Extrude with taper on any Workplane return a wrong extrude length

[trunglebao]

Generic use of the extrude(until = x, taper = y) command will result in extrude length less than x.

a = list(range(0,60))
z = []
for angle in a:
    part = cq.Solid(cq.Workplane('XY').rect(1,1).extrude(1, taper = angle).toOCC())
    bb = part.BoundingBox()
    z.append(bb.zlen)

Running the code above create a bunch of extrude with height 1, with taper ranging from 0 to 60 degree.
Plotting the taper angle (0 to 60 degree) with resulted extrude height (suppose to be 1):

Only taper = 0 return the correct height = 1, the rest is all less than 1.
Any thought/alternative solution to get the correct height?
Thanks!

[jmwright]

It's because the sides converge to a point before they get to the extruded length. The zlen can be calculated via the cosine of the taper angle as long as there is not a point convergence.

import cadquery as cq
from math import cos, radians

a = list(range(0,30))
for angle in a:
    part = cq.Workplane('XY').rect(100.0,100.0).extrude(100.0, taper = angle)
    bb = part.val().BoundingBox()
    print(str(angle) + " : " + str(bb.zlen) + " : " + str(100.0 * cos(radians(angle))))

Outputs:

0 : 100.00000020000002 : 100.0
1 : 99.98476971563919 : 99.98476951563913
2 : 99.93908290190964 : 99.93908270190957
3 : 99.86295367545753 : 99.86295347545739
4 : 99.75640522598249 : 99.75640502598242
5 : 99.61947000917462 : 99.61946980917456
6 : 99.45218973682736 : 99.45218953682733
7 : 99.25461536413223 : 99.2546151641322
8 : 99.02680707415715 : 99.02680687415703
9 : 98.76883425951381 : 98.76883405951378
10 : 98.48077550122083 : 98.4807753012208
11 : 98.16271854476643 : 98.1627183447664
12 : 97.81476027338061 : 97.81476007338057
13 : 97.43700667852359 : 97.43700647852353
14 : 97.02957282759971 : 97.02957262759965
15 : 96.59258282890686 : 96.59258262890683
16 : 96.12616979383193 : 96.12616959383189
17 : 95.63047579630357 : 95.63047559630354
18 : 95.10565182951544 : 95.10565162951535
19 : 94.55185775993174 : 94.55185755993169
20 : 93.96926227859088 : 93.96926207859084
21 : 93.35804284972023 : 93.35804264972018
22 : 92.71838565667879 : 92.71838545667875
23 : 92.05048554524413 : 92.05048534524404
24 : 91.3545459642601 : 91.35454576426008
25 : 90.63077890366509 : 90.63077870366499
26 : 89.87940482991672 : 89.8794046299167
27 : 89.1006526188368 : 89.10065241883679
28 : 88.29475948589273 : 88.2947592858927
29 : 87.46197091393961 : 87.46197071393958

[trunglebao]

That explained it. Thank you very much!
Most of the CAD software that i use set the extrude length to be zlen regardless of the taper amount. While cq use the length of the tapered edge.

Regardless, very useful library. Hat off to you guys.

[adam-urbanczyk]

I think we need to fix this. Me expectation was that this param is height, that is confirmed by the docstring:

The distance to extrude, normal to the workplane plane

[jmwright]

@trunglebao This has been fixed in the latest commit, (see linked PR) but the CI system will need a few hours to build and deploy.