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- Merged in recent ZDBSP fixes:

Browse source 
- Added code to explicitly handle outputting overlapping segs when
   building GL nodes with ZDBSP, removing the check that discarded
   them early on.
 - AddIntersection() should convert to doubles before subtracting the vertex
   from the node, not after, to avoid integer overflow. (See cah.wad, MAP12
   and MAP13.) A simpler dot product will also suffice for distance calculation.
 - Splitters that come too close to a vertex should be avoided. (See cata.wad.)
 - Red-Black Tree implementation was broken and colored every node red.
 - Moved most of the code for outputting degenerate GL subsectors into another
   function.


SVN r160 (trunk)
This commit is contained in:
Randy Heit 2006-06-01 01:43:16 +00:00
parent fd1a239c66
commit 4325fb8993
7 changed files with 213 additions and 291 deletions
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17
docs/rh-log.txt
View file

@ -1,3 +1,20 @@
May 31, 2006
- Merged in recent ZDBSP fixes:
- Added code to explicitly handle outputting overlapping segs when
building GL nodes with ZDBSP, removing the check that discarded
them early on.
- AddIntersection() should convert to doubles before subtracting the vertex
from the node, not after, to avoid integer overflow. (See cah.wad, MAP12
and MAP13.) A simpler dot product will also suffice for distance calculation.
- Splitters that come too close to a vertex should be avoided. (See cata.wad.)
- Red-Black Tree implementation was broken and colored every node red.
- Moved most of the code for outputting degenerate GL subsectors into another
function.
- Removed forgotten debugging file dump from WriteSSectors2().
- Enabled reference optimization and COMDAT folding in the linker for a slightly
smaller executable.
May 31, 2006 (Changes by Graf Zahl)
- Fixed: Ammo items dropped by monsters that have a default amount of 1 didn't
contain any ammo at all.

165
src/nodebuild.cpp
View file

@ -88,6 +88,7 @@ void FNodeBuilder::BuildTree ()
C_InitTicker ("Building BSP", FRACUNIT);
HackSeg = DWORD_MAX;
HackMate = DWORD_MAX;
CreateNode (0, bbox);
CreateSubsectorsForReal ();
C_InitTicker (NULL, 0);
@ -320,7 +321,11 @@ bool FNodeBuilder::CheckSubsector (DWORD set, node_t &node, DWORD &splitseg, int
} while (seg != DWORD_MAX);
if (seg == DWORD_MAX)
{ // It's a valid subsector
{ // It's a valid non-GL subsector, and probably a valid GL subsector too.
if (GLNodes)
{
return CheckSubsectorOverlappingSegs (set, node, splitseg);
}
return false;
}
@ -331,20 +336,60 @@ bool FNodeBuilder::CheckSubsector (DWORD set, node_t &node, DWORD &splitseg, int
// from multiple sectors, and it seems ZenNode does something
// similar. It is the only technique I could find that makes the
// "transparent water" in nb_bmtrk.wad work properly.
//
// The seg is marked to indicate that it should be forced to the
// back of the splitter. Because these segs already form a convex
// set, all the other segs will be in front of the splitter. Since
// the splitter is formed from this seg, the back of the splitter
// will have a one-dimensional subsector. SplitSegs() will add two
// new minisegs to close it: one seg replaces this one on the front
// of the splitter, and the other is its partner on the back.
//
// Old code that will actually create valid two-dimensional sectors
// is included below for reference but is not likely to be used again.
return ShoveSegBehind (set, node, seg, DWORD_MAX);
}
// When creating GL nodes, we need to check for segs with the same start and
// end vertices and split them into two subsectors.
bool FNodeBuilder::CheckSubsectorOverlappingSegs (DWORD set, node_t &node, DWORD &splitseg)
{
int v1, v2;
DWORD seg1, seg2;
for (seg1 = set; seg1 != DWORD_MAX; seg1 = Segs[seg1].next)
{
if (Segs[seg1].linedef == -1)
{ // Do not check minisegs.
continue;
}
v1 = Segs[seg1].v1;
v2 = Segs[seg1].v2;
for (seg2 = Segs[seg1].next; seg2 != DWORD_MAX; seg2 = Segs[seg2].next)
{
if (Segs[seg2].v1 == v1 && Segs[seg2].v2 == v2)
{
if (Segs[seg2].linedef == -1)
{ // Do not put minisegs into a new subsector.
swap (seg1, seg2);
}
D(Printf("Need to synthesize a splitter for set %d on seg %d (ov)\n", set, seg2));
splitseg = DWORD_MAX;
return ShoveSegBehind (set, node, seg2, seg1);
}
}
}
// It really is a good subsector.
return false;
}
// The seg is marked to indicate that it should be forced to the
// back of the splitter. Because these segs already form a convex
// set, all the other segs will be in front of the splitter. Since
// the splitter is formed from this seg, the back of the splitter
// will have a one-dimensional subsector. SplitSegs() will add one
// or two new minisegs to close it: If mate is DWORD_MAX, then a
// new seg is created to replace this one on the front of the
// splitter. Otherwise, mate takes its place. In either case, the
// seg in front of the splitter is partnered with a new miniseg on
// the back so that the back will have two segs.
bool FNodeBuilder::ShoveSegBehind (DWORD set, node_t &node, DWORD seg, DWORD mate)
{
SetNodeFromSeg (node, &Segs[seg]);
HackSeg = seg;
HackMate = mate;
if (!Segs[seg].planefront)
{
node.x += node.dx;
@ -353,79 +398,6 @@ bool FNodeBuilder::CheckSubsector (DWORD set, node_t &node, DWORD &splitseg, int
node.dy = -node.dy;
}
return Heuristic (node, set, false) != 0;
#if 0
// If there are only two segs in the set, and they form two sides
// of a triangle, the splitter should pass through their shared
// point and the (imaginary) third side of the triangle
if (setsize == 2)
{
FPrivVert *v1, *v2, *v3;
if (Vertices[Segs[set].v2] == Vertices[Segs[seg].v1])
{
v1 = &Vertices[Segs[set].v1];
v2 = &Vertices[Segs[seg].v2];
v3 = &Vertices[Segs[set].v2];
}
else if (Vertices[Segs[set].v1] == Vertices[Segs[seg].v2])
{
v1 = &Vertices[Segs[seg].v1];
v2 = &Vertices[Segs[set].v2];
v3 = &Vertices[Segs[seg].v2];
}
else
{
v1 = v2 = v3 = NULL;
}
if (v1 != NULL)
{
node.x = v3->x;
node.y = v3->y;
node.dx = v1->x + (v2->x-v1->x)/2 - node.x;
node.dy = v1->y + (v2->y-v1->y)/2 - node.y;
return Heuristic (node, set, false) != 0;
}
}
bool nosplit = true;
int firsthit = seg;
do
{
seg = firsthit;
do
{
if (Segs[seg].linedef != -1 &&
Segs[seg].frontsector != sec &&
Segs[seg].frontsector == Segs[seg].backsector)
{
node.x = Vertices[Segs[set].v1].x;
node.y = Vertices[Segs[set].v1].y;
node.dx = Vertices[Segs[seg].v2].x - node.x;
node.dy = Vertices[Segs[seg].v2].y - node.y;
if (Heuristic (node, set, nosplit) != 0)
{
return true;
}
node.dx = Vertices[Segs[seg].v1].x - node.x;
node.dy = Vertices[Segs[seg].v1].y - node.y;
if (Heuristic (node, set, nosplit) != 0)
{
return true;
}
}
seg = Segs[seg].next;
} while (seg != DWORD_MAX);
} while ((nosplit ^= 1) == 0);
// Give up.
return false;
#endif
}
// Splitters are chosen to coincide with segs in the given set. To reduce the
@ -525,6 +497,7 @@ int FNodeBuilder::Heuristic (node_t &node, DWORD set, bool honorNoSplit)
int side;
bool splitter = false;
unsigned int max, m2, p, q;
double frac;
Touched.Clear ();
Colinear.Clear ();
@ -616,6 +589,13 @@ int FNodeBuilder::Heuristic (node_t &node, DWORD set, bool honorNoSplit)
}
}
// Splitters that are too close to a vertex are bad.
frac = InterceptVector (node, *test);
if (frac < 0.001 || frac > 0.999)
{
score -= int(1 / frac);
}
counts[0]++;
counts[1]++;
if (test->linedef != -1)
@ -900,15 +880,24 @@ void FNodeBuilder::SplitSegs (DWORD set, node_t &node, DWORD splitseg, DWORD &ou
DWORD newback, newfront;
newback = AddMiniseg (seg->v2, seg->v1, DWORD_MAX, set, splitseg);
newfront = AddMiniseg (Segs[set].v1, Segs[set].v2, newback, set, splitseg);
if (HackMate == DWORD_MAX)
{
newfront = AddMiniseg (Segs[set].v1, Segs[set].v2, newback, set, splitseg);
Segs[newfront].next = outset0;
outset0 = newfront;
}
else
{
newfront = HackMate;
Segs[newfront].partner = newback;
Segs[newback].partner = newfront;
}
Segs[newback].frontsector = Segs[newback].backsector =
Segs[newfront].frontsector = Segs[newfront].backsector =
Segs[set].frontsector;
Segs[newback].next = outset1;
outset1 = newback;
Segs[newfront].next = outset0;
outset0 = newfront;
}
set = next;
}

7
src/nodebuild.h
View file

@ -133,6 +133,7 @@ private:
TArray<FSplitSharer> SplitSharers; // Segs collinear with the current splitter
DWORD HackSeg; // Seg to force to back of splitter
DWORD HackMate; // Seg to use in front of hack seg
FLevel &Level;
bool GLNodes; // Add minisegs to make GL nodes?
@ -143,7 +144,7 @@ private:
int SelectVertexExact (FPrivVert &vertex);
void BuildTree ();
void MakeSegsFromSides ();
FPrivSeg *CheckSegForDuplicate (const FPrivSeg *check);
int CreateSeg (int linenum, int sidenum);
void GroupSegPlanes ();
void FindPolyContainers (TArray<FPolyStart> &spots, TArray<FPolyStart> &anchors);
bool GetPolyExtents (int polynum, fixed_t bbox[4]);
@ -153,7 +154,8 @@ private:
int CreateSubsector (DWORD set, fixed_t bbox[4]);
void CreateSubsectorsForReal ();
bool CheckSubsector (DWORD set, node_t &node, DWORD &splitseg, int setsize);
int SelectSplitter (DWORD set, node_t &node, DWORD &splitseg, int step, bool nosplit);
bool CheckSubsectorOverlappingSegs (DWORD set, node_t &node, DWORD &splitseg);
bool ShoveSegBehind (DWORD set, node_t &node, DWORD seg, DWORD mate); int SelectSplitter (DWORD set, node_t &node, DWORD &splitseg, int step, bool nosplit);
void SplitSegs (DWORD set, node_t &node, DWORD splitseg, DWORD &outset0, DWORD &outset1);
DWORD SplitSeg (DWORD segnum, int splitvert, int v1InFront);
int Heuristic (node_t &node, DWORD set, bool honorNoSplit);
@ -173,6 +175,7 @@ private:
int CloseSubsector (TArray<seg_t> &segs, int subsector, vertex_t *outVerts);
DWORD PushGLSeg (TArray<seg_t> &segs, const FPrivSeg *seg, vertex_t *outVerts);
void PushConnectingGLSeg (int subsector, TArray<seg_t> &segs, vertex_t *v1, vertex_t *v2);
int OutputDegenerateSubsector (TArray<seg_t> &segs, int subsector, bool bForward, double lastdot, FPrivSeg *&prev, vertex_t *outVerts);
static int STACK_ARGS SortSegs (const void *a, const void *b);

12
src/nodebuild_events.cpp
View file

@ -37,12 +37,15 @@
**
*/
#include <string.h>
#include "doomtype.h"
#include "nodebuild.h"
FEventTree::FEventTree ()
: Root (&Nil), Spare (NULL)
{
memset (&Nil, 0, sizeof(Nil));
Nil.Color = FEvent::BLACK;
}
FEventTree::~FEventTree ()
@ -85,7 +88,7 @@ void FEventTree::LeftRotate (FEvent *x)
y->Left->Parent = x;
}
y->Parent = x->Parent;
if (x->Parent != &Nil)
if (x->Parent == &Nil)
{
Root = y;
}
@ -110,7 +113,7 @@ void FEventTree::RightRotate (FEvent *x)
y->Right->Parent = x;
}
y->Parent = x->Parent;
if (x->Parent != &Nil)
if (x->Parent == &Nil)
{
Root = y;
}
@ -176,7 +179,7 @@ void FEventTree::Insert (FEvent *z)
z->Right = &Nil;
z->Color = FEvent::RED;
while (z != Root && z->Parent->Color != FEvent::RED)
while (z != Root && z->Parent->Color == FEvent::RED)
{
if (z->Parent == z->Parent->Parent->Left)
{
@ -219,10 +222,11 @@ void FEventTree::Insert (FEvent *z)
}
z->Parent->Color = FEvent::BLACK;
z->Parent->Parent->Color = FEvent::RED;
RightRotate (z->Parent->Parent);
LeftRotate (z->Parent->Parent);
}
}
}
Root->Color = FEvent::BLACK;
}
void FEventTree::Delete (FEvent *z)

183
src/nodebuild_extract.cpp
View file

@ -246,126 +246,16 @@ int FNodeBuilder::CloseSubsector (TArray<seg_t> &segs, int subsector, vertex_t *
// to the start seg.
// A dot product serves to determine distance from the start seg.
seg = &Segs[SegList[first].SegNum];
double x1 = Vertices[seg->v1].x;
double y1 = Vertices[seg->v1].y;
double dx = Vertices[seg->v2].x - x1, dx2;
double dy = Vertices[seg->v2].y - y1, dy2;
double lastdot = 0, dot;
bool firstside = seg->planefront;
// Stage 1. Go forward.
for (i = first + 1; i < max; ++i)
{
double bestdot = DBL_MAX;
FPrivSeg *bestseg = NULL;
for (j = first + 1; j < max; ++j)
{
seg = &Segs[SegList[j].SegNum];
if (seg->planefront != firstside)
{
continue;
}
dx2 = Vertices[seg->v1].x - x1;
dy2 = Vertices[seg->v1].y - y1;
dot = dx*dx2 + dy*dy2;
if (dot < bestdot && dot > lastdot)
{
bestdot = dot;
bestseg = seg;
}
}
if (bestseg != NULL)
{
if (prev->v2 != bestseg->v1)
{
PushConnectingGLSeg (subsector, segs, &outVerts[prev->v2], &outVerts[bestseg->v1]);
count++;
}
seg->storedseg = PushGLSeg (segs, bestseg, outVerts);
count++;
prev = bestseg;
lastdot = bestdot;
}
}
count += OutputDegenerateSubsector (segs, subsector, true, 0, prev, outVerts);
// Stage 2. Go backward.
lastdot = DBL_MAX;
for (i = first + 1; i < max; ++i)
{
double bestdot = -DBL_MAX;
FPrivSeg *bestseg = NULL;
for (j = first + 1; j < max; ++j)
{
seg = &Segs[SegList[j].SegNum];
if (seg->planefront == firstside)
{
continue;
}
dx2 = Vertices[seg->v1].x - x1;
dy2 = Vertices[seg->v1].y - y1;
dot = dx*dx2 + dy*dy2;
if (dot > bestdot && dot < lastdot)
{
bestdot = dot;
bestseg = seg;
}
}
if (bestseg != NULL)
{
if (prev->v2 != bestseg->v1)
{
PushConnectingGLSeg (subsector, segs, &outVerts[prev->v2], &outVerts[bestseg->v1]);
count++;
}
seg->storedseg = PushGLSeg (segs, bestseg, outVerts);
count++;
prev = bestseg;
lastdot = bestdot;
}
}
count += OutputDegenerateSubsector (segs, subsector, false, DBL_MAX, prev, outVerts);
// Stage 3. Go forward again.
lastdot = -DBL_MAX;
for (i = first + 1; i < max; ++i)
{
double bestdot = 0;
FPrivSeg *bestseg = NULL;
for (j = first + 1; j < max; ++j)
{
seg = &Segs[SegList[j].SegNum];
if (seg->planefront != firstside)
{
continue;
}
dx2 = Vertices[seg->v1].x - x1;
dy2 = Vertices[seg->v1].y - y1;
dot = dx*dx2 + dy*dy2;
if (dot < bestdot && dot > lastdot)
{
bestdot = dot;
bestseg = seg;
}
}
if (bestseg != NULL)
{
if (prev->v2 != bestseg->v1)
{
PushConnectingGLSeg (subsector, segs, &outVerts[prev->v2], &outVerts[bestseg->v1]);
count++;
}
seg->storedseg = PushGLSeg (segs, bestseg, outVerts);
count++;
prev = bestseg;
lastdot = bestdot;
}
}
count += OutputDegenerateSubsector (segs, subsector, true, -DBL_MAX, prev, outVerts);
}
if (prev->v2 != firstVert)
{
PushConnectingGLSeg (subsector, segs, &outVerts[prev->v2], &outVerts[firstVert]);
@ -375,6 +265,73 @@ int FNodeBuilder::CloseSubsector (TArray<seg_t> &segs, int subsector, vertex_t *
return count;
}
int FNodeBuilder::OutputDegenerateSubsector (TArray<seg_t> &segs, int subsector, bool bForward, double lastdot, FPrivSeg *&prev, vertex_t *outVerts)
{
static const double bestinit[2] = { -DBL_MAX, DBL_MAX };
FPrivSeg *seg;
int i, j, first, max, count;
double dot, x1, y1, dx, dy, dx2, dy2;
bool wantside;
first = Subsectors[subsector].firstline;
max = first + Subsectors[subsector].numlines;
count = 0;
seg = &Segs[SegList[first].SegNum];
x1 = Vertices[seg->v1].x;
y1 = Vertices[seg->v1].y;
dx = Vertices[seg->v2].x - x1;
dy = Vertices[seg->v2].y - y1;
wantside = seg->planefront ^ !bForward;
for (i = first + 1; i < max; ++i)
{
double bestdot = bestinit[bForward];
FPrivSeg *bestseg = NULL;
for (j = first + 1; j < max; ++j)
{
seg = &Segs[SegList[j].SegNum];
if (seg->planefront != wantside)
{
continue;
}
dx2 = Vertices[seg->v1].x - x1;
dy2 = Vertices[seg->v1].y - y1;
dot = dx*dx2 + dy*dy2;
if (bForward)
{
if (dot < bestdot && dot > lastdot)
{
bestdot = dot;
bestseg = seg;
}
}
else
{
if (dot > bestdot && dot < lastdot)
{
bestdot = dot;
bestseg = seg;
}
}
}
if (bestseg != NULL)
{
if (prev->v2 != bestseg->v1)
{
PushConnectingGLSeg (subsector, segs, &outVerts[prev->v2], &outVerts[bestseg->v1]);
count++;
}
seg->storedseg = PushGLSeg (segs, bestseg, outVerts);
count++;
prev = bestseg;
lastdot = bestdot;
}
}
return count;
}
DWORD FNodeBuilder::PushGLSeg (TArray<seg_t> &segs, const FPrivSeg *seg, vertex_t *outVerts)
{
seg_t newseg;

19
src/nodebuild_gl.cpp
View file

@ -63,24 +63,7 @@ double FNodeBuilder::AddIntersection (const node_t &node, int vertex)
// Calculate signed distance of intersection vertex from start of splitter.
// Only ordering is important, so we don't need a sqrt.
FPrivVert *v = &Vertices[vertex];
double dx = double(v->x - node.x);
double dy = double(v->y - node.y);
double dist = dx*dx + dy*dy;
if (node.dx != 0)
{
if ((node.dx > 0 && dx < 0.0) || (node.dx < 0 && dx > 0.0))
{
dist = -dist;
}
}
else
{
if ((node.dy > 0 && dy < 0.0) || (node.dy < 0 && dy > 0.0))
{
dist = -dist;
}
}
double dist = (double(v->x) - node.x)*(node.dx) + (double(v->y) - node.y)*(node.dy);
FEvent *event = Events.FindEvent (dist);
if (event == NULL)

101
src/nodebuild_utility.cpp
View file

@ -125,14 +125,8 @@ int FNodeBuilder::SelectVertexExact (FPrivVert &vertex)
void FNodeBuilder::MakeSegsFromSides ()
{
FPrivSeg *share1, *share2;
FPrivSeg seg;
int i, j;
seg.next = DWORD_MAX;
seg.loopnum = 0;
seg.partner = DWORD_MAX;
if (Level.NumLines == 0)
{
I_Error ("Map is empty.\n");
@ -140,28 +134,9 @@ void FNodeBuilder::MakeSegsFromSides ()
for (i = 0; i < Level.NumLines; ++i)
{
share1 = NULL;
if (Level.Lines[i].sidenum[0] != NO_SIDE)
{
seg.linedef = i;
seg.sidedef = Level.Lines[i].sidenum[0];
seg.frontsector = Level.Lines[i].frontsector;
seg.backsector = Level.Lines[i].backsector;
seg.v1 = (int)(size_t)Level.Lines[i].v1;
seg.v2 = (int)(size_t)Level.Lines[i].v2;
seg.nextforvert = Vertices[seg.v1].segs;
seg.nextforvert2 = Vertices[seg.v2].segs2;
share1 = CheckSegForDuplicate (&seg);
if (share1 == NULL)
{
j = (int)Segs.Push (seg);
Vertices[seg.v1].segs = j;
Vertices[seg.v2].segs2 = j;
}
else
{
Printf ("Linedefs %d and %d share endpoints.\n", i, share1->linedef);
}
CreateSeg (i, 0);
}
else
{
@ -170,55 +145,49 @@ void FNodeBuilder::MakeSegsFromSides ()
if (Level.Lines[i].sidenum[1] != NO_SIDE)
{
seg.linedef = i;
seg.sidedef = Level.Lines[i].sidenum[1];
seg.frontsector = Level.Lines[i].backsector;
seg.backsector = Level.Lines[i].frontsector;
seg.v1 = (int)(size_t)Level.Lines[i].v2;
seg.v2 = (int)(size_t)Level.Lines[i].v1;
seg.nextforvert = Vertices[seg.v1].segs;
seg.nextforvert2 = Vertices[seg.v2].segs2;
share2 = CheckSegForDuplicate (&seg);
if (share2 == NULL)
j = CreateSeg (i, 1);
if (Level.Lines[i].sidenum[0] != NO_SIDE)
{
j = (int)Segs.Push (seg);
Vertices[seg.v1].segs = j;
Vertices[seg.v2].segs2 = j;
if (Level.Lines[i].sidenum[0] != NO_SIDE && share1 == NULL)
{
Segs[j-1].partner = j;
Segs[j].partner = j-1;
}
}
else if (share1 == NULL || share2->linedef != share1->linedef)
{
Printf ("Linedefs %d and %d share endpoints.\n", i, share2->linedef);
Segs[j-1].partner = j;
Segs[j].partner = j-1;
}
}
}
}
// Check for another seg with the same start and end vertices as this one.
// Combined with its use above, this will find two-sided lines that are shadowed
// by another one- or two-sided line, and it will also find one-sided lines that
// shadow each other. It will not find one-sided lines that share endpoints but
// face opposite directions. Although they should probably be a single two-sided
// line, leaving them in will not generate bad nodes.
FNodeBuilder::FPrivSeg *FNodeBuilder::CheckSegForDuplicate (const FNodeBuilder::FPrivSeg *check)
int FNodeBuilder::CreateSeg (int linenum, int sidenum)
{
DWORD segnum;
FPrivSeg seg;
int segnum;
// Check for segs facing the same direction
for (segnum = check->nextforvert; segnum != DWORD_MAX; segnum = Segs[segnum].nextforvert)
{
if (Segs[segnum].v2 == check->v2)
{
return &Segs[segnum];
}
seg.next = DWORD_MAX;
seg.loopnum = 0;
seg.partner = DWORD_MAX;
if (sidenum == 0)
{ // front
seg.frontsector = Level.Lines[linenum].frontsector;
seg.backsector = Level.Lines[linenum].backsector;
seg.v1 = (int)(size_t)Level.Lines[linenum].v1;
seg.v2 = (int)(size_t)Level.Lines[linenum].v2;
}
return NULL;
else
{ // back
seg.frontsector = Level.Lines[linenum].backsector;
seg.backsector = Level.Lines[linenum].frontsector;
seg.v2 = (int)(size_t)Level.Lines[linenum].v1;
seg.v1 = (int)(size_t)Level.Lines[linenum].v2;
}
seg.linedef = linenum;
seg.sidedef = Level.Lines[linenum].sidenum[sidenum];
seg.nextforvert = Vertices[seg.v1].segs;
seg.nextforvert2 = Vertices[seg.v2].segs2;
segnum = (int)Segs.Push (seg);
Vertices[seg.v1].segs = segnum;
Vertices[seg.v2].segs2 = segnum;
return segnum;
}
// Group colinear segs together so that only one seg per line needs to be checked