Wednesday, February 22, 2017

Creating a Stencil from a Bitmap Image

I began searching online for a Minecraft stencil when my son (5th grade) had a school assignment to find a way to earn money to donate to the Make a Wish Foundation. He wanted to use bleach to stencil tee-shirts with MineCraft artwork to sell to his friends at school. We supported his efforts and purchased a small supply of shirts from the local variety store (Dollar Tree).

We first tried cutting stencils using Freezer Paper and ironing that onto the shirt. It worked for some shirts but the bleach had little impact on some colors. So back to the craft store for some fabric spray paint. The freezer paper stencils did not hold up well when the kids used the shirt spray paint. The stencil wanted to peel off of the shirt. We discussed coming up with a reusable stencil that would give us more detail and hold up better. 

This seemed like a great job for the laser cutter but I was not sure how to make a stencil. It took an an entire evening of working with a drawing program but I finally figured out a process that seems to work. Here are the steps I followed.

First, find a bitmap of the Minecraft item that you like. I used a Google Image search to find the image below. 


Here is how that image appears in CorelDraw. I will leave all of objects selected to make it more clear when we shift between a bitmap and objects. 


Next use your favorite drawing program to auto-trace the image. For CorelDraw you can get good results using Trace: Outline for High Quality Image. Move the Detail slider all the way to the right to ensure you get all of the squares. Smoothing did not seem to matter and I used 0 (slider all the way to the left) for Corner Smoothness. If the tracing went well you will get a group of objects that looks similar to the original image. Note the new selection handles for the newly created objects.



Next select all of the traced objects and apply a black fill.


Now change the Outline to 8.0 pt, solid line, in white.



This looks like a stencil and can be used if you want to print it and cut by hand. I read that you can get great results if you print on card-stock and laminate the card-stock before cutting the pattern out with an Xacto knife. But I want to use a laser cutter so there are still a few steps left. 

The laser cutter wants lines showing where the beam should cut. So we want lines that define the outline of the black boxes in the image above. But those black boxes are actually the fill inside the boxes defined by the 8.0 pt white rectangles. So we need to convert the fill of each square into it's own object without the white outline. 

I suspect there are easier ways to convert the black fill to objects but I did it by converting the objects to a bitmap (150 dpi, Black and White, 1-bit) and then tracing the new bitmap. Below is the bitmap produced when you convert the objects to bitmap. 



Next we Trace Outline for High Quality Art.  Set the Detail slider full right, and both Smoothing and Corner smoothness full left. You should get a group of objects like shown below. Again notice the selection handles for the new objects created by tracing.



Finally, we select all of the objects and set the Outline to Hairline, solid black line, set the fill to none and adjust the size to match the desired stencil size. You are limited only by the cutting bed size of your laser cutter. Here is the pickaxe on a letter size sheet. 

Finally save in the preferred file format for your laser cutter, I used DXF, and you should be ready to cut the stencil on your laser cutter.

The laser will cut along the lines, giving you a stencil. I ended up using thin plastic sheet that worked well. Below are some photos of the stencils and shirts the kids make using them. The green images are fabric paint. The lighter images were made applying bleach to black fabric.






















We were pleased with the results. I hope this workflow can help you create your own stencils!






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