View Full Version : Screen coordinates in the programs

SeskaPeel

02-23-2004, 04:56 AM

I'm trying to do some basic screen space hatching (same mapping as OGL automatic texcoo generation using GL_EYE_THING).

My implementation doesn't work well ...

The vertex program :

# ---------------- SCREEN COORDINATES ---------------- #

OUTPUT outScreenCoo = result.texcoord[6] ;

PARAM mvp[4] = { state.matrix.mvp } ;

PARAM Constants = {0.5, 1.0, 5.0, 2.0} ;

TEMP ScrCoo ;

DP4 ScrCoo.x, mvp[0], inPosition ;

DP4 ScrCoo.y, mvp[1], inPosition ;

MAD outScreenCoo, ScrCoo, Constants.x, Constants.x ;

and for the fragment program :

# -------------------- HATCH TEXTURE ------------------- #

ATTRIB inScreenCoo = fragment.texcoord[6] ;

TEMP R1 ;

TEX R1, inScreenCoo, texture[4], 2D ;

I get some output, but the mapping coordinates are definetely not correct ... where did I go wrong ?

Thanks,

SeskaPeel.

SeskaPeel

02-23-2004, 10:13 AM

Actually, I fixed some stuff (normalized the output), and I got some nearly correct values.

But the texture is still warped in the lower right and upper left corner ... any idea ?

VP :

ATTRIB inPosition = vertex.position ;

PARAM mvp[4] = { state.matrix.mvp } ;

OUTPUT outScreenCoo = result.texcoord[4] ;

PARAM Constants = {0.5, 1.0, 5.0, 2.0} ;

DP4 outScreenCoo.x, mvp[0], inPosition ;

DP4 outScreenCoo.y, mvp[1], inPosition ;

DP4 outScreenCoo.z, mvp[2], inPosition ;

DP4 outScreenCoo.w, mvp[3], inPosition ;

FP :

ATTRIB inScreenCoo = fragment.texcoord[4] ;

PARAM Constants = {0.5, 1.0, 5.0, 2.0} ;

TEMP R1 ;

DP3 R1.w, inScreenCoo, inScreenCoo ;

RSQ R1.w, R1.w ;

MUL R1, inScreenCoo, R1.w ;

MAD R1, R1, Constants.x, Constants.x ;

TEX R1, R1, texture[4], 2D ;

Thanks,

SeskaPeel.

zeckensack

02-23-2004, 10:22 AM

Try TXP in the fragment program. And don't normalize.

SeskaPeel

02-24-2004, 01:57 AM

It works fine, thanks.

I first tried to divide by w, but I substituted the RCP with a RSQ ...

SeskaPeel

02-24-2004, 02:45 AM

I tried to divide in the vertex program but I had some nasty warping effects due to interpolation.

here is the method I choosed

VP :

OUTPUT outScreenCoo = result.texcoord[4] ;

PARAM Constants = {0.5, 1.0, 0.0, 4.0} ;

TEMP ScrCoo ;

DP4 ScrCoo.x, mvp[0], inPosition ;

DP4 ScrCoo.y, mvp[1], inPosition ;

DP4 ScrCoo.w, mvp[3], inPosition ;

MAD outScreenCoo, ScrCoo, Constants.yyyw, ScrCoo.wwwz ;

FP :

ATTRIB inScreenCoo = fragment.texcoord[4] ;

TEMP R1 ;

TXP R1, inScreenCoo, texture[4], 2D ;

Thanks,

SeskaPeel.

zeckensack

02-24-2004, 08:03 AM

That's expected. The perspective divide is needed per fragment. Dividing per vertex and interpolating the result is quite different.

A lerp(A,B,0.5) B

s:=0 s:=1 s:=2

w:=1 w:=3 w:=5

___________________________________

s/w=0 0.2 s/w=0.4

0.2!=1/3!

I first tried to divide by w, but I substituted the RCP with a RSQ ...My bad, I didn't even see that. The real "mistake" was that you did the MAD after the normalization step.

Actually, normalizing the texcoord in the fragment program and then doing TEX is equivalent to TXP. I'd still expect TXP to generally perform better.

edited again because the first edit ate the first part of my post ...

[This message has been edited by zeckensack (edited 02-24-2004).]

[This message has been edited by zeckensack (edited 02-24-2004).]

SeskaPeel

02-24-2004, 09:37 AM

Explanations of my programs :

V(x,y,z,w) is the position after being mvp'ed

P(s,t) is the screen coordinate I'm trying to get, in the range [0, 1]

s = (x / w) * 0.5 + 0.5

s = (x + w) / (2 * w)

the vertex prog outputs to the frag prog

(x+q, y+q, 0, 2w), and the frag prog performs the divide in the TXP.

Values are big and close enough to give a nice approximation of the lerp(A/w1, B/w2, l) == lerp(A, B, l) / lerp (w1, w2, l)

It interpolates near to perfect and let the fragment program cool down.

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