Layered Paper: Masking the Cornell Painted Warblers

After getting better at aligning printed sheets for laser cuttery and learning how to use GIMP’s Path Edit tool, a pair of Cornell Painted Warblers emerged!

We call this one “Happy Bird”:

And his companion “Pensive Bird”:

The page numbers refer to the PDF file containing the outlines of all six birds.

The wing feathers and butterfly require finer resolution than the black mask layer allows, so the first layer is printed on white paper. I put the berries and nut-like objects on that layer, too, mostly to reduce the number of layers.

After considerable fumbling, it turns out GIMP provides a straightforward way to create the outlines required for the top Black Mask, from which all the other layers descend.

A closeup of Happy Bird’s head as imported into GIMP from the PDF at 300 DPI = 11.8 pixel/mm:

Because the Cornell Warblers were intended as a paint-by-number project, the PDF has (gray) color numbers in (most of the) regions. Selecting the numbers with GIMP’s Select by Color tool and hitting Delete clears the region for Bucket Fill colors copied from the key sheet with the Color Picker tool:

I cropped the images to emphasize the birds and scaled the result to fit the 8×10 inch frame size. The picture is 172×220 mm within the red 200×250 mm frame cut from Letter size sheets of Art Cover paper stock. The corner targets come into play while aligning the paper with the laser pattern.

Creating the Black Mask layer also starts from the cropped / scaled image. Select the black lines by color, then Grow the selection by 7 pixels = 0.6 mm to make the selection about 1.5 mm wide:

Use Quick Mask to edit the selection. For example, I reduced the eye to a cutout through the mask revealing the white spot on the printed layer:

Eventually the selection will neatly bracket the borders between the colors:

The black legs and feet must be edited down to their original size, as became obvious on the first iteration:

The fixed-up Quick Mask looks like this:

The mask includes a 6 pixel = 0.5 mm border line around the perimeter of the image which serves as the outer edge of the cut-out areas and helps match the selections to the image.

The process of growing & tweaking the selection will create orphan pixels:

Each orphan pixel becomes a tiny triangle when imported into LightBurn:

You’ll first notice those when the laser hesitates while cutting the outlines, which may overcook the adjacent paper. I cannot reliably detect these other than by watching the laser, because a single pixel is simply too small to see on-screen at normal zoom sizes. You can delete them in the LightBurn file or, if you prefer a single source of truth, in GIMP to regenerate the whole thing.

The end result of all this is a single, rather complex, Selection that you must convert into a Path, which you then export to an SVG file and import into LightBurn for laser cuttery.

Next: Aligning and layering