130 lines
5.5 KiB
Plaintext
130 lines
5.5 KiB
Plaintext
DESCRIPTION OF PROBLEM:
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=======================
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This example map demonstrates what a relatively large value for plane distance
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epsilon can do as far as destroying draw surfaces. The plane distance
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epsilon was 0.01 at the time of this writing. This means that two planes
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with the same normal and a distance 0.00999 apart are considered to be the
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same plane.
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The recommended value of plane distance epsilon is more along the lines of
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0.001; however, we can't currently change this epsilon to that value due to
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lack of resolution in 32 bit floating point integers in the 2^16 number range.
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The smallest epsilon in 32 bit float land that can be added to 2^16 that
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results in a value greater than 2^16 is approximately 0.007, which is already
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practically the current default plane distance epsilon (0.01).
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Brush 0 in the example map is a 6-sided brush with red top face. The red top
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face is defined such in the .map file:
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( -127 256 513 ) ( -127 384 513 ) ( 1 384 512 )
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During the -bsp stage,
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In ParseRawBrush() for brush 0
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Side 4:
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(-127.000000 256.000000 513.000000)
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(-127.000000 384.000000 513.000000)
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(1.000000 384.000000 512.000000)
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normal: (0.0078122616 0.0000000000 0.9999694824)
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dist: 511.9921875000
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Now brush 1, the 4-sided brush with the red top face, has the following
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defined as the red top face:
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( -128 0 513 ) ( -128 128 513 ) ( 0 128 512 )
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It's almost the same plane, only off by a distance of about 0.01.
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During compiling:
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In ParseRawBrush() for brush 1
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Side 1:
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(-128.000000 0.000000 513.000000)
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(-128.000000 128.000000 513.000000)
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(0.000000 128.000000 512.000000)
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normal: (0.0078122616 0.0000000000 0.9999694824)
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dist: 511.9921875000
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Note that the normal and dist are identical to the plane above, even though
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the two are different. This leads to multiplying errors later on, when this
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side is intersected with the bottom green face. In particular, the blue face
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disappears from the map. The blue face:
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In CreateBrushWindings() for brush 1
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Handling side 2 on the brush
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Before clipping we have:
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(-262144.00000000 0.00000000 262144.00000000)
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(262144.00000000 0.00000000 262144.00000000)
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(262144.00000000 -0.00000000 -262144.00000000)
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(-262144.00000000 0.00000000 -262144.00000000)
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After clipping w/ side 0 we have:
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(-262144.00000000 0.00000000 262144.00000000)
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(262144.00000000 0.00000000 262144.00000000)
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(262144.00000000 0.00000000 -19977.00073172)
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(-262144.00000000 0.00000000 20983.00073758)
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After clipping w/ side 1 we have:
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(262144.00000000 0.00000000 -1535.99218726)
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(262144.00000000 0.00000000 -19977.00073172)
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(-128.11106773 0.00000000 513.00868046)
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After clipping w/ side 3 we have:
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(0.00000000 0.00000000 503.00000293)
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(-128.11106773 0.00000000 513.00868046)
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(-0.00000000 0.00000000 512.00781274)
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We see the error is greater than 0.1 in the X value -128.11106773. This
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causes the draw surface to be discarded in processing later on. We must
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not allow such a great error. An error greater than or equal to 0.1 is
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considered to be very bad.
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This disappearing blue triangle can be fixed simply by deleting brush 0,
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the 6-sided brush that causes the bad plane alias. I would suggest deleting
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this brush by editing the .map file manually (and then renumbering the brushes
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manually).
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After deleting this brush:
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In ParseRawBrush() for brush 0
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Side 1:
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(-128.000000 0.000000 513.000000)
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(-128.000000 128.000000 513.000000)
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(0.000000 128.000000 512.000000)
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normal: (0.0078122616 0.0000000000 0.9999694824)
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dist: 511.9843750000
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In CreateBrushWindings() for brush 0
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Handling side 1 on the brush
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Before clipping we have:
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(262132.00000000 262136.00000000 -1535.90625143)
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(262132.00000000 -262136.00000000 -1535.90625143)
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(-262124.00048828 -262136.00000000 2559.84375238)
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(-262124.00048828 262136.00000000 2559.84375238)
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After clipping w/ side 0 we have:
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(262132.00000000 262136.00000000 -1535.90625143)
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(262132.00000000 -262136.00000000 -1535.90625143)
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(-127.99993289 -262136.00000000 512.99999805)
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(-127.99993289 262136.00000000 512.99999805)
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After clipping w/ side 2 we have:
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(262132.00000000 262136.00000000 -1535.90625143)
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(262132.00000000 -0.00000000 -1535.90625143)
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(-127.99993289 -0.00000000 512.99999805)
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(-127.99993289 262136.00000000 512.99999805)
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After clipping w/ side 3 we have:
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(0.00000000 0.00000000 511.99999857)
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(-127.99993289 -0.00000000 512.99999805)
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(-127.99993289 127.99993289 512.99999805)
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Note that we're way more accurate now, and we're nowhere near to approaching
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the 0.1 error. All it took was the deletion of a seemintly unrelated brush.
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SOLUTION TO PROBLEM:
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====================
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The suggested fix is to increase the resolution of plane distance from 32 bit
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float to 64 bit float for the relevant brush processing operations. After that
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is done, decrease the default plane distance epsilon from 0.01 to 0.001. Note
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that even though plane distance epsilon can be specified on the command-line,
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it will do no good (even if set to 0) if the plane distance is close to
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2^16.
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