zero thickness membranes in Preview as a result of difference()

When I difference two objects that have a common length on one dimension, I see a thin membrane in Preview rather than a hole: Here's sample code: $fn=40; dist=3; difference(){ cube([5,5,dist],center=true); cylinder(r=2,h=dist,center=true); } I know that this membrane goes away when the model is rendered but I'm producing complex objects where the rendering process can take several minutes. I'm also using the customizer so the membrane is redisplayed each time a parameter is changed. I have worked around this behavior by liberally peppering my code with references to a global fudge factor: fudge=0.005. I add this factor to any object that will be used to difference another object. Often, I have to add a translate() call just to move the resultant object down by fudge/2 to ensure that the hole is uncovered on both sides of the first object. Since I'm 3D printing the final model, this fudge factor is in the noise but it makes a mess of my code. How have you all solved this problem? Remember, requiring a render step isn't a solution. Thanks, Ken

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When I difference two objects that have a common length on one dimension, I see a thin membrane in Preview rather than a hole: Here's sample code: $fn=40; dist=3; difference(){ cube([5,5,dist],center=true); cylinder(r=2,h=dist,center=true); } I know that this membrane goes away when the model is rendered but I'm producing complex objects where the rendering process can take several minutes. I'm also using the customizer so the membrane is redisplayed each time a parameter is changed. I have worked around this behavior by liberally peppering my code with references to a global fudge factor: fudge=0.005. I add this factor to any object that will be used to difference another object. Often, I have to add a translate() call just to move the resultant object down by fudge/2 to ensure that the hole is uncovered on both sides of the first object. Since I'm 3D printing the final model, this fudge factor is in the noise but it makes a mess of my code. How have you all solved this problem? Remember, requiring a render step isn't a solution. Thanks, Ken

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Hi,

Your approach is the conventional one. But we usually call the variable epsilon :)

This is actually pretty common in 3D printing and other fields. You'll find the same approach occasionally used in vector graphics, for example. Floating point precision errors cause headaches all around.

I know you say rendering is not an option, but you might check out the render() primitive, which allows you to force rendering for particular parts of your model. Depending on the geometry you're working with, you may be able to render the outermost parts so you can see the interior details, without rendering the whole thing.

Best of luck!

On Wed, Apr 22, 2020, 5:25 PM khackbarth <[hidden email]> wrote:

When I difference two objects that have a common length on one dimension, I see a thin membrane in Preview rather than a hole: Here's sample code: $fn=40; dist=3; difference(){ cube([5,5,dist],center=true); cylinder(r=2,h=dist,center=true); } I know that this membrane goes away when the model is rendered but I'm producing complex objects where the rendering process can take several minutes. I'm also using the customizer so the membrane is redisplayed each time a parameter is changed. I have worked around this behavior by liberally peppering my code with references to a global fudge factor: fudge=0.005. I add this factor to any object that will be used to difference another object. Often, I have to add a translate() call just to move the resultant object down by fudge/2 to ensure that the hole is uncovered on both sides of the first object. Since I'm 3D printing the final model, this fudge factor is in the noise but it makes a mess of my code. How have you all solved this problem? Remember, requiring a render step isn't a solution. Thanks, Ken

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I call it itsy (as it itsy bitsy). Any small number where the precise value
isn't of importance.


Max Bond wrote
> Your approach is the conventional one. But we usually call the variable
> epsilon :)





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This is a general principle of computing : “any program such depends on (a == b) is broken”

 

Manufacturing has a similar concept : tolerance.

I disagree with the claim that only “difference” needs a tolerance.  It’s also necessary when doing a “union” of two objects with a shared boundary.  

 


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-Kenneth Sloan

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