109 lines
4.0 KiB
Plaintext
109 lines
4.0 KiB
Plaintext
---
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title: "CadQuery Direct API"
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author: "James Pace"
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date: "2024/02/06"
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---
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[CadQuery][cadquery-homepage] is a "CAD via Python" library.
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Instead of using a WYSIWYG editor to generate CAD models, when using CadQuery
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you write Python code that does the same operations.
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The most popular code based CAD tool is [OpenSCAD][openscad-homepage].
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Using CadQuery feels much closer to using a "normal" CAD package, in my opinion,
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than using OpenSCAD.
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With CadQuery and normal CAD packages you think in terms of operations on sketches
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or faces that compose into the final part.
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OpenSCAD doesn't feel like that, at least in the little bit I've tried.
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CadQuery has three API layers (in order from highest to lowest level: fluent, direct, and OCCT).
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Fluent is the suggested level with the most documentation.
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It feels the closest to normal CAD.
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The mental model is you start with a workplane which is a root of a tree and then
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perform operations which add elements to the tree with the modifications needed to build
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the desired part.
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For example:
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```python
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import cadquery as cq
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result = (
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cq.Workplane("front")
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.circle(2.0)
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.pushPoints([(1.5, 0), (0, 1.5), (-1.5, 0), (0, -1.5)])
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.circle(0.25)
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.extrude(0.125)
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)
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```
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will:
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1. Make a workplane.
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2. Add a single 2D circle.
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3. Add four points to the stack of the tree.
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4. Operate on those four points by drawing smaller circles at each point.
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5. Extrude the result.
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This may seem confusing, but if you've worked with Fusion360 or Creo, you've basically done
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the same thing (see the timeline in the bottom of the window in Fusion360 or the history in Creo).
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I've found that it once the idea clicks what's going on isn't that hard to work out.
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What is a pain to work out is when something goes wrong.
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Its very hard to introspect what's happening at each step.
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Particularly when so much is wirtten on basically one line, which isn't a requirment per se,
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but definitely the suggested workflow.
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Regardless, the actions in each of the lines is extremely dependent on state hidden in the workplane
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that is basically impossible to tease out.
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What I've found more challenging is it is really hard to organize things in a way that allows for code
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reuse.
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Specifically, I've worked on two projects where I've wanted to reuse sketches in multiple places,
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and in neither case could I really find a nice way to do that while using the Fluid API.
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I've found the Direct API to make this much easier.
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The Direct API is much closer to "normal" OOP programming, where objects are made by calling
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constructors, and therefore is more amenable to be organized using normal software engineering
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practices.
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Unfortunately, the documentation and examples for the Direct API are lacking.
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One of the things I found confusing was the class hiearchy.
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My understanding from looking at examples is roughly:
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* "Edges" are generated by connecting points, and are combined into "Wires".
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* "Wires" are combined into "Faces",
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* "Faces" can be extruded directly into "Solids" OR
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* "Faces" can be combined into "Shells" which can then be turned in "Solids".
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Or graphically (and a little more completely):
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```{mermaid}
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flowchart LR
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points ---> Edge
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Edge ---> Wire
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Wire ---> Face
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Face ---> Solid
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Face ---> Shell
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Shell ---> Solid
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Wire ---> Solid
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```
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The best example I found for using the Direct API was in a Gitlab issue [linked here][github-link].
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An example that I wrote which forms a cube by extruding a square from the "YZ" plane
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is below.
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``` python
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import cadquery as cq
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edge1 = cq.Edge.makeLine((0.0, 0.0, 0.0), (0.0, 0.0, 1.0))
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edge2 = cq.Edge.makeLine((0.0, 0.0, 1.0), (0.0, 1.0, 1.0))
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edge3 = cq.Edge.makeLine((0.0, 1.0, 1.0), (0.0, 1.0, 0.0))
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edge4 = cq.Edge.makeLine((0.0, 1.0, 0.0), (0.0, 0.0, 0.0))
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edges = [edge1, edge2, edge3, edge4]
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wires = cq.Wire.assembleEdges(edges)
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face = cq.Face.makeFromWires(wires)
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result = cq.Solid.extrudeLinear(face, (1.0, 0.0, 0.0))
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```
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[cadquery-homepage]: https://cadquery.readthedocs.io/en/latest/index.html
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[openscad-homepage]: https://openscad.org/
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[github-link]: https://github.com/CadQuery/cadquery/issues/497
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