three -bears plaque - full process to develop from bitmap

Converting a bitmap picture to a multipart, multilayer print file.

picture from here:

https://soundcloud.com/listenbearilybear/a-story-of-goldilocks-and-the-three-bears-1

This was an experiment in using build123d to build a multipart printable model from a bitmap via an Inkscape svg route. The build123d attempt was not really successful, as it is well-near impossible to get the positive and negative shape flow correct for the busy detail in the model. In the end I used Blender, which did a reasonable job. Perhaps detailing the process might be useful for someone.

I have this fixation on making printable multipart models from SVG files, mostly using inkscape as drawing tool. I have yet to find the perfect workflow for complex designs with lots of detail. Yes, you can do multicolour by using the slicer's paint-on tools, but this does not really support complex detail models, for non-destructive work flows.

In this case I wanted flat coaster-like part, in three layers: the frame, the three-bear shape and line detail on the bears, respectively in layer thicknesses of 2, 2.6, and 3.2 mm.

Using inkscape to trace the bitmap was not easy because of gradients in the image (even after extensive image manipulation). The tracing algorithm leaves you with too many node control point clutter to edit manually. So I ended up doing a simple manual trace on the outline details I required. These lines were then given width (0.5 mm) and transformed from strokes to paths. This gives the finite printable width shapes. From these line traces the outline shape was formed by splitting the trace lines at the perimeter and joining them (this process is not super exact and some misalignment appears). Three svg files were exported: the frame, the outline and the trace lines.

Importing into build123d went well, but the shapes did not work well: there was no consistency in the various shapes' positive and negative rotation, hence the fill did not work well. Inkscape has a means to see the rotation of the shapes, but I was not successful in finding the mechanism. It was evident that build123d can handle it, but the export from inkscape was not consistent. Aborted this workflow, I might come back to it later.

Edit note: in inkscape, to see and change the shape path direction do this: (1) You can visualize the path direction by going to Edit > Preferences > Tools > Node Tool and selecting the option to "Show path direction on outlines". Arrows will appear on the path when selected with the Node tool, indicating the current direction. (2) Select the path using the Selector tool or the Node tool. Go to the menu and select Path > Reverse. The path's start and end points will flip, reversing the direction,

Blender can import svg files as 2-D shapes from which extrusions can be formed, see here https://www.youtube.com/watch?v=shCZ2zjR5CU. In this case with the many line traces, the attempt to join the traces was not working well because of, again, the shape rotations - so I just edited the trace shapes one at a time. Follow the instructions in the youtube video by making a mesh from the 2-D shape and then extrude. I found the best way to extrude is to use the solidify modifier https://www.youtube.com/watch?v=rodbBWVMZno.

One of the issues with inkscape to build123d or Blender transfer is that the coordinate references are lost and you have to align the meshes manually once imported into Blender. This is best done by using x-ray mesh view to see al the vertices and edges - a bit messy but it work ok.

Once the meshes are extruded, check for errors in the 3-D print toolbox. Not all errors are fixed this way, but it helps a little. Export to STL, import into the slicer and if necessary, fix errors again.

The 3mf file has a colour change to print the top three layers (the trace lines) in another colour from the rest.