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97 lines
2.4 KiB
97 lines
2.4 KiB
# Tetrahedron |
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# Tetrahedron bounding box size: |
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# Figure 1: Side View |
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# |
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# *_ - |
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# / \ ^ |
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# / | \_ |
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# / | \ depth |
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# / | \_ v |
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# *----0-------* - |
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# |
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# Figure 2: Top View |
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# |
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# *_B |
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# | \_ |
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# |\ \_ |
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# | \ \_ |
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# | \ \_ |
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# | \A \__ |
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# |<a> *-<b>-__*D |
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# | / _/ |
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# | / _/ |
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# | / _/ |
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# |/ _/ |
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# |_/ |
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# *C |
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# From these diagrams, you can tell that the width is the height of an |
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# equilateral triangle. If we assume the edge of the tetrahedron is s, then: |
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define @half_height s / 2 |
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# For the depth, a few calculations are necessary: |
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# |
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# a + b = s * sin(⅓π) |
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# a² + (s/2)² = b² |
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define @a (s/2)*tan(⅙π) |
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define @b s * (sin(⅓π) - tan(⅙π)/2) |
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# a² + d² = (s*sin(⅓π))² |
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define @depth s * sqrt((sin(⅓π))^2 - ((1/2)*tan(⅙π))^2) |
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# Considering that the origin is in the center of the base, the min x is -a, |
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# the min y is -s/2, and the min z is 0 |
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-@a < x < @b |
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-@half_height < y < @half_height |
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0 < z < @depth |
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# A can be given by: |
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# |
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# (0,0,depth) |
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# B and C can be given by: |
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# |
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# (-a,±half_height,0) |
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# D can be given by: |
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# |
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# (b,0,0) |
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# The tetrahedron can be given by the intersection of 4 semi-spaces. |
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# However, one of the semi-spaces is z >= 0, which is implicit in the |
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# declaration of the z boundaries, so it will not require a condition. |
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# Cross Product Cheatsheet |
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# |
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# a ⨯ b = | i j k | |
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# | ax ay az | |
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# | bx by bz | |
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# |
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# = (ay bz - az by, az bx - ax bz, ax by - ay bx) |
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# The normal vector for the plane ABC is the cross product of AB and AC |
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# AB = (-a, half_height, -depth) |
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# AC = (-a, -half_height, -depth) |
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# |
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# vABC = (half_height * (-depth) - (-depth) * (-half_height), |
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# (-depth) * (-a) - (-a) * (-depth), |
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# (-a) * (-half_height) - half_height * (-a)) = |
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# (- s * depth, 0, s * a) |
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x * (- s * @depth) + (z - @depth) * (s * @a) ≤ 0 |
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# The normal vector for the plane ADB is the cross product of AD and AB |
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# |
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# AD = (b,0,-depth) |
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# AB = (-a, half_height, -depth) |
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# |
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# vABD = (depth * half_height, depth * a + b * depth, b * half_height) |
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x * @depth * @half_height + y * @depth * (@a + @b) + (z - @depth) * @b * @half_height ≤ 0 |
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# The normal vector for the plane ACD is equal to the previous one but |
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# reflected on the y axis. Thus, we only have to flip a plus to a minus. |
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# v |
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x * @depth * @half_height - y * @depth * (@a + @b) + (z - @depth) * @b * @half_height ≤ 0
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