Automated Cookie Cutters: Image Preparation Checklist

With the Bash script and OpenSCAD source in hand, here’s how you go about producing a grayscale image that works as the height map file to produce a cookie press and matching cookie cutter.

The Big Picture description: the grayscale height map image looks a lot like a photo of the final cookie on a plate in front of you. The darkest regions mark the thinnest parts of the cookie: black lines emboss deep trenches. Gray regions denote thicker sections and very light gray will be thickest. Because image files are rectangular, a pure white area surrounds the region that will become the cookie press and acts as a mask to remove the rectangular border.

If you start by hand-drawing the shape of the cookie press at full size inside a 5 inch square (chosen to match the 3D printer’s build platform and in inches because that’s how it got measured; it’s not my printer) with a 1.5 or 2 mm black marker, then the marker lines will be just about exactly the right width to ensure good plastic fill (for a printer producing a 0.5 mm thread width, anyway) and printable walls. You can scale the drawing for smaller (my Thing-O-Matic) or larger (M2, Series One) platforms, but the thread width and minimum wall thickness do not scale: a tiny 1 inch push mold must still have 2 mm walls.

The workflow looks like this:

  • Draw cookie press lines at full scale with fat black marker on white paper
  • Scan a 5×5 inch (127×127 mm) square around the image at 300-ish dpi → 1500×1500 pixel image
  • Convert a full-color scan to grayscale now (better to scan in grayscale)
  • Resize image to 317×317 pixel, optionally set 2.5 pixel/mm = 63.5 dpi for proper display size
  • Set color levels to blow out the contrast; auto probably works fine
  • Threshold to reduce to just two colors: 0% = black and 100% = white
  • Clean up the image: remove specks and single-pixel bumps, fill small gaps

Some sample images to show what happens along the way…

A hand-drawn image, derived from The Original Tux by crudely tracing the outline with a fat Sharpie, including some areas outside the box for pedagogic purposes:

TuxTrace - raw scan
TuxTrace – raw scan

The interior edge of the black box is exactly 5×5 inches. I created a 5×5 inch blank white image at 300 dpi, enlarged the canvas to 5.2×5.2 inches with the blank white image centered in the middle, set a black background, flattened the image to fill the border, and printed it out. That produces a piece of blank paper with a black square suitable for full-scale drawing.

It does not, however, confer any artistic eptitude whatsoever, so for this drawing I imported one of the Tux drawings, isolated the edges with one of The GIMP’s edge detectors, and traced over the thin lines with the aforementioned fat Sharpie. You can draw whatever you want, however you want it. If you already have an image file, you need not print it out and scan it back in; just resize it appropriately.

Pro tip: Ensuring that the drawing doesn’t touch the black square will greatly simplify the next half hour of your life.

A note on Sharpies. I used a Fine Point Marker, which is much fatter than a Fine Point Pen. The whole, uh, point is to produce a line about 2 mm wide that will become an actual plastic wall; you can’t print anything much finer than that.

A note on blackness. There’s no requirement for any black lines whatsoever. For most cookie presses, however, you want distinct walls that emboss lines into the dough, which is what the black lines will do. If you want to mold a cookie (or anything else, like a butter pat), you can produce a gently curved push mold by using only grayscale areas. For example, a circular area with a radial fill going from very light gray on the exterior to very dark gray in the center will produce a round cookie with a conical dent in the middle.

Given a drawn image, scan the area just inside the black square at 300 dpi to produce a nominally 1500×1500 pixel image, then resize it to 317×317 pixel at 63.5 dpi:

TuxTrace - crop resize
TuxTrace – crop resize

The magic number 317 comes from not wanting OpenSCAD to fall over dead after chewing on a larger image for an hour or two. Given the restriction to maybe 330×330 pixels, max, I picked a nice round 2.5 pixel/mm scaling factor that converts a 5 inch = 127 mm distance into 317 pixels:

317 pixel = 127 mm x 2.5 pixel/mm

The magic number 63.5 comes from wanting the image to print (on paper) and display (on screen) at the proper size:

5 inch = 317 pixel / 63.5 pixel/inch

Given a properly sized image, blow out the contrast so the background is mostly white and the lines are mostly black. This gets rid of background cruft:

TuxTrace - color levels
TuxTrace – color levels

Then apply a threshold to get rid of all the gray levels. The threshold level determines the line width (the edges shade from black to gray to white), so you can tune for best width. The result doesn’t look much different than the blown contrast version, but the lines will become thinner and more jagged.  Remember that you want the lines to be at least three pixels wide:

TuxTrace - threshold
TuxTrace – threshold

Do whatever cleanup is required; eliminate single-pixel bunps and dents, fatten (or, rarely, thin) lines as needed. If you draw with a 3 pixel wide pen, the line will print just over 1 mm wide, which is about the thinnest possible wall and may encounter problems at corners. Use pure 0% black and pure 100% white.

If you possess powerful art-fu, you can draw that kind of image directly in the graphics program. Those of us with weak art-fu must rescale a found image of some sort. Should you draw a new image or rescale an old one, then:

  • Start with a 317×317 pixel grayscale canvas in 100% white
  • Draw lines with a 3 pixel (probably a square) 0% black pen

Now you have a clean black and white image of the cookie press lines; it’s still a grayscale image, but using only two colors.

  • Use color levels to reduce the white to about 95% gray; this avoids interior islands
  • Bucket-fill the exterior with 100% white (interior remains 95%): no anti-aliasing or blending
  • Fill interior regions with grays to set cookie press depths: dark = low, light = high, no 100% white, no anti-aliasing
  • Save as PNG to avoid compression artifacts

By reducing the overall white level to 95%, you get rid of all that pure white in the whole interior. Remember that pure white marks the area outside of the press, so any white inside the press will produce bizarre islands. You could pour 95% white into all the interior areas, but if you miss one, you have an island.

Having reduced all the whites, pouring pure 100% white around the press restores the exterior mask color. Turn off the anti-aliasing / blending / feathering options, because you want crisp edges rather than nice-looking gray transitions.

If all you want is a press with lines, you’re done. Save the image and proceed to make the cutter & press.

If you want a press that produces a cookie with different thicknesses, do some gray pours. For example:

TuxTrace - grayscale height map
TuxTrace – grayscale height map

That’s obviously contrived, but the general idea is that the feet and beak will be the thickest part of the cookie, the tummy next, and the body will be the thinnest part. The glint above one eye will become a bizarre peak, but that’s to show why you probably don’t want to do that. It’s not obvious, but the eyeball pupil and sclera areas will be recessed into the body.

If you’re doing a push mold, elaborate grayscaling will make a lot more sense. For a cookie press, black is where it’s at.

That process produces a clean grayscale image. Save it as a PNG file to avoid JPEG image compression artifacts: you want crisp lines and flat areas that define heights, not a small file. It’ll be small enough, anyway, compared to the eventual STL files.

To review, the grayscale height map image must satisfy this checklist:

  • Maximum 317×317 pixels: smaller is OK and will print at 2.5 pixel/mm; larger may not work
  • Exterior pure white: 100% = 255/255
  • Four corners must be 100% white to allow proper auto-cropping
  • No interior pixels are 100%: at most 99.6% = 254/255 will be fine
  • All lines at least 3 pixels wide: will print at 1.2 mm = (3 pixel / 2.5 pixel/mm)
  • No speckles or stray dots
  • Clean lines with no single-pixel bumps or dents: they’re hard to print
  • Saved as PNG to preserve crisp lines and areas

Then hand the file to the Bash script, do something else for an hour, and get a pair of STL files.

To get higher resolution, you could use Shapeways’s online 2D-to-3D Converter, although it seems to produce STL files with many reversed normals. The press and cutter would require different height map images, of course, but I betcha ImageMagick could produce them for you. The PNG23D project may be of more than passing interest. Note that their recommended resolution matches up just about exactly with my 2.5 pixel/mm default, so higher resolution may not pay off the way you think.

In any event, for this example the height map file shown above is TuxTrace.png and all the output files use TuxTrace as a prefix.

The cookie press (TuxTrace-press.stl):

TuxTrace-press - solid model
TuxTrace-press – solid model

Notice that Tux has been reversed from left-to-right, the darkest parts of the original image correspond to the tallest lines, and that glint over the eye became a triangular depression. All that makes sense when you imagine pressing this shape onto a layer of dough rolled out over the kitchen cutting board.

The cookie cutter (TuxTrace-cutter.stl), with a stiffening lip extending on both sides of the cutting blade:

TuxTrace-cutter - solid model
TuxTrace-cutter – solid model

The press probably won’t slide into the cutter, because I set things up to use the same dimensions, and certainly won’t fit inside the inner lip on the build platform. Another Minkowski around the press to add half a millimeter or so would let them nest together, at the cost of even more computation time.

Those nicely shaded blue images come from MeshLab screenshots, which you can (and should!) install on your Linux box without any hassle at all.

The “blade” isn’t particularly sharp, due to the fact that we’re printing blocky pixels. I produced a very thin blade for the original Tux Cutter by using a finicky collection of settings, but that won’t produce a corresponding press.

The surface that OpenSCAD builds from the height map image has slightly tapering walls, because that’s how it ensures a 2-manifold 3D object. The base of the walls will be slightly wider than the grayscale line width and the top will be slightly narrower. This produces a tapered edge, which is probably what you want for a cookie cutter, but it means you must make the lines wide enough to ensure good fill along the top of the wall.

The G-Code produced from the height map image above looks like this at the base of the walls on the press (as always, clicky for more dots):

TuxTrace-press - G-Code Layer 27
TuxTrace-press – G-Code Layer 27

The same walls become much thinner on the top layer, including a few single-thread sections:

TuxTrace-press - G-Code Layer 35
TuxTrace-press – G-Code Layer 35

Moral of the story: draw with a chunky marker!

Bonus lesson: always analyze the G-Code before you build anything…

The Bash script produces several intermediate images and data files along the way; delete them if you like.

A cropped / rotated / de-commented / contrast-stretched image (TuxTrace_prep.png):


An image (TuxTrace_plate.pgm and .dat) that defines the outside edge, with no interior detail, to shape the cutter outline:


An image (TuxTrace_map.pgm and .dat) that defines the height map for the press surface:


That one is actually identical to the incoming PNG file, just converted to an ASCII image file format.