Digitising hand drawn sketches

Every so often I want to quickly sketch out what should be a simple diagram. Irrespective of what drawing program I use, I always seem to take much more time than I intend for a result that does not even remotely resemble what I want.

So I decided to give up and find a way to use hand-drawn sketches instead. Here’s the method I ended up with, based almost entirely on Marc Liberatore’s "Whiteboard Diagrams as PDFs" post and the wonderful ImageMagick and Potrace utilities.

My drawings still look fairly terrible, but at least they convey what I want them to and are quick to produce! :)

Ingredients

  • Paper / whiteboard
  • Marker
  • Phone with camera
  • ImageMagick: brew install imagemagick
  • Potrace. I used the binary distribution, although it’s also on Homebrew.

Method

Commit sin against art

Here’s a sample result of me unleashing my inner da Vinci on a poor, defenceless bit of paper.

I don't know much about art, but I repeat myself
I don’t know much about art, but I repeat myself

I’ve found a thick texta/marker works well, but standard ball-point pens can come out alright too.

Photo cameras: not just for tweeting lunch

Next, take a photo with ye olde phone camera. I avoid using the flash - even light (no shadows) is best. (Try holding the paper up vertically so your phone does not cast a shadow over the paper.)

My phone auto-uploads photos to an online thingoe from which I can quickly crop the image and download to my Mac.

Post-processing

Next up I want to convert the photo to a grayscale bitmap and turn up the brightness and contrast to wash out the background and bring out the marker lines. I’m using ImageMagick’s convert to quickly do this from the console.

We’ll then run the bitmap through potrace as described in Marc’s post to create a nice SVG. We can stop there, or use ImageMagick again to get a PNG out.

Here’s the original photo, which I’ve cropped and saved as sketch.jpg:

Taken with no flash. Cropped and downloaded with no processing.
Taken with no flash. Cropped and downloaded with no processing.

Then the adjustments:

# Convert to grayscale BMP. Dial up brightness (20) and contrast (10)
% convert sketch.jpg -colorspace Gray -brightness-contrast 20x10 sketch.bmp

# Convert to SVG (-s), set a reasonable height, smooth speckles (-t 10)
% potrace -s -H 400pt -t 10 sketch.bmp

# Convert SVG to PNG (using 256 colours)
% convert sketch.svg PNG8:sketch.png

And here’s the output:

The end result
The end result

You may need to tweak settings like brightness, contrast, dimensions, PNG quality/size1 and so on.


  1. Might be worth also running Pngcrush or similar optimiser over the resulting PNG: brew install pngcrush; png crush sketch.png sketch2.png. A GUI option for Mac is ImageOptim.

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