MisterDave wrote:I'm just wrestling with a bug that seems to make anything that moves behind the camera repeat in an endless corridor in front of the camera and trying to determine what is causing it.
It seems to be in the Parallax version of nehe lesson 10 (lesson 9 in the parallaxnehe naming convention) has that bug too. It runs fine on lxdream, but on the real hardware the problem happens. I'm just trying at the moment to determine if it is something wrong in that code (such as clipping being set wrong) or in KOS itself.
Yes BlueCrab beat me to it somehow ( my last post didnt submit? )
You will need to clip the vertices to the Near-Z Plane for use with the PVR, the results of not doing so, as you have experienced, can be quite bizarre indeed.
KGL includes a mechanism for Clipping on the Near-Z Plane, that might be sufficient for your project.
However, I noticed many deficiencies with the algorithm, so I decided to write my own implementation for my build of OpenGL.
My algorithm ( currently only written for clipping Triangle Strips, but will be easy to add support for triangles ) will break a Triangle Strip down into individual triangles, clipping them against the Near-Z plane, and output the results as an array of triangles.
It is not yet completed, as there are some cases where a strip will be clipped when it shouldn't, but I am not sure if the problem is in my clip code, or in the stage that clipping occurs in my pipeline( post transformation )
Any rate, here is my code, if you know of a better method I would like to hear about it.
Code: Select all
/*
** Gl-Clip.c (C) Josh PH3NOM Pearson 2013
** *Work In Progress
*/
#include <dc/pvr.h>
#define NONE 0x0000 /* Clip Codes */
#define FIRST 0x0001
#define SECOND 0x0010
#define THIRD 0x0100
#define ALL 0x0111
#define FIRST_TWO_OUT 0x0011
#define FIRST_AND_LAST_OUT 0x0101
#define LAST_TWO_OUT 0x0110
#define ALPHA 0xFF000000 /* Color Components using PVR's Packed 32bit int */
#define RED 0x00FF0000
#define GREEN 0x0000FF00
#define BLUE 0x000000FF
#define CLIP_ZNEAR 0.00001
#define CLIPZ 0xF
inline void VertexCopy( pvr_vertex_t *src, pvr_vertex_t *dst )
{
*dst = *src;
}
inline void VertexClipZNear( pvr_vertex_t *v1, pvr_vertex_t *v2 )
{
float MAG = (CLIP_ZNEAR - v1->z)/(v2->z-v1->z); /* Displacement Mag */
/* Linear interpolate the U/V data */
v1->u = ((v2->u-v1->u)*MAG)+v1->u;
v1->v = ((v2->v-v1->v)*MAG)+v1->v;
/* Linear interpolate the Vertex data */
v1->x = ((v2->x-v1->x)*MAG) + v1->x;
v1->y = ((v2->y-v1->y)*MAG) + v1->y;
v1->z = ((v2->z-v1->z)*MAG) + v1->z;
/* Extract Color Components, Apply Linear Interpolation, then Pack it up */
BYTE a1 = (v1->argb & ALPHA)>>24;
BYTE r1 = (v1->argb & RED)>>16;
BYTE g1 = (v1->argb & GREEN)>>8;
BYTE b1 = (v1->argb & BLUE)>>0;
BYTE a2 = (v2->argb & ALPHA)>>24;
BYTE r2 = (v2->argb & RED)>>16;
BYTE g2 = (v2->argb & GREEN)>>8;
BYTE b2 = (v2->argb & BLUE)>>0;
BYTE a = ((a2-a1)*MAG)+ a1;
BYTE r = ((r2-r1)*MAG)+ r1;
BYTE g = ((g2-g1)*MAG)+ g1;
BYTE b = ((b2-b1)*MAG)+ b1;
v1->argb = ( (a<<24) | (r<<16) | (g<<8) | (b<<0) );
}
/* Wowsers, what a run this has been. Initially forgot to modify vertex flags */
inline int ClipZNearTriStrip( pvr_vertex_t *in, pvr_vertex_t *out, unsigned int n )
{
unsigned short int clip = 0, vertices_inside = 0;
unsigned int v = 0, i;
for( i=0; i<(n-2); i++ )
{
/* Compute clip codes for each vertex of triangle */
( in[i+0].z < CLIP_ZNEAR ) ? clip |= FIRST : ++vertices_inside;
( in[i+1].z < CLIP_ZNEAR ) ? clip |= SECOND : ++vertices_inside;
( in[i+2].z < CLIP_ZNEAR ) ? clip |= THIRD : ++vertices_inside;
switch(vertices_inside)
{
case 0: /* All Vertices are outside of clip plane */
break;
case 3: /* All Vertices are inside of clip plane */
VertexCopy( &in[i+0], &out[v+0] );
VertexCopy( &in[i+1], &out[v+1] );
VertexCopy( &in[i+2], &out[v+2] );
out[v+0].flags = out[v+1].flags = PVR_CMD_VERTEX;
out[v+2].flags = PVR_CMD_VERTEX_EOL;
v+=3;
break;
case 1: /* 1 Vertex is inside of clip window, 2 are out */
VertexCopy( &in[i+0], &out[v+0] );
VertexCopy( &in[i+1], &out[v+1] );
VertexCopy( &in[i+2], &out[v+2] );
switch(clip)
{
case FIRST_TWO_OUT:
VertexClipZNear( &out[v], &out[v+2] );
VertexClipZNear( &out[v+1], &out[v+2] );
break;
case FIRST_AND_LAST_OUT:
VertexClipZNear( &out[v], &out[v+1] );
VertexClipZNear( &out[v+2], &out[v+1] );
break;
case LAST_TWO_OUT:
VertexClipZNear( &out[v+1], &out[v] );
VertexClipZNear( &out[v+2], &out[v] );
break;
}
out[v+0].flags = out[v+1].flags = PVR_CMD_VERTEX;
out[v+2].flags = PVR_CMD_VERTEX_EOL;
v+=3;
break;
case 2: /* 2 Vertices are inside of clip window, 1 is out */
switch(clip)
{
case FIRST:
VertexCopy( &in[i+0], &out[v+0] );
VertexCopy( &in[i+1], &out[v+1] );
VertexClipZNear( &out[v+0], &out[v+1] );
out[v+0].flags = out[v+1].flags = PVR_CMD_VERTEX;
v+=2;
VertexCopy( &in[i+0], &out[v+0] );
VertexCopy( &in[i+2], &out[v+1] );
VertexClipZNear( &out[v+0], &out[v+1] );
out[v+0].flags = PVR_CMD_VERTEX_EOL;
++v;
VertexCopy( &out[v-2], &out[v+0] );
VertexCopy( &out[v-1], &out[v+1] );
VertexCopy( &in[i+2] , &out[v+2] );
out[v+0].flags = out[v+1].flags = PVR_CMD_VERTEX;
out[v+2].flags = PVR_CMD_VERTEX_EOL;
v+=3;
break;
case SECOND:
VertexCopy( &in[i+0], &out[v+0] );
VertexCopy( &in[i+1], &out[v+1] );
VertexClipZNear( &out[v+1], &out[v+0] );
out[v+0].flags = out[v+1].flags = PVR_CMD_VERTEX;
v+=2;
VertexCopy( &in[i+2], &out[v+0] );
out[v+0].flags = PVR_CMD_VERTEX_EOL;
++v;
VertexCopy( &out[v-2], &out[v+0] );
VertexCopy( &out[v-1], &out[v+1] );
VertexCopy( &in[i+1] , &out[v+2] );
VertexClipZNear( &out[v+2], &out[v+1] );
out[v+0].flags = out[v+1].flags = PVR_CMD_VERTEX;
out[v+2].flags = PVR_CMD_VERTEX_EOL;
v+=3;
break;
case THIRD:
VertexCopy( &in[i+0], &out[v+0] );
VertexCopy( &in[i+1], &out[v+1] );
VertexCopy( &in[i+2], &out[v+1] );
VertexClipZNear( &out[v+2], &out[v+1] );
out[v+0].flags = out[v+1].flags = PVR_CMD_VERTEX;
out[v+2].flags = PVR_CMD_VERTEX_EOL;
v+=3;
VertexCopy( &out[v-3], &out[v+0] );
VertexCopy( &out[v-1], &out[v+1] );
out[v+0].flags = out[v+1].flags = PVR_CMD_VERTEX;
v+=2;
VertexCopy( &in[i+2], &out[v] );
VertexClipZNear( &out[v+0], &out[v-2] );
out[v+0].flags = PVR_CMD_VERTEX_EOL;
++v;
break;
}
break;
}
vertices_inside = clip = 0;
}
//printf("clip in/out: %i / %i\n", n, v);
return v;
}
