# Artificial Textures: 2D and 3D textures the "easy" way

This article is an introduction to the method of texturing objects (usually 3D objects) that is most commonly used in computer graphics today. Although its principles are very simple, it can be used to create a wide range of textures such as marble, water and clouds. A good example is the Persistence of Vision (PoV) Raytrace package which uses these algorithms, or even some of the ground-breaking films from Hollywood. Remember the funny water-creature in "The Abyss" ? The water textures for it were creating with these very same techniques.

The ideas behind the methods involve simple maths, and on this disk you should find some example source code to demonstrate how it works. Unfortunately it's in Pascal (!!) which most of you won't have. However, rather than supply a version in BASIC or C, the Pascal shows better the underlying principles. Pascal is very easy to "read" because of the structures it uses, and certainly the finished code looks a good deal better than the equivalent code in any other language I've seen. Some notes on how to convert the code to the language of your choice are provided with the source code.

## Basic Principle

The basic thing we need is a mathematical function that looks random but varies smoothly from point to point, can repeat itself or go one indefinitely.

The original idea for artificial textures came from the creation of fractals and fractal landscapes, where a surface is divided up into triangles with random heights, then subdivided again to give increasing levels of detail (see the game "Midwinter" for a good example of these techniques.)

However, a technique was needed for 3D rendering which required a seemingly random value, which varied slightly in neighbouring points to give an impression of texture, but not too much texture. We will then use this "random" value to create textures later on. The creator of this technique called such a function the "Noise" function. In mathematical language we get:

`noise(x,y,z) --> [a real number]`

## The Noise Function

The way to create the Noise Function is to split up any 2D or 3D space into a grid and assign various values to a series of all points at a certain interval. Let's take 2D space as a starting point, because that is easier to represent on screen. The x-values are the horizontal scale, the y-values the vertical:-

`(0,0)+--+--+--+--+--+(5,0) `

: : : : : :

+--+--+--+--+--+

: : : : : :

+--+--+--+--+--+

: : : : : :

+--+--+--+--+--+

: : : : : :

+--+--+--+--+--+

: : : : : :

(0,5)+--+--+--+--+--+(5,5)

Now we assume that our texture will never be more than 5x5 units. If it is, the texture will "wrap round" and start again. Now for each point on this lattice we have created, we will create and store 3 random values: an x-gradient, a y-gradient and a "height" value. In fact this height value is only to help us to visualise this concept: it isn't really a height at all.

If we imagine a flat landscape, where at the corner of each square one value is a "height" we can see that for any value of x or y not lying on these vertices, by using a bit of maths we can create another "height" value from the values of the 4 corners. But how do we create the in-between values?

## 1. Linear Interpolation

One way is to simply draw a straight lines between the corners (so called "linear interpolation" - "interpolation" as in finding the point in-between, "linear" as in using straight lines.)

Let us look at point (0.5,0.5) from above our "landscape" and assume that the values are as follows:

`(0,0) (1,0) `

2-----4----- ---...

: : :

: X : : X marks the spot we want

: : :

6-----8----- ---...

(0,1) (1,1)

This is where the x- and y- gradient values mentioned above come in. First we interpolate the x values. Since we are taking x=0.5 we only need to find the halfway point between them: first from 2 to 4, giving 3; then from 6 to 8, giving 7. Then we find the halfway point in the y direction. Halfway from 3 to 7 gives 5. Hence at (0.5,0.5) the value is 5.

Simple enough, but if we apply this to textures, it gives a curious "straight line" appearance that isn't very satisfactory. What we need is a smoother way of creating in-between values...

## 2. Polynomial Interpolation

Now, don't panic... this is a lot easier than it sounds... in fact, I've done all the maths for you in this article.

If you read the above text again, you will notice that when I described each point on the lattice structure, I mentioned *three* values rather than the one we have used above. This is to describe a curve between lattice points rather than a straight line. Let's look at the interpolation along the x-lattices again. Imagine we are looking at the landscape again, but this time from the side. The "heights" now look like this:

` 4 `

? : ^

2 : : :

: : : :Height axis

(0,0) (0.5,0) (1,0) :

--+---------+---------+-----> X axis

To give a smoother interpolation, we give each point an x-gradient, that is how steep the "slope" at this point is. Let's say that at (0,0) this is 0 (ie flat) but at (1,0) this is 1 (ie at an angle of 45 degrees)

Now I said that I've done all the maths for you.

If v0 and v1 are the heights at 0 and 1 respectively, and g0 and g1 are the gradients then we need an equation which describes this curve. Not only this, but to get a genuinely smooth translation, the "gradient of the gradient" ie. how the gradient is changing from one point to the next, must be 0 at both points.

This assumed, then the equation to define this curve is:

` new_height = (-2*v1 + 2*v0 + g0 + g1 ) * x*x*x `

+ (-2*g0 - g1 - 3*v0 + 3*v1) * x*x

+ (g0) * x

+ (v0)

We also need to interpolate the gradients between these points:

` new_gradient = 3 * (-2*v1 + 2*v0 + g0 + g1 ) * x*x `

+ 2 * (-2*g0 - g1 - 3*v0 + 3*v1) * x

+ (g0)

(Who says Maggie never gives you anything? These equations took me hours. Proof available on request for a small fee)

Now all we need to do is interpolate the new heights and gradients at (0.5,0) and (0.5,1), then do this again to give the value at (0.5,0.5) Voila! the noise function is complete.

## The Turbulence Function

Alternatively, we can use the results of the Noise function in more complicated ways (what do you mean, "even" more?) You can make a very realistic marble effect by making what is called a Turbulence function. In pseudocode, the basic algorithm for this is something like:

`------------------------------------------------- `

function (x,y,z) : real

turbulence = 0 { set to zero to start }

scale = 1

while (scale > some_size) { some_size is how far we want to go }

turbulence = turbulence +

(abs (Noise (x/scale,y/scale,z/scale) * scale ))

scale = scale / 2

wend

return turbulence

end function

-------------------------------------------------

The function repeatedly "zooms" the Noise function, but the bigger the scale the less notice is taken of the result. Also if the value of "Noise" allows negative results, the use of "abs" here will cause discontinuities which give effects such as ridges or veins in the textures. Suitable code is provided in the example on disk.

## So how is this useful?

Firstly, you can use this technique to create some very pretty patterns. If we simply limit the height values to between 0 and say 15, then we use the result as a colour value to create a smooth texture - good for colour-cycling effects, too! Alternatively, many overlaid Noise functions give realistic impressions of lapping water (which you could even animate)

In addition, you can extend this method to cover 3 dimensions, as POV or NeoN-3D do. This is extremely hairy to code in something like STOS though, and the "height" analogy I used to explain the 2D method will fall down. However, converting it to 3D gives you the idea of the "solid" texture for objects made of wood, stone etc.

If you have 3D, then the Noise can make surface ripples for bump mapping or texture mapping, some of the sexiest techniques around in graphics at the moment (especially on an ST!)

Using the Turbulence function gives a "mottled" look suitable for marble, other water effects or even cloud formations, the choice is yours. More detailed examples (using a kind of bastardized C code) are available in the original paper by Perlin (see the bibliography)

## Conclusion

Hopefully you will have some idea of how the textures in the above packages work now, and with a bit of practice can create your own.

Problems with getting it to work can be sent to:

Steve Tattersall

6 Derwent Drive

Littleborough

Lancs OL15 0BT

England

s.j.tattersall@cms.salford.ac.uk

...and I'll try to fix them.

## Bibliography

- "An Image Synthesizer" (Perlin) "Computer Graphics" (Proceedings of SIGGraph Volume 19 Number 3, 1985), pages 287-296. Try the Periodicals section of your local library under "Computer Graphics." This was in my University library, so if Salford have it most other places will too?
- "Computer Graphics: Principles and Practice" (Foley, Van Dam, Feiner, Hughes) Addison-Wesley Publishing ISBN 0-201-12110-7. Good all-round reference; (dodgy) translations into other languages are also available. The German version is rubbish though: it even misses chapters out
- "Introduction to Computer Graphics" (Foley, Van Dam, Feiner, Hughes, Phillips) Addison-Wesley Publishing ISBN 0-201-60921-5. Not as good as (2) on most areas of graphics - but cheaper
- Any decent A-level book on maths to do the interpolation!

## Notes on reading Pascal source code

These are some notes to explain how Pascal source code is structured, and to give some tips on how to convert it to other languages (especially C or BASIC)

A ready-compiled test program should be available: please run this in ST low resolution!

## Pascal Structure

Due to the way Pascal is compiled the structure may seem strange to those used to languages such as BASIC. The basic structure is:

`PROGRAM name of program; `

TYPE data structure declarations

CONST constant declarations

VAR variable declarations

FUNCTION/PROCEDURE subprogram code

BEGIN

main program code

END.

This should be fairly familiar to anyone with experience of C. Other notes:

- All variables and data structures must be declared unlike GFA or STOS. Then all subroutines are declared and described, including what must go in and out from the routine (ie. its parameters) and finally there is the main core of the program itself. Program cores and functions are all contained within BEGIN and END commands.
- The number type "real" denotes any variable that does not necessarily hold an integer. "Integer" is the equivalent of a% in GFA or a normal variable in STOS.
- In this example "records" should used for each point on the lattice - but to aid conversion to BASIC I have used a 3-dimensional array of numbers. It could also be replaced by 3 data arrays eg. DIM xgrad(20,20), ygrad(20,20), val(20,20).
- Pascal functions may have their own local variables, and pass back one value to the main program. This is best converted by means of a DEF FN statement (especially in GFA - STOS will probably need a subroutine)
- The code is designed for HighSpeed Pascal or Turbo Pascal on the PC (yuk!) so I've commented the lines that initialise Pascal-only graphics routines. These should be replaced by your own routines in other languages.
- The semicolon is used to separate statements; just ignore them if they are missing (this depends on what Pascal deems a "statement" which is different from C or BASIC)
- Other functions: "trunc" equivalates to "int" in BASIC (takes the integer part) and "mod" finds the modulus.

Steve Tattersall

s.j.tattersall@cms.salford.ac.uk

## PERLIN2.PAS

`PROGRAM Perlin; `

{

Example code to demonstrate artificial texture creation

Steve Tattersall for (Maggie 22)

Program and routines to implement noise and turbulence functions

as described in Perlin (1985) See the article in this issue

for more help

This is purely an example: those wishing to develop a full texture

system should think about storing values in the lattice in the range

0 to 1 rather than as integer values, for example. Also the code is

completely unoptimised to better show the algorithm used.

email: s.j.tattersall@cms.salford.ac.uk

smail: 6 Derwent Drive, Littleborough, Lancs OL15 0BT England

}

USES Dos, Crt, Graph;

CONST

LatticeSize = 20; { Lattice size }

VAR

x,y, { FOR..NEXT type counters }

Driver,Mode : Integer; { used for graphics - ignore }

Lattice : ARRAY [0..LatticeSize,0..LatticeSize,1..3] OF integer;

{ ------------------ Subprograms ----------------------------------}

PROCEDURE Init_Lattice;

VAR

X, Y : integer;

BEGIN

FOR Y:= 0 TO LatticeSize-1 DO

FOR X:= 0 TO LatticeSize-1 DO

BEGIN

Lattice [X,Y,1] := Random (256) - 128;

Lattice [X,Y,2] := Random (256) - 128;

Lattice [X,Y,3] := Random (256) - 128;

END

END;

{ ------------------------------------------------------------ }

PROCEDURE Get_Lattice ( x,y : integer;

VAR value, xgrad, ygrad : real );

BEGIN

xgrad := Lattice [x MOD LatticeSize , y MOD LatticeSize, 1];

ygrad := Lattice [x MOD LatticeSize , y MOD LatticeSize, 2];

value := Lattice [x MOD LatticeSize , y MOD LatticeSize, 3];

END;

{ ------------------------------------------------------------ }

FUNCTION Interpolate_Cubic

( v0, v1, g0, g1, x : real ) : real;

BEGIN

Interpolate_Cubic := (-2*v1 + 2*v0 + g0 + g1 ) * x*x*x

+ (-2*g0 - g1 - 3*v0 + 3*v1) * x*x

+ (g0) * x

+ (v0)

END;

{ ------------------------------------------------------------ }

FUNCTION Interpolate_Square

( v0, v1, g0, g1, x : real ) : real;

BEGIN

Interpolate_Square := 3 * (-2*v1 + 2*v0 + g0 + g1 ) * x*x

+ 2 * (-2*g0 - g1 - 3*v0 + 3*v1) * x

+ (g0)

END;

{ ------------------------------------------------------------ }

{ This procedure interpolates both value and new gradient }

PROCEDURE Interpolate1

( VAR value1, value2, xgrad1, xgrad2,

ygrad1, ygrad2,

xfrac : real;

VAR newvalue, newygrad : real );

BEGIN

newvalue := Interpolate_Cubic ( value1, value2, xgrad1, xgrad2, xfrac);

newygrad := Interpolate_Cubic ( value1, value2, ygrad1, ygrad2, xfrac);

END;

{ ------------------------------------------------------------ }

{ Interpolates the final value only. }

PROCEDURE Interpolate2

( VAR value1, value2,

ygrad1, ygrad2,

yfrac : real;

VAR newvalue : real );

BEGIN

newvalue := Interpolate_Cubic ( value1, value2, ygrad1, ygrad2, yfrac);

END;

{ ------------------------------------------------------------ }

FUNCTION Noise2D ( x, y : real ) : real;

VAR

value1, value2, value3, value4,

xgrad1, xgrad2, xgrad3, xgrad4,

ygrad1, ygrad2, ygrad3, ygrad4 : real;

newvalue1, newvalue2,

newygrad1, newygrad2 : real;

finalvalue : real;

xint, yint : integer;

xfrac, yfrac : real;

{ Method:

- Take the 4 corners' xgrad,ygrad and values.

- Interpolate [x,y] to [x+1,y], giving new value and ygrad

- Interpolate [x,y+1] to [x+1,y+1], giving same

- Use two values and ygrads to calculate final value.

}

BEGIN

xint := trunc (x); yint := trunc (y);

xfrac := x - xint ; yfrac := y - yint;

Get_Lattice ( xint, yint, value1, xgrad1, ygrad1 );

Get_Lattice ( xint+1, yint, value2, xgrad2, ygrad2 );

Get_Lattice ( xint, yint+1, value3, xgrad3, ygrad3 );

Get_Lattice ( xint+1, yint+1, value4, xgrad4, ygrad4 );

Interpolate1 ( value1, value2, xgrad1, xgrad2,

ygrad1, ygrad2, xfrac, newvalue1, newygrad1 );

Interpolate1 ( value3, value4, xgrad3, xgrad4,

ygrad3, ygrad4, xfrac, newvalue2, newygrad2 );

Interpolate2 ( newvalue1, newvalue2, newygrad1,

newygrad2, yfrac, finalvalue );

Noise2D := finalvalue;

END;

{ ------------------------------------------------------------ }

FUNCTION Turb2D ( x, y : real ) : real;

VAR

T, size : real;

BEGIN

T := 0;

size := 1 ;

WHILE size > 0.01 DO

BEGIN

T := T + abs ( Noise2D(x/size,y/size) * size );

size := size * 0.5

END;

Turb2D := T;

END;

{ ------------------ End of subprograms ------------------------- }

{ This is now the main program code - subprograms are called from this }

BEGIN

{ Set up the lattice of points }

Init_Lattice; { initialize lattice values }

{ Put in graphics mode }

Driver := DETECT; { these lines init the graphics }

InitGraph(Driver,Mode,''); { add path if using TurboPascal }

{ Main plotting loop. To look at the turbulence function, change

"Noise2D" to "Turb2D" and recompile }

FOR Y := 0 TO 199 DO

IF NOT KeyPressed THEN

FOR X := 0 TO 319 DO

PutPixel ( X,Y, trunc (Noise2D(X*0.01,Y*0.01)) MOD 16 );

readln; { wait for a keypress }

CloseGraph; { shut down graphics, quit }

END.