Floating polygons in output

[bgamari]

For some reason, this model seems to produce some very strange floating polygons. This appears to happen at all resolutions although the size of the polygons decreases with resolution; the attached image was taken of a mesh with a resolution of 1.

[bgamari]

It seems that the rotate on line 41 is responsible for the floating polygons: commenting this line out is sufficient to eliminate them.

[bgamari]

In fact, the following is sufficient to reproduce the issue,

rotate([0,30,0])
cylinder(r=10, h=20);

In general it seems that cylinders are extremely fragile under rotation.

[colah]

That's an interesting object!

Sorry I haven't been able to look into this before now. Super super busy.

I've seen issues similar to this before. They were related to mesh simplification and went away when I disabled it...

I'll try to look into this properly in a bit!

[colah]

I confirmed that if you comment out the tesseltateLoop case starting at line 43 of Graphics/Implicit/Export/Render/TesselateLoop.hs , which is responsible for generating squares (which are the merged if possible, and then converted into triangles) the problem goes away.

{-tesselateLoop res obj [[a,_],[b,_],[c,_],[d,_]] | centroid [a,c] == centroid [b,d] =
    let
        b1 = normalized $ a ^-^ b
        b2 = normalized $ c ^-^ b
        b3 = b1 `cross3` b2
    in [Sq (b1,b2,b3) (a ⋅ b3) (a ⋅ b1, c ⋅ b1) (a ⋅ b2, c ⋅ b2) ]-}

But disabling the square merging code doesn't fix it, so there are some cases in which squareToTris doesn't accurately turn squares into triangles. squareToTris (Graphics/Implicit/Export/Render/HandleSquares.hs

squareToTri (Sq (b1,b2,b3) z (x1,x2) (y1,y2)) =
    let
        zV = b3 ^* z
        (x1V, x2V) = (x1 *^ b1, x2 *^ b1)
        (y1V, y2V) = (y1 *^ b2, y2 *^ b2)
        a = zV ^+^ x1V ^+^ y1V
        b = zV ^+^ x2V ^+^ y1V
        c = zV ^+^ x1V ^+^ y2V
        d = zV ^+^ x2V ^+^ y2V
    in
        [(a,b,c),(c,b,d)]

Since they appear to be shifting along the normal, clearly b3 and its dot products are implicated. My suspicion is some sort of floating point error...

[julialongtin]

I'm seeing holes, but no floating squares.

[julialongtin]

Reproduced.

[oneacik]

Btw. Thank you @julialongtin for reviewing these bugs. I hope that after I get with speed on the project I will be able to help.

[julialongtin]

I've confirmed that disabling the tesserateLoop above removes this failure. I'm going to leave it disabled, while i come up to speed on the math in those two functions.

[julialongtin]

`width=88; height=6; length=6; center_offset=5.3;

linear_extrude (width/2, translate(z)=(0,-z/center_offset)) polygon ( [(0,0), (0,center_offset+length), (height, center_offset+length)]); `

produces the same problem as the initial reporter's issue, but is much shorter. the vertical squares are perfect, the squares not-quite-aligned with the Y axis are perfect, but the squares on the underside of the object are on the right plane, but off of their x-y position.

[julialongtin]

those squares are more off the further from (0,0,0) they are.