Category Archives: vector

How To Laser Cut Really Big Halftone Photos

Posted on by KevinB

Wanting to up the ante a bit after having the Makerspace laser cutter chop out hundreds of city blocks to form a big map of MKE, I decided to laser cut a  24” by 18” halftone image!  As it required the laser cutter to carve 10368 circles out of an off-yellow piece of 98 Lb paper, the cutting took 1.3 hours and produced quite a bit of confetti.  I’ll display this with a purple (rather than black) paper behind the off-yellow laser cut paper.  In person there is an interesting transition from an abstract purple/yellow shape into a black and white image as one moves further away from the image.  You may even want to sit back from your monitor to improve the “image quality.”

Laser_Cut_Halftone_Image_color2

Check out this video of the laser cutter in the middle of cutting 10000 circles!  Note the mysterious logic employed by the laser cutter to determine the order of its cuts.

halftone_cutting

I imported a photo into GIMP, and desaturated it to produce a black and white image.  After bumping up the contrast and darkening it slightly to nearly saturate the darkest areas (and avoid any totally white areas), I brought it into Inkscape.  Inkscape can create halftones in a two step, manual process.  The first step is to draw an 8 pixel by 8 pixel circle in the upper left corner of the 1133×720 pixel image, and select Edit->Clone-> Create Tiled Clones.  To create a rectangular grid of halftone dots whose sizes are set by the color of the image below, use these settings:

Rectangular_array

From a quick test cut of a particularly dark area, I found that I needed to add an offset between each row and each column to account for the kerf of the laser.  I.e. the laser beam has a cutting width that is wider than that of the line, and so in the darkest areas of the photo the halftone dots overlapped, causing a large section of the paper to fully detach. That led me to make this test strip with 11 shades of grayscale, evenly spaced between pure black and white.  I laser cut this test strip with various offset distances between the rows and columns in order to arrive at the optimal 10% extra offset between adjacent rows and columns shown in the above settings. Note also that the smallest size circles may not even be exported from Inkscape due to their infinitesimal dimensions (i.e. if you export as a .pdf).  The minimum gap between circles with 42% speed and 100% power on an 1133 pixel wide image blown up to 24″ is 0.79 pixels, which is 0.017″.

test_tripApplying these same settings to the image created a 128 by 81 array of circles, for a grand total of 10368 vector objects.  In my first trial run last weekend, I found that sending this much data to our 60 Watt Universal Laser takes 5 minutes and results in a print error I noticed only after hitting start!  After 1.3 hours of vector cutting, I found that a few of the rows and columns were shifted a bit from their intended location.  It’s not clear whether this had to do with the print error, or if the paper moved slightly during the cutting process.

In order to improve the second version (shown at top), I chose to move away from the rectangular grid of halftone dots – recall that Kays and London teach that hexagonal close packing is for champions.  The reason to abandon the rectangular spacing is to improve the dynamic range (i.e. to make the blacks blacker).  For example, rectangular grids of circles pack at an “efficiency” of Pi/4, which is 79%, whereas hexagonal close packing results in a pi/6*sqrt(3) packing, or 91%.  That means that the darkest sections of the image will be darker, as more of the light colored “front” piece of paper can be cut away.  See the image below, and note that the hexagonal pattern does indeed appear darker.

rectangular_vs_hexagonal2

It turns out that Inkscape doesn’t easily permit this.  I ended up spending an hour or two fiddling with the column and row offset settings using my 11 black/white tone test strip to find settings that gave the hexagonal offset with the closest, even hexagonal spacing between adjacent circles.  The following settings worked great for an 8 by 8 dot on the darkest square of the test strip:

hex_arrayI test cut this yesterday, sending ¼ of the data at a time to the laser to avoid printing errors. However, part way through the cutting, cut-out paper circles stuck to the long air assist nozzle of the laser head (ironically) hit a washer I was using to weigh down the paper to prevent movement while cutting. The paper shifted by about 1mm, which was enough to make some adjacent halftone dots overlap and cause others to have a visibly wider spacing.

In the process of cutting that photo, Shane happened by and mentioned that vector cutting 10368 objects may be just as fast as the typically-very-slow raster cutting time.  With three clicks, I turned off the vector outline of the halftone dots, and selected a fill color.  After test cutting a row, I found that he was right.  Check out the difference between raster  (100% speed, 100% power) and vector (42% speed, 100% power) in the darkest section of the image – the area with the closest spaced circles:

vector_vs_raster2

The vector halftone dots are perfectly circular, though the edges are a bit rough.  Some of them have a very small border and so are a bit fragile.  The raster halftone dots are not very circular, but the edges are very smooth and the boarders are slightly wider.  I chose to raster cut the 24″ x 18″ image, and found that the raster cutting time of 1.4 hours was nearly equal to the 1.3 hour vector cutting time.

Note that many programs can create halftones, though often the results will not be suitable for laser cutter use:

halftone_error

The next step is to laser cut this image into wood.  Also, Inkscape will let you draw any shape to create tiled clones from – so please do share photos of any halftone images you create with star shapes!

Giant Laser Cut Map of Milwaukee

Posted on by KevinB

I’ve been on a laser cutting kick lately.  In the last two weeks, I made 9 travel coasters, two of which feature neighborhood maps of places I’ve lived.  Though I could have just raster cut these very small coasters, generating the vector version allowed me to create this big map of Milwaukee, Wisconsin!  This wall hanging map is the maximum size of our largest laser cutter: 24″ by 18″!  Boom!

Big_MKE

This map was inspired by a project made by my friend NJStacie a while back.  While she has both the infinite patience and the limitless awesome that allowed her to use an X-acto knife to cut out all the city blocks of Boston from an actual map, I used a laser cutter and software. To create images for my roadtrip coasters, I simply took screen captures of google maps, and processed them into vector files using GIMP and Inkscape.  There are so many extraneous details in google maps (lines for buildings, the text labeling street names, etc), that it was clear I needed an alternate approach for making this map.

Its easy to get a small google map without text labels, check out the url of this page.  My first approach to get more than 512×512 pixels was to use the Google Maps API, which is a toolset to imbed an interactive google map into webpages using Java.  The great thing about it is that the rendering style is completely configurable. Even better, there is a GUI to quickly configure your desired style, and automatically generate the JSON object to pass to the style property of the MapOptions on your webpage.  Instead of investing 10 or 15 minutes reading about how to integrate all these steps, I just created the style, and took a few .png screen captures.  I opened them as layers in GIMP and combined them to create the following grey and black image:

milwaukee_whole

I saved it as a .png, and imported it into Inkscape, selecting Embed Upon Import. I created vector data from this raster image by first selecting Path -> Trace Bitmap, opening a dialog box with many choices.  I really only experimented with the top two import choices, Edge Detect and Brightness Cutoff.  I found that Edge Detect gives two outlines, one of the streets and one of the city blocks.  For this reason, Edge Detect seems to be the best choice to create the widest streets, and therefore the strongest paper cutout.  It required some cleanup though, so I selected Path -> Break Apart, adjusted the Fill and Stroke, and then just deleted all the street outlines (thereby widening the spaces between buildings, which is effectively the streets).  As some of the streets were to narrow to really form one continuous outline, they formed a lot of smaller street segments that I deleted in five or ten minutes of fast and furious clicking.  After all those steps, a vector version of the following image was produced:

vector_lines_of_milwaukee

I did a few test cuts to find a power/speed that cut all the way though some colorful, 98lb, 25″ by 19″ acid-free archival paper I picked up.  The goal is to use enough power to cut though, without using too much power, which widens the kerf (laser cut width), thereby undesirably narrowing the streets.  This ended up being 100% power, with 52% speed. Check out the laser cutter in this real-time (not sped up) video.  Note that one typical problem of having both air assist and super-power fume-removal suction while cutting is that the laser cut bits tend to flip over into the cutting path, potentially resulting in an incomplete cut.  That meant when the laser cutting was complete, I had to carefully punch out the 15 or 20 stubborn city blocks that weren’t completely cut though.

GIANT Laser Cut Milwaukee Map!

I also cut this design into a coaster, and made one with my old Massachusetts neighborhood too.  Naturally this was a lot of data on a small surface, but the results are pretty good despite the vector cutting time approaching that of the raster cutting time! I cut these at 100% power, 100% speed, like the other coasters.

Map_Coasters_Improved

After I completed all these steps, I learned about a way to access vector map data directly.  The Open Street Map site allows export of .svg vector data just by clicking the share button on the right side of the page!  Even better, one can zoom in to Milwaukee, and press the big green Export button on the upper left to export an .osm database of the visible section of the map.  This OpenStreetMap archive can be opened in Maperative! and a style can be applied to the rendered map.  Maperative has several styles built-in, and I simply edited the google maps-like style to omit all the buildings, and draw all roads, highways, on-ramps, etc in a black with no border.  Maperative can export .svg files, but I found the content of these files are a bit of a wreck.  For example, each different road type is a separate vector path, meaning that there are many separate paths in the file.  Ultimately I found I’d taken the wrong approach, as I should have rendered all the city blocks as black vector outlines, and omitted the roads – as that is what I really need to laser cut.  With a bit more work, using Maperative would likely be a quite quick path from map to laser cutter. However, I abandoned this approach as I’d already created a somewhat reasonable workflow.