Dec 092017

A diagram and discussion on how to write programs employing texture mapping. | |||
---|---|---|---|

File Name | File Size | Zip Size | Zip Type |

IDEA.GIF | 8120 | 8120 | stored |

TEXTURE.TXT | 6239 | 2770 | deflated |

# Download File TEXTMAP.ZIP Here

## Contents of the TEXTURE.TXT file

From: [email protected] (Robert Schmidt)

Newsgroups: rec.games.programmer

Subject: Re: Texture mapping sources wanted

This is not a source file, but rather a short document containing what

little I know about texture mapping arbitrarily positioned and rotated

prallellograms in 3D space. It also contains a GIF file which probably

is needed to understand what is going on. I know one person who has

used the formulas presented to implement correct texture mapping

of 3D triangles.

Concerning Digital Image Warping: on the subjects of texture mapping,

it contains little more than what you can read below. It does provide

some faster approximations though, but they don't look to good in the

general case.

---------------------------------------------- use a chainsaw here

My idea of texture mapping

Mapping a 2D bitmap to a 3D parallellogram

by Robert Schmidt ([email protected]) of Ztiff Zox Softwear

This is an introductory text which might not be particularly useful, but it

might get you started, and might be the start of some good (I mean FAST!)

public domain source code for texture mapping. No source code is

included, but a 640x480 B&W GIF-sketch is appended uuencoded at the

end of this text.

See the accompanying IDEA.GIF when you get bewildered. I'll use lower

case letters for scalars, and capital letters for vectors in this text.

I drew the image before trying to incorporate underlining in this ASCII

text... 🙂 Thus:

In this text:

a is a scalar,

A is a vector (NOT a matrix).

In the drawing:

a is a scalar,

a-underlined is the vector.

Assume the following:

o The origo (0,0,0) of our world is positioned right in your eye.

(Ouch!) Choose left or right yourself. The x axis points right of

your head, the y axis points up, and the z axis points straight ahead

of you.

o A four sided parallellogram, is spanned out in 3D space by two vectors

U and V. The common base of the two vectors is given by the base vector

B.

o Your computer screen is positioned a fixed distance from your

eye/origo. The origo of the screen is at (0,0,zs). The xy-plane of

your screen is parallell to the world xy-plane.

o Stored somewhere else is a bitmap, which is to be mapped onto the

parallellogram, so that the base of the bitmap coincides with the base

(B) of the parallellogram, and the edges of the two fall together.

Now our goal is made up of two smaller goals:

1) Map the bitmap to the parallellogram.

2) Map the parallellogram to the screen.

I assume you are familiar with drawing a 3D polygon on screen, i.e.

performing a perspective transform of the coordinates and rasterizing

the edges. This process, i.e. goal 2, isn't really the issue here.

The idea is that for each point S= on the screen that is

contained in the polygon, we have to find the coordinates (u,v) along

the vectors (U,V). The corresponding point in space is given by

P = B + uU + vV

u and v will be in the interval [0,1] if P is within the polygon. This

is crucial.

Now if the 3D point P is to map to the screen pixel S, the vectors P and

S have to be parallell. Moreover, since they both are based in origo,

they lie on the same line in space, thus:

P = tS

for some constant t. Thus:

tS = B + uU + vV

Now we have 3 equations in 3 unknowns, t, u and v:

t sx = bx + u ux + v vx

t sy = by + u uy + v vy

t sz = bz + u uz + v vz

t is of no interest to us, so I'll just show the solutions for u and v:

d sx + e sy + f sz

u = ------------------

a sx + b sy + c sz

(1)

g sx + h sy + i sz

v = ------------------

a sx + b sy + c sz

where a,b,c,d,e,f,g,h,i are all constants which are calculated once each

time the bitmap is moved/redrawn:

a = uy vz - vy uz

b = vx uz - ux vz

c = ux vy - vx uy

d = vy bz - by vz

e = bx vz - vx bz

f = vx by - bx vy

g = by uz - uy bz

h = ux bz - bx uz

i = bx uy - ux by

The straightforward algorithm to draw the bitmap is as follows:

for (ys=0; ys<200; ys++)

for (xs=0; xs<320; xs++)

calculate u,v from (1)

if (u in [0,1) and v in [0,1)

putpixel (xs, ys, bitmap[u*xsize, v*ysize])

This will scan through each pixel on the screen, check wether the pixel

is mapped inside the bitmap, and plot the bitmap pixel to the screen

if it is.

Calculating (1) for each pixel is time consuming, but there are facts to

exploit for significant speed gains:

1) sz (the eye z-coordinate of the screen pixel) is constant, so all

products involving sz can be calculated outside all loops.

2) sy is constant on each raster line, so the products involving sy need

be calculated only once per line, i.e. outside the xs loop.

3) Scan convert the polygon (parallellogram) to screen coordinates, and

calculate (u,v) only for pixels inside the polygon. I'm not going to

say more about this, but it's the obvious way to go!

4) The denominators in (1) are equal for u and v, so it need only be

evaluated once for each pixel.

5) Try to incrementalize. Instead of calculating (d sx) when x

increases, just add d to the previous value, for example.

The problem is I still can't get away with less than 1 divsion and 2

multiplications per pixel, alternatively 2 divisions. There are ways to

approximate this, for example by subdividing the polygon and using first

or second degree Taylor polynomials, combined with the use of forward

differences. Third degree polynomials don't give much quality gain over

second degree polynomials, and take more initial calculations. These are

all approximations, and will give visible artifacts and errors.

Subdividing helps on this, but is expensive.

I'm looking for an incremental algorithm similar to the Bresenham

algorithms for drawing straight lines or ellipses. This would give an

exact perspective mapping and *not* an approximation. At the moment, I'm

literaly halfway there, it's just that the second half seems impossible

to figure out. I am able to get it right as long as each and every pixel

in the texture bitmap is used at least once. If one or more bitmap

pixels are to be skipped, my algorithm fails. Interested persons

mail me.

Mail any comments and ideas to [email protected]

Newsgroups: rec.games.programmer

Subject: Re: Texture mapping sources wanted

This is not a source file, but rather a short document containing what

little I know about texture mapping arbitrarily positioned and rotated

prallellograms in 3D space. It also contains a GIF file which probably

is needed to understand what is going on. I know one person who has

used the formulas presented to implement correct texture mapping

of 3D triangles.

Concerning Digital Image Warping: on the subjects of texture mapping,

it contains little more than what you can read below. It does provide

some faster approximations though, but they don't look to good in the

general case.

---------------------------------------------- use a chainsaw here

My idea of texture mapping

Mapping a 2D bitmap to a 3D parallellogram

by Robert Schmidt ([email protected]) of Ztiff Zox Softwear

This is an introductory text which might not be particularly useful, but it

might get you started, and might be the start of some good (I mean FAST!)

public domain source code for texture mapping. No source code is

included, but a 640x480 B&W GIF-sketch is appended uuencoded at the

end of this text.

See the accompanying IDEA.GIF when you get bewildered. I'll use lower

case letters for scalars, and capital letters for vectors in this text.

I drew the image before trying to incorporate underlining in this ASCII

text... 🙂 Thus:

In this text:

a is a scalar,

A is a vector (NOT a matrix).

In the drawing:

a is a scalar,

a-underlined is the vector.

Assume the following:

o The origo (0,0,0) of our world is positioned right in your eye.

(Ouch!) Choose left or right yourself. The x axis points right of

your head, the y axis points up, and the z axis points straight ahead

of you.

o A four sided parallellogram, is spanned out in 3D space by two vectors

U and V. The common base of the two vectors is given by the base vector

B.

o Your computer screen is positioned a fixed distance from your

eye/origo. The origo of the screen is at (0,0,zs). The xy-plane of

your screen is parallell to the world xy-plane.

o Stored somewhere else is a bitmap, which is to be mapped onto the

parallellogram, so that the base of the bitmap coincides with the base

(B) of the parallellogram, and the edges of the two fall together.

Now our goal is made up of two smaller goals:

1) Map the bitmap to the parallellogram.

2) Map the parallellogram to the screen.

I assume you are familiar with drawing a 3D polygon on screen, i.e.

performing a perspective transform of the coordinates and rasterizing

the edges. This process, i.e. goal 2, isn't really the issue here.

The idea is that for each point S=

contained in the polygon, we have to find the coordinates (u,v) along

the vectors (U,V). The corresponding point in space is given by

P = B + uU + vV

u and v will be in the interval [0,1] if P is within the polygon. This

is crucial.

Now if the 3D point P is to map to the screen pixel S, the vectors P and

S have to be parallell. Moreover, since they both are based in origo,

they lie on the same line in space, thus:

P = tS

for some constant t. Thus:

tS = B + uU + vV

Now we have 3 equations in 3 unknowns, t, u and v:

t sx = bx + u ux + v vx

t sy = by + u uy + v vy

t sz = bz + u uz + v vz

t is of no interest to us, so I'll just show the solutions for u and v:

d sx + e sy + f sz

u = ------------------

a sx + b sy + c sz

(1)

g sx + h sy + i sz

v = ------------------

a sx + b sy + c sz

where a,b,c,d,e,f,g,h,i are all constants which are calculated once each

time the bitmap is moved/redrawn:

a = uy vz - vy uz

b = vx uz - ux vz

c = ux vy - vx uy

d = vy bz - by vz

e = bx vz - vx bz

f = vx by - bx vy

g = by uz - uy bz

h = ux bz - bx uz

i = bx uy - ux by

The straightforward algorithm to draw the bitmap is as follows:

for (ys=0; ys<200; ys++)

for (xs=0; xs<320; xs++)

calculate u,v from (1)

if (u in [0,1) and v in [0,1)

putpixel (xs, ys, bitmap[u*xsize, v*ysize])

This will scan through each pixel on the screen, check wether the pixel

is mapped inside the bitmap, and plot the bitmap pixel to the screen

if it is.

Calculating (1) for each pixel is time consuming, but there are facts to

exploit for significant speed gains:

1) sz (the eye z-coordinate of the screen pixel) is constant, so all

products involving sz can be calculated outside all loops.

2) sy is constant on each raster line, so the products involving sy need

be calculated only once per line, i.e. outside the xs loop.

3) Scan convert the polygon (parallellogram) to screen coordinates, and

calculate (u,v) only for pixels inside the polygon. I'm not going to

say more about this, but it's the obvious way to go!

4) The denominators in (1) are equal for u and v, so it need only be

evaluated once for each pixel.

5) Try to incrementalize. Instead of calculating (d sx) when x

increases, just add d to the previous value, for example.

The problem is I still can't get away with less than 1 divsion and 2

multiplications per pixel, alternatively 2 divisions. There are ways to

approximate this, for example by subdividing the polygon and using first

or second degree Taylor polynomials, combined with the use of forward

differences. Third degree polynomials don't give much quality gain over

second degree polynomials, and take more initial calculations. These are

all approximations, and will give visible artifacts and errors.

Subdividing helps on this, but is expensive.

I'm looking for an incremental algorithm similar to the Bresenham

algorithms for drawing straight lines or ellipses. This would give an

exact perspective mapping and *not* an approximation. At the moment, I'm

literaly halfway there, it's just that the second half seems impossible

to figure out. I am able to get it right as long as each and every pixel

in the texture bitmap is used at least once. If one or more bitmap

pixels are to be skipped, my algorithm fails. Interested persons

mail me.

Mail any comments and ideas to [email protected]

December 9, 2017
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