ImplicitCAD

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Hi devolopers and maintainers of this great project,

I have come across a bug in the refine function, which generates broken faces in some models. For now, I don't understand the full idea of the function so I don't come with a fix proposal, so I provide the reproduction code (which creates an internal helic involute gear):

module Main where

import Prelude
import Graphics.Implicit
import Graphics.Implicit.Definitions

involute x y = - (x * cos angle + y * sin angle - 1)
  where angle = sqrt$ abs$ x**2 + y**2 - 1

circ x y = x**2 + y**2 - 1

invcirc :: SymbolicObj2
invcirc = flip implicit ((-5, -5), (5, 5)) $ \(x, y) -> maximum
  [ min (involute x y) (maximum [x - 1, y - 1.5, -x - 3])
  , -y
  , circ (x/4.9) (y/4.9)
  ]

double :: a -> (a, a)
double x = (x, x)

involuteGear :: ℝ -> ℕ -> ℝ -> ℝ-> SymbolicObj2
involuteGear pressureAngle toothCount modulus gapExpansion = scale scale_ $
  intersect
    [ circle (1 + 2/toothCountℝ)
    , union$ [circle (1 - 2*5/4/toothCountℝ)] ++
        [ rotate ((fromℕtoℝ i)*toothAngle) $
            intersect [toothHalf, rotate toothAngle$ scale (1, -1) toothHalf]
        | i <- [0..toothCount - 1]
        ]
    ]
  where
    toothCountℝ = fromℕtoℝ toothCount
    scale_ = double$ modulus * toothCountℝ / 2

    toothHalf = rotate (halfGapAngle)$ scale (double involuteScale) invcirc
    involuteScale = cos pressureAngle

    toothAngle = 2 * pi / toothCountℝ
    halfGapAngle = (toothAngle / 2 - 2 * (tan pressureAngle - pressureAngle)) / 2
      + gapExpansion

main = writeSTL 1 "test.stl" obj
  where
    pressureAngle = 30*pi/180
    modulus = 2
    gearProfile toothCount gapExpansion =
      involuteGear pressureAngle toothCount modulus gapExpansion

    gearLike toothCount profile = extrudeRM
      0
      (Right$ \z -> abs$ 360/toothCount' - 360/toothCount'/5*2*z)
      (Left 1)
      (Left (0, 0))
      profile
      (Left 5)
      where toothCount' = fromℕtoℝ toothCount

    gear toothCount gapExpansion = gearLike toothCount$
      gearProfile toothCount gapExpansion

    obj = gearLike 54$ difference [circle$ 54*modulus/2 + 10, gearProfile 54 (-0.003)]

The broken faces were no more there when I removed the refine function application here, which can be used as a simple workaround to this bug:

-    in map (refine res obj) . filter notPointLine $ case (x1y2 <= 0, x2y2 <= 0,
-                                                          x1y1 <= 0, x2y1 <= 0) of
+    in filter notPointLine $ case (x1y2 <= 0, x2y2 <= 0, x1y1 <= 0, x2y1 <= 0) of

Good catch! do you mind if i add your example code to the examples in implicitcad? It would help me know people depend on that part of the API during release testing.

Thanks again!

Sure you can add it as an example, I will also publish an involute gear library soon :) And... am I using some obsolete part of the API? Is there some more recent approach of modelling? I followed the examples and it looks like I should just use the basic primitives combined with implicit if the primitives are not enough. Then output via writeSTL.

So far as i know, you're using the up to date part. I just don't have any examples testing it that thoroughly, hense why i asked to add yours. :)

On Thu, Nov 21, 2019 at 8:55 PM p4l1ly [email protected] wrote:

Sure you can add it as an example, I will also publish an involute gear library soon :) And... am I using some obsolete part of the API? Is there some more recent approach of modelling? I followed the examples and it looks like I should just use the basic primitives combined with implicit if the primitives are not enough, then output via writeSTL.

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After poking at this, the change reduces the polygon count of all objects, making them slightly smaller (read: less accurate). I'm going to have to dig deep into the example you've given me directly, and see if i can find the root cause.

still, thanks. this at least points me in the right direction.

On Thu, Nov 21, 2019 at 8:57 PM Julia Longtin [email protected] wrote:

So far as i know, you're using the up to date part. I just don't have any examples testing it that thoroughly, hense why i asked to add yours. :)

On Thu, Nov 21, 2019 at 8:55 PM p4l1ly [email protected] wrote:

Sure you can add it as an example, I will also publish an involute gear library soon :) And... am I using some obsolete part of the API? Is there some more recent approach of modelling? I followed the examples and it looks like I should just use the basic primitives combined with implicit if the primitives are not enough, then output via writeSTL.

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It is true, but I think it's not a very big deal because the accuracy can be still controlled by the resolution parameter. If you do e.g. 3D printing, it is OK just to pick a resolution a bit smaller than the resolution of your 3D printer and you don't care about further accuracy.

I can see that the refine function uses some derivations but my function is not differentiable.