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Fixing "disappearing_sliver" bug.
A regression test is included. This is a bug in q3map2. Please read regression_tests/q3map2/disappearing_sliver/README.txt for more info. git-svn-id: svn://svn.icculus.org/gtkradiant/GtkRadiant/trunk@363 8a3a26a2-13c4-0310-b231-cf6edde360e5
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rambetter
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4 changed files with 252 additions and 5 deletions
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@ -127,7 +127,7 @@ void _VectorCopy (vec3_t in, vec3_t out)
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}
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vec_t VectorNormalize( const vec3_t in, vec3_t out ) {
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vec_t length, ilength;
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vec_t length;
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length = (vec_t)sqrt (in[0]*in[0] + in[1]*in[1] + in[2]*in[2]);
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if (length == 0)
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@ -136,10 +136,9 @@ vec_t VectorNormalize( const vec3_t in, vec3_t out ) {
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return 0;
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}
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ilength = 1.0f/length;
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out[0] = in[0]*ilength;
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out[1] = in[1]*ilength;
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out[2] = in[2]*ilength;
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out[0] = in[0]/length;
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out[1] = in[1]/length;
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out[2] = in[2]/length;
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return length;
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}
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164
regression_tests/q3map2/disappearing_sliver/README.txt
Normal file
164
regression_tests/q3map2/disappearing_sliver/README.txt
Normal file
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@ -0,0 +1,164 @@
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DESCRIPTION OF PROBLEM:
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=======================
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The example map, maps/disappearing_sliver.map, contains an example of this bug.
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There are 6 walls in the map, and one tall thin triangular sliver brush in the
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middle of the room (7 brushes total). Only one face of the sliver in the
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middle of the room is a draw surface. The bug is that this sliver surface is
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not rendered in the compiled BSP. Note that the sliver brush was hand-crafted
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to demonstrate the bug. If you re-save the map, Radiant might adjust the
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order in which the planes on the brush are defined, and this might, as a side
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effect, get rid of the immediate bug.
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To trigger the bug, compile the map; you don't need -vis or -light. Only
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-bsp (the first q3map2 stage) is necessary to trigger the bug. The only
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entities in the map are 2 lights and a single info_player_deathmatch, so the
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map will compile for any Q3 mod.
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SOLUTION TO PROBLEM:
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====================
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Several days were spent studying this problem in great detail.
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The fix for this problem was to make the outcome of the VectorNormalize()
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function libs/mathlib/mathlib.c more accurate. The previous code in this
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function looks something like this:
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vec_t length, ilength; // vec_t is a typedef for 32 bit float.
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/* Compute length */
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ilength = 1.0f/length;
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out[0] = in[0]*ilength;
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out[1] = in[1]*ilength;
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out[2] = in[2]*ilength;
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As you can see, we are introducing a lot of extra error into our normalized
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vector by multiplying by the reciprocal length instead of outright dividing
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by the length. The new fixed code looks like this:
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out[0] = in[0]/length;
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out[1] = in[1]/length;
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out[2] = in[2]/length;
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And we get rid of the recpirocal length ilength altogether. Even the
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slightest math errors are magnified in successive calls to linear algebra
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functions.
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POSSIBLE SIDE EFFECTS:
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======================
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The only negative side effect is that compilation of a map might take longer
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due to an increased number of divide operations. (I'm actually not sure if
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that is indeed the case.) Another side effect might be that if you're used
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to a map being broken (missing triangles) or having "sparklies" between
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brushes, those might be gone now. :-)
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IN-DEPTH DISCUSSION:
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====================
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VectorNormalize() is used very frequently in Radiant and tools code. My goal
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for this fix was to make the least amount of code change but still be able to
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demonstrate a significant improvement in math accuracy (including a fix to
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the test case). At the same time don't risk that any other bugs pop up as a
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side effect of this change.
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Here is the sequence of calls (stack trace) that cause the example bug to
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happen:
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main() in main.c -->
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BSPMain() in bsp.c -->
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LoadMapFile() in map.c -->
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ParseMapEntity() in map.c -->
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ParseBrush() in map.c -->
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FinishBrush() in map.c -->
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CreateBrushWindings() in brush.c -->
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ChopWindingInPlace() in polylib.c
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What basically happens in this sequence of calls is that a brush is parsed
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out of the map file, "infinite" planes are created for each brush face, and
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then the planes are "intersected" to find the exact vertex topologies of each
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brush face. The vertex topology of the visible face of the sliver (in the
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example map) gets computed with a significant amount of math error. If we
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did our math with infinite precision, the sliver face would have the following
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vertices:
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(67 -1022 0)
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(88 -892 -768)
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(134 -1015 0)
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In fact, if you open the map file (disappearing_sliver.map), you can actually
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see these exact points embedded in the third plane defined on brush 0.
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I managed to print out the actual computed vertices of the sliver face before
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and after this bug fix. Basically this is printed out after all the
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ChopWindingInPlace() calls in the above stack trace:
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(66.984695 -1021.998657 0.000000)
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(87.989571 -891.969116 -768.174316)
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(133.998917 -1014.997314 0.000000)
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(If you want to print this out for yourself, print out the coordinates of the
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winding_t "w" parameter right after the ChopWindingInPlace() call in
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CreateBrushWindings() in brush.c.)
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The same vertices after the bugfix have the following coordinates:
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(67.000229 -1021.998657 0.000000)
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(88.000175 -891.999146 -767.997437)
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(133.999146 -1014.998779 0.000000)
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As you can see, the vertices after the fix are substantially more accurate,
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and all it took was an improvement to VectorNormalize().
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The problem before the fix was the Z coordinate of the second point, namely
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-768.174316. There is a lot of "snap to nearest 1/8 unit" and "epsilon 0.1"
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code used throughout q3map2. 0.174 is greater than the 0.1 epsilon, and that
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is the problem.
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main() in main.c -->
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BSPMain() in bsp.c -->
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ProcessModels() in bsp.c -->
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ProcessWorldModel() in bsp.c -->
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ClipSidesIntoTree() in surface.c -->
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ClipSideIntoTree_r() in surface.c -->
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ClipWindingEpsilon() in polylib.c
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Now what ClipWindingEpsilon() does is, since -768.174316 reaches below the
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plane z = -768 (and over the 0.1 epsilon), it clips the winding_t and creates
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two points where there used to be only one.
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main() in main.c -->
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BSPMain() in bsp.c -->
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ProcessModels() in bsp.c -->
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ProcessWorldModel() in bsp.c
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FixTJunctions() in tjunction.c
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FixBrokenSurface() in tjunction.c
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FixBrokenSurface() realizes that there are two points very close together
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(in our case, since they were "snapped", the are coincident in fact).
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Therefore it reports the surface to be broken. The drawable surface is
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deleted as a result.
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RELATED BUGS:
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=============
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A lot of the math operations in the Radiant codebase cut corners like this
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example demonstrates. There is a lot more code like this that can be
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improved upon. In fact, it may make sense to use 64 bit floating points in
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some important math operations (and then convert back to 32 bit for the return
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values). Plans are to look at similar code and improve it.
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The following "issue" was discovered while doing research for this bug.
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If FixBrokenSurface() sees two points very close together, it attempts to
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partially fix the problem (the code is not complete) and then returns false,
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which means that the surface is broken and should not be used. So in fact
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it attempts to fix the problem partially but none of the fixes are used.
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It seems that FixBrokenSurface() should be fixed to completely fix the case
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where there are two close points, and should report the surface as fixed.
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This might be a destabilizing change however, so if this is indeed fixed, it
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may make sense to activate the fix only if a certain flag is set.
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@ -0,0 +1,84 @@
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// entity 0
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{
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"classname" "worldspawn"
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// brush 0
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{
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( -704 -1152 0 ) ( 4480 -1152 0 ) ( -704 -1152 -768 ) common/caulk 0 0 0 0.500000 0.500000 134217728 4 0
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( 67 -1022 0 ) ( 67 -4096 0 ) ( 88 -892 -768 ) common/caulk 0 0 0 0.500000 0.500000 134217728 4 0
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( 67 -1022 0 ) ( 88 -892 -768 ) ( 134 -1015 0 ) radiant_regression_tests/tile 0 0 0 0.500000 0.500000 134217728 0 0
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( 88 -892 -768 ) ( 88 -4096 -768 ) ( 134 -1015 0 ) common/caulk 0 0 0 0.500000 0.500000 134217728 4 0
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( 134 -1015 0 ) ( 134 -4096 0 ) ( 67 -1022 0 ) common/caulk 0 0 0 0.500000 0.500000 134217728 4 0
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}
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// brush 1
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{
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( 584 -640 -768 ) ( 520 -640 -768 ) ( 520 -1160 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 528 -1160 320 ) ( 528 -640 320 ) ( 592 -640 320 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 528 -1152 0 ) ( 592 -1152 0 ) ( 592 -1152 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 520 -1160 0 ) ( 520 -640 0 ) ( 520 -640 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 584 -640 0 ) ( 520 -640 0 ) ( 520 -640 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 512 -640 0 ) ( 512 -1160 0 ) ( 512 -1160 -768 ) radiant_regression_tests/tile 0 0 0 0.500000 0.500000 0 0 0
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}
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// brush 2
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{
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( 512 -552 -768 ) ( -296 -552 -768 ) ( -296 -640 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -296 -640 320 ) ( -296 -552 320 ) ( 512 -552 320 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -296 -640 320 ) ( 512 -640 320 ) ( 512 -640 -768 ) radiant_regression_tests/tile 0 0 0 0.500000 0.500000 0 0 0
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( 512 -640 320 ) ( 512 -552 320 ) ( 512 -552 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 528 -632 320 ) ( -280 -632 320 ) ( -280 -632 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -512 -536 320 ) ( -512 -624 320 ) ( -512 -624 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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}
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// brush 3
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{
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( -512 -648 -768 ) ( -536 -648 -768 ) ( -536 -1152 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -536 -1152 320 ) ( -536 -648 320 ) ( -512 -648 320 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -536 -1152 320 ) ( -512 -1152 320 ) ( -512 -1152 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -512 -1152 320 ) ( -512 -648 320 ) ( -512 -648 -768 ) radiant_regression_tests/tile 0 0 0 0.500000 0.500000 0 0 0
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( -512 -640 320 ) ( -536 -640 320 ) ( -536 -640 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -520 -648 320 ) ( -520 -1152 320 ) ( -520 -1152 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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}
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// brush 4
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{
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( 512 -640 320 ) ( 24 -640 320 ) ( 24 -1152 320 ) radiant_regression_tests/tile 0 0 0 0.500000 0.500000 0 0 0
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( 48 -1152 328 ) ( 48 -640 328 ) ( 536 -640 328 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 24 -1152 440 ) ( 512 -1152 440 ) ( 512 -1152 328 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 512 -1152 440 ) ( 512 -640 440 ) ( 512 -640 328 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 512 -640 440 ) ( 24 -640 440 ) ( 24 -640 328 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -512 -640 464 ) ( -512 -1152 464 ) ( -512 -1152 352 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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}
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// brush 5
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{
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( -328 -896 -776 ) ( -512 -896 -776 ) ( -512 -1152 -776 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -512 -1152 -768 ) ( -512 -896 -768 ) ( -328 -896 -768 ) radiant_regression_tests/tile 0 0 0 0.500000 0.500000 0 0 0
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( -512 -1152 -768 ) ( -328 -1152 -768 ) ( -328 -1152 -776 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -512 -896 -768 ) ( -512 -1152 -768 ) ( -512 -1152 -776 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 512 -1152 -768 ) ( 512 -896 -768 ) ( 512 -896 -776 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -368 -640 -824 ) ( -856 -640 -824 ) ( -856 -640 -960 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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}
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// brush 6
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{
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( -512 -1168 320 ) ( -512 -1584 320 ) ( -512 -1584 0 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 1152 -1152 320 ) ( 176 -1152 320 ) ( 176 -1152 0 ) radiant_regression_tests/tile 0 0 0 0.500000 0.500000 0 0 0
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( 512 -1576 320 ) ( 512 -1160 320 ) ( 512 -1160 0 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 208 -1160 320 ) ( 1184 -1160 320 ) ( 1184 -1160 0 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( 176 -1568 320 ) ( 176 -1152 320 ) ( 1152 -1152 320 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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( -256 -1152 -768 ) ( -520 -1152 -768 ) ( -520 -1192 -768 ) common/caulk 0 0 0 0.500000 0.500000 0 4 0
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}
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}
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// entity 1
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{
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"light" "3000"
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"origin" "0 -768 -480"
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"classname" "light"
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}
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// entity 2
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{
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"angle" "270"
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"origin" "0 -712 -640"
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"classname" "info_player_deathmatch"
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}
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// entity 3
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{
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"classname" "light"
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"origin" "0 -832 160"
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"light" "3000"
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}
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