picture. Cross-eyed
viewers will see the effect in reverse.
We'll take the pyramids again, but this time copied across the screen
with a single space between each layer -
. . . . .
/|\ /|\ /|\ /|\ /|\
/ |'\ / |'\ / |'\ / |'\ / |'\
/ |'/ / |'/ / |'/ / |'/ / |'/
/___|/ /___|/ /___|/ /___|/ /___|/
. . . . .
/|\ /|\ /|\ /|\ /|\
/ |'\ / |'\ / |'\ / |'\ / |'\
/ |'/ / |'/ / |'/ / |'/ / |'/
/___|/ /___|/ /___|/ /___|/ /___|/
. . . . .
/|\ /|\ /|\ /|\ /|\
/ |'\ / |'\ / |'\ / |'\ / |'\
/ |'/ / |'/ / |'/ / |'/ / |'/
/___|/ /___|/ /___|/ /___|/ /___|/
The change in depth between each layer is smoother, but equally is less
pronounced.
I aligned the middle columns for symmetry, there's no actual need to do
this, sometimes it looks better if the images don't line up. In the above
example, the pyramids don't get out of step by more than a couple of
squares, but as we will see in the next section, it can become a problem
ensuring that different layers don't drift too far horizontally from each
other.
When it comes to deciding depths, it's worth remembering that most
viewers will expect the images at the bottom of the screen to appear
closer than those at the top.
__________________________________________________________________________
5. Multiple layers
Changing the gaps between columns produces different layers, but the
previous example is a bit cold and disjointed - the individual images are
flat and they don't interact with each other. A much better 3D effect is
gained when layers of objects at different depths are laid on top of each
other, or when one of the images contains more than one layer.
When discussing layers, I often use the term one layer deeper or one
layer nearer. If I say I've pushed a set of images one layer deeper into
the screen, it means I've added one extra column of spaces in between the
images. Bringing images one layer out of the screen involves removing a
column of spaces between the images. If the layer is continuous, then I've
either reduced it or widened it by one column.
Let's look at a simple example of the images containing multiple layers
first. Two squares (in case you weren't sure!) -
____________ ____________
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
|____________| |____________|
Completely flat and uninteresting, so lets add another square in the
middle -
____________ ____________
| ________ | | ________ |
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
| |________| | | |________| |
|____________| |____________|
Still flat, so lets try and make the inner square appear deeper in the
picture by making it further apart than the outer one -
____________ ____________
| ________ | | ________ |
|| | | | | | |
|| | | | | | |
|| | | | | | |
|| | | | | | |
||________| | | |________| |
|____________| |____________|
Now you get the feeling of looking into a pit, so let's add some more
squares -
____________ ____________
| ________ | | ________ |
|| ____ | | | | ____ | |
|| | | | | | | | || |
|| |[] | | | | | | [] || |
|| |____| | | | | |____|| |
||________| | | |________| |
|____________| |____________|
Now you have a reasonably convincing lift (elevator) shaft. Why the
stagger in placing the squares? Personal opinion. This is how the picture
looks if you only move the squares from one image -
____________ ____________
| ________ | | ________ |
|| ____ | | | | ____ | |
||| | | | | | | | | |
|||[] | | | | | | [] | | |
|||____| | | | | |____| | |
||________| | | |________| |
|____________| |____________|
Notice how the shaft now seems tilted to the left? (Did you just catch
yourself shifting your head to the right? Convincing isn't it?!!) If I am
drawing a double-image stereogram, I try to make changes to the layers on
both the left and right images, otherwise one of the images becomes very
distorted when compared to the other. It also makes for a more centred 3D
effect.
Now onto the hardest type of stereogram drawing, the multi-layer
continuous picture (sorry, I just love inventing names for things!).
First we need to decide on some layers. My favourite way of drawing a
multi-layer 3D picture is to start with the background layer and add the
foreground layers on top as I need them. We'll start with a simple two
layer picture to show the basic principles involved.
Here's the background, a line of street lights, nine lines high -
1 __ __ __ __
2 .===|__| .===|__| .===|__| .===|__|
3 || || || ||
4 || || || ||
5 || || || ||
6 || || || ||
7 [] [] [] []
8 [] [] [] []
9 [] [] [] []
Thin vertical columns work well in stereograms as they allow you to
fill the spaces in between with other layers.
Now here's the foreground, a four line convoy of jeeps -
A _ _ _ _ _ _ _ _
B __// |___. __// |___. __// |___. __// |___.
C |'_ '--' _ | |'_ '--' _ | |'_ '--' _ | |'_ '--' _ |
D `(_)----(_)'= `(_)----(_)'= `(_)----(_)'= `(_)----(_)'=
The jeeps are two layers above the streetlights for extra effect. The
numbers are to make identification easier later on.
First we put the two layers below each other to see how they line up
horizontally (it's worth doing this now because once we start to overlap
the layers it gets awkward to move them horizontally) -
__ __ __ __
.===|__| .===|__| .===|__| .===|__|
|| || || ||
|| || || ||
|| || || ||
|| || || ||
[] [] [] []
[] [] [] []
[] [] [] []
_ _ _ _ _ _ _ _
__// |___. __// |___. __// |___. __// |___.
|'_ '--' _ | |'_ '--' _ | |'_ '--' _ | |'_ '--' _ |
`(_)----(_)'= `(_)----(_)'= `(_)----(_)'= `(_)----(_)'=
I've moved the streetlights so we get to see one of them between the
jeeps.
As the jeeps are 4 lines high, we'll overlap the top 3 lines of the
jeeps with the bottom 3 of the lights. This is the method I use.
Copy the top line from the foreground layer and insert it above the
layer it will overlap on the background layer as shown (I've trimmed the
rest of the layers to make it clearer) -
6 || || || ||
A _ _ _ _ _ _ _ _
7 [] [] [] []
We want to merge the middle line with the bottom line (above). Copy the
bottom line and edit the copy, inserting the characters from the middle
line as you go, making sure the new line matches both the old bottom line
and the middle line -
6 || || || ||
A _ _ _ _ _ _ _ _
A+7 [] _ _ [] _ _ [] _ _ []_ _
7 [] [] [] []
Now delete the old bottom and middle lines -
6 || || || ||
A+7 [] _ _ [] _ _ [] _ _ []_ _
And copy these lines back into the original background -
__ __ __ __
.===|__| .===|__| .===|__| .===|__|
|| || || ||
|| || || ||
|| || || ||
6 || || || ||
A+7 [] _ _ [] _ _ [] _ _ []_ _
[] [] [] []
[] [] [] []
And the next line -
A+7 [] _ _ [] _ _ [] _ _ []_ _
B __// |___. __// |___. __// |___. __// |___.
8 [] [] [] []
Now we have a situation where the foreground characters are on top of
the background characters, so we simply delete any covered background
letters and replace them with the foreground ones -
A+7 [] _ _ [] _ _ [] _ _ []_ _
B __// |___. __// |___. __// |___. __// |___.
B+8 [] __// |___. [] __// |___. [__// |___. __// |___.
8 [] [] [] []
Putting the wanted lines back into the background -
__ __ __ __
.===|__| .===|__| .===|__| .===|__|
|| || || ||
|| || || ||
|| || || ||
|| || || ||
A+7 [] _ _ [] _ _ [] _ _ []_ _
B+8 [] __// |___. [] __// |___. [__// |___. __// |___.
[] [] [] []
And so on until -
__ __ __ __
.===|__| .===|__| .===|__| .===|__|
|| || || ||
|| || || ||
|| || || ||
|| || || ||
A+7 [] _ _ [] _ _ [] _ _ []_ _
B+8 [] __// |___. [] __// |___. [__// |___. __// |___.
C+9 [] |'_ '--' _ | []|'_ '--' _ | |'_ '--' _ | |'_ '--' _ |
`(_)----(_)'= `(_)----(_)'= `(_)----(_)'= `(_)----(_)'=
One multi-layer continuous stereogram! Phew!
Yes this technique is a bit laborious, but you'll find that you make
fewer mistakes when copying layers on top of each other. If you want more
layers, you simply overlap them as shown.
__________________________________________________________________________
6. To mask or mix
'The Phantom of The Opera is here...'. Well, almost the right type of
mask, but no star prize. Masking is a technique I sometimes use when I'm
working on complicated stereograms.
The 3D effect relies on the brain being able to overlap two images to
form a three dimensional one. Well sometimes, layers overlap in such a
way to leave only one image. Let's take a look at the bottom of the jeep
drawing again -
|| || || ||
[] _ _ [] _ _ [] _ _ []_ _
[] __// |___. [] __// |___. [__// |___. __// |___.
[] |'_ '--' _ | []|'_ '--' _ | |'_ '--' _ | |'_ '--' _ |
`(_)----(_)'= `(_)----(_)'= `(_)----(_)'= `(_)----(_)'=
|_____
\
Notice that the street light is obscured here. Looked at in 3D, this
part of the street light is now difficult to see properly as it has
nothing to overlap with.
The simplest way to cure this problem is to shift the jeeps one column
to the left to obscure the street light. However, this is difficult in an
overlapped drawing and it just causes new problems with other clashing
layers.
Instead, the next best thing to do is try to mask off the offending
characters. By this I mean draw an imaginary blank layer around the fore-
ground image - in this case, the jeep. This gives you -
|| || || ||
[] _ _ [] _ _ [ _ _ _ _
[] __// |___. [] __// |___. __// |___. __// |___.
[] |'_ '--' _ | []|'_ '--' _ | |'_ '--' _ | |'_ '--' _ |
`(_)----(_)'= `(_)----(_)'= `(_)----(_)'= `(_)----(_)'=
Now this is not the best example as I've deleted a lot of characters,
but it does illustrate the how the picture can be 'cleaned up' by trying
to remove characters that don't appear to fit properly. This is very much
personal judgement, particularly since once you start to draw more
complicated drawings with three or four complicated layers, you will get
character clashes that *cannot* be removed with masks, no matter how hard
you try!
Unfortunately, there is a drawback with masking - you can be left with
large holes in your stereogram that can spoil the picture. The other way
to deal with character clashes is to borrow (steal) an idea from Row and
use an alternative Ascii character that best approximates the two clashing
characters.
In the above example, the clash is fairly simple to mix. The two
Ascii characters that need to be mixed are the '[' of the streetlight and
the '_' of the jeep's bonnet. The obvious character to use to represent
both of these Ascii shapes is the '[' as it contains the streetlight shape
plus enough of a low horizontal line to fool the eye. The result of this
mix can be seen below -
|| || || ||
[] _ _ [] _ _ [] _ _ []_ _
[] __// |___. [] __// |___. []_// |___. _[// |___.
[] |'_ '--' _ | []|'_ '--' _ | |'_ '--' _ | |'_ '--' _ |
`(_)----(_)'= `(_)----(_)'= `(_)----(_)'= `(_)----(_)'=
In this drawing, the mixing effect gives a much better overall look. In
other cases, the masking effect is a better way to solve character clashes.
Of course, the decision on which effect appears better is very subjective,
so don't always expect to be able to make a quick decision when you see a
character clash.
Good examples of character clashes that aren't worth fixing can be
seen in one of my earlier drawings, shown below -
. . . .
: : : :
: . : . : . :
.' . . .' . . .' . . .' .
.' .' O.' . .' .O .' . .' O.' .' . .'O .'
.' .' .' .. .' .' .' .. .' .' .' .. .' .'
:: : .O .. :: : O.' .. :: :O .' .. :: O:
: \. ' : : \. .' : : \. .' : : \. .'
\, ;\\,, :O '. \, ;\\,, O: '. \, ;\\,, O : '. \, ;\\,,O'
\\::333:o . '' \\::333:o '. '' \\::333:o '. '' \\::333:o
, /:33333:< : , /:33333:< , : /:33333:<, :: : /:33333:<
' '///'' .:/ ' '///'' .:/ : ' '///''.:/ .' : ' '///'' ::
/ :: ,,///;, ,/ / ,,///;, ,/ .: / ,,///;, ,/ '. .: / ::
.' o:33333::// o:33333::// . : o:33333::// '. : .''
.' >:3333333:\\ >:3333333:\\ :: >:3333333:\\ :: .' .
.' ''\\\\\" '\ ''\\\\\" '\ : ''\\\\\" '\ :: .' :
:: '. ';\ ' '\ :: ';\ . '\ ' :: ';\ . '\ .' ' ::
':.. ' ,. '' : ':.. ' ,.:'' : ':.. ' ,.:'' : ':..
::: .'' :: ::: .'' :: ::: .'' :: :::
::: .:' . .' ::: .:' . .' ::: .:' . .' :::
::: ::: :::: ::: ::: :::: ::: ::: :::: ::: :
DR J DR J
Look at the seaweed above the foreground fish's tail, notice the clash
caused by the fish covering the seaweed? Well I could try and sort out a
way of curing the problem, but what for? The clash is almost unnoticeable
and has virtually no effect on the overall look of the picture.
As an aside, this picture is clearer when viewed as a stereogram
because the 3D separates the fish from the seaweed better than the brain
can when looking at the flat picture!
When drawing random-letter stereograms, many artists add two markers at
the top of the drawing to show at what point the images should overlap. I
decided to incorporate this idea in my drawings by adding my name in 3D.
The DR J's are always one layer above the main picture, so you should look
at them first to get an idea of the picture's depth. Once you've got them,
look at the main image and you should be able to see it as a stereogram
immediately.
Still here? You are keen, aren't you?!! ;-)
Well this concludes the sections on the basic drawing techniques, the
next section lists some of the common problems that arise when drawing
stereograms.
__________________________________________________________________________
7. Possible traps and pitfalls
This section is based on personal opinion more than any of the others.
Too many layers
Once you start drawing multi-layer stereograms, the temptation is to
keep adding more and more layers. Up to about five layers is fine, but go
beyond that and the difference in spacing between the characters in the
top layer and the bottom layer will be so great as to cause the layers to
'break up' while being viewed. This is when the characters on one layer
obviously no longer overlap while the viewer is concentrating on another
layer.
For example, look at the stereogram below -
(-O-) (-O-) (-O-)
|-O-| |-O-| |-O-|
(-O-) (-O-) (-O-)
|-O-| |-O-| |-O-|
(-O-) (-O-) (-O-)
|-O-| |-O-| |-O-|
(-O-) (-O-) (-O-)
If you lock onto the top layer, and then glance downwards, you will see
that each layer down becomes further and further disjointed. At first, the
effect is not too noticeable, but by the time you reach the bottom layer,
the images are so badly misaligned that the 3D effect is missing
completely.
Images too close together
As I mentioned earlier, there is a 'limit' to the distance between
repeating images in a stereogram before the pattern becomes too wide to be
seen. The reverse effect is also true, if the repeating patterns are too
close together, then the 3D effect becomes difficult to 'lock onto'. For
example, take the picture shown below -
___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
:o: :o: :o: :o: :o: :o: :o: :o: :o: :o: :o: :o:
:o: :o: :o: :o: :o: :o: :o: :o: :o: :o: :o: :o:
:o: :o: :o: :o: :o: :o: :o: :o: :o: :o: :o: :o:
| | | | | | | | | | | |
| | | | | | | | | | | |
When the brain tries to unfocus the eyes by only a small amount, to
view a closely spaced stereogram, it is still receiving visual information
about the correct distance to the screen. If the information is 'strong'
enough, then the eyes 'snap back' to the correct focusing point and the 3D
effect is lost. This is often what happens when you view stereograms for
the first time, the 3D illusion isn't strong enough to fool the brain so
it re-aligns the eyes making the correct images overlap.
Continous lines
When the brain overlaps images, it is looking for visual cues to use
as it aligns the two images. Normally, the images can only be overlapped
at certain points, such as the line shown below -
----------||----------||----------||----------||----------||----------
However, when drawing Ascii pictures, sometimes you may want to draw a
continuous line, for example for a bridge or a road. This can cause
problems when viewed in 3D.
For example, taking the line above, we can see that the brain will want
to overlap this part with this part, giving the line a specific depth -
\_____ \___
| |
----------||----------||----------||----------||----------||----------
Now take a continous line like so -
----------------------------------------------------------------------
The brain no longer has any visual cues on which to rely on, so when
it tries to align this character, it can align it with any of these ones -
\_ ______________________/
| | |
----------------------------------------------------------------------
The result of this is that the line does not stay at the depth that you
want and instead changes depth as the brain attempts to align the image.
For this reason, I always try to break up any continous lines with other
characters, to ensure that the line stays at the depth I want.
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8. Conclusion
Well, what is there to conclude? This guide started out as simply a way
of answering some of the general questions that get asked about Ascii
stereograms. All the techniques listed above are ones that I use for my
own artwork, but that doesn't mean that it's a complete list of the
available techniques.
I hope that this guide will be useful, not just to novices, but to
Ascii artists of all levels. I think that we are still a long long way
from exhausting the possibilities of Ascii art and the stereogram is
arguably the least developed of all the Ascii styles.
Maybe it's time to redress the balance.
Have fun and get drawing!
DR J
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A. Viewing 3D pictures
If you can't see stereograms then this is the key to the whole guide,
if you can't get beyond this part, nothing I've said in the previous
sections is going to make much sense!
Humans have two eyes, roughly Critical Design
level and separated by only a few Feature
centimetres, but it is this 'design
feature' that lets us see in three ->| |<- 6-8cm
dimensions.
|||||||
The key to stereoscopic or three / \
dimensional vision is the ability | (o) (o) |
to 'line up' the separate views of | . |
an object seen from each eye to form \ \_/ /
a single image. \_____/
Normally, when looking at something, the brain is presented with two
images, one from each eye. In order to make sense of the information it
is receiving, the brain alters the angle of the two eyes until the images
overlap. The brain then uses the muscles that surround the eyeballs to
alter their shape to create a sharp image. Using the eyeballs to provide
a sharp image is an independent action from lining up the eyes and does
not contribute to the 3D effect, just the clarity. This is known as
focusing. The brain then measures the angle between the eyes and using
simple trigonometry, calculates the distance to the object in focus. The
point at which the two images overlap is called the focal point and the
distance is referred to as the focal distance.
The two pictures below show how the eyes normally see an image on a
monitor. The right-hand '3-D' seen by the right eye overlaps with the
right-hand '3-D' seen by the left eye (same for the left-hand one) and
the brain locks the eyes as the image is focused. The diagram on the left
shows the angle between the eyes whilst focusing on a screen image. The
diagram on the right shows what the brain sees and what distance it
perceives it to be.
screen ________3-D___3-D________ ________3-D___3-D________
.
. .
. .
. .
eye level <O> <O> <O> <O>
The problem for the brain is that it can only focus by comparing the
two images that it sees. It needs two images of an object to focus upon it
properly (making it difficult to focus on a point in mid air). Normally,
the brain is only presented with two images of each object, so is able to
overlap them easily. It is when the brain is unable to decide at what
point the images should overlap that it can be fooled.
As stated earlier, aligning the two eyes onto an object and obtaining a
sharp image are separate functions for the brain. This means that the eyes
can view an object clearly without locking onto it.
If we use the same two images as above, the brain will normally focus
correctly on the screen. However with a little training, it is possible to
let the eyes 'drift' outwards, so that they are effectively focusing on a
point *behind* the screen.
The best way to learn how to 'unfocus' is to stand at arms' length in
front of a wall, keep your elbows against your chest and hold both hands
in front of your face, with both index fingers extended vertically.
If you focus on your fingers, _ _
you should simply see them both |_| |_|
as shown on the right. The wall |L| |R|
behind will be blurred and out | | | |
of focus. Solid Solid
Now focus on the wall. You will _ _ _ _
probably see four translucent |_| |_| |_| |_|
(semi-transparent) fingers (don't |L| |L| |R| |R|
worry, they're alright!). This is | | | | | | | |
because the brain is now aligning
the images of the wall instead of Translucent Translucent
your fingers.
Next, keep focusing on the wall, _ _ _
but step backwards or forwards |_| |_| |_|
until the two centre fingers meet |L| |L| |R|
and overlap, forming a solid finger | | |R| | |
in the middle. Even though you are
still looking at the wall, it should ^ ^ ^
be possible to see the 'combined' | Solid |
finger clearly. '-Translucent-'
It is this skill, being able to look behind an object but still keeping
it in focus, that allows the brain to be fooled into seeing 3D pictures.
Returning to our screen with our new-found (and practiced!) skill, we
allow the eyes to focus behind the screen as shown below. Initially, the
brain sees two different images. However, as the eyes focus further back,
the right-eye's view of the left-hand '3-D' and the left-eye's view of the
right-hand '3-D' overlap (Yes, go back and re-read that again!). At this
point the brain becomes confused. It sees a solid combined image at the
centre of its vision (the only bit it is interested in) despite the rest
of the image being jumbled. If the viewer holds his or her eyes in that
position, the brain simply resolves the paradox by deciding that it has
locked on correctly to the object in front.
virtual screen ________3-D___3-D________
.
. . 3D image-> 3-D
real screen ________3-D___3-D________
. .
. .
. .
. .
eyes <O> <O> <O> <O>
Now convinced that it has the correct lock-on angle, the brain will
focus the eyeballs for a clear image and then recalculate the distance to
the object, but with an incorrect angle. Because the eyes are at a wider
angle, *everything* appears further back (although it might not seem that
way) but more importantly, the combined image appears in front of all the
other screen images - the brain appears to view everything unfocused as
being even further back.
It is worth noting again that crossing your eyes achieves a 3D effect
in reverse. This is not advised because the eyes have difficulty in
focusing on the two images and if the wind changes, you'll stay that way!
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