How to use the
linedraw.py module to vectorise images¶
linedraw.py module converts bitmap images to vectors.
Typically, you will find it more convenient not to run linedraw on the Raspberry Pi itself. Even on a reasonably powerful computer, vectorising images can take several seconds or even minutes. On a Pi, that will be slower still.
In addition, you will often want to check the result of the vectorisation before drawing it, so it makes much more sense to do it on a computer with a screen attached, which may not be the case for your plotter.
The main use you will have for linedraw is to take a bitmap image file as input, and save the vectorised version as:
an SVG file, to check it
a JSON file, to draw with the BrachioGraph.
linedraw, you’ll first need to install
opencv-python-headless (it’s not included in
pip install opencv-python-headless
Using key functions¶
Launch a Python shell, and import
linedraw so you have the functions at your disposal:
from linedraw import *
Convert an image to JSON using
images directory, you’ll find an image named
To convert it to a JSON file in the same directory, run:
image_to_json("africa", draw_contours=2, draw_hatch=16)
find a file in
draw its contours, and hatch lines
create a JSON file called
create an SVG file called
A value of 0 for
draw_hatch will turn them off.
Smaller values mean more detail, higher values mean less. It’s worth experimenting with these values. Start with a
draw_contours of 2, and then values between 0.5 and 4. Start with a
draw_hatch of 16, and then values between 8
Emphasise the edges¶
You can also provide a value for
repeat_contours (or even
repeat_hatch, though this is less useful).
repeat_contours=3 means that the contour data will be added to the JSON file three times in
succession; the effect will be to draw them three times instead of just once, so the edges of the final image stand
out. This is especially effective with pencil drawings as in the example below.
Use the SVG file to check¶
Check the results by opening the SVG file. You can draw the JSON file with
Convert an image to lines using
If you’d like to get hold of the lines in code to process them in a shell or script, use
lines = vectorise("africa.jpg", draw_hatch=16, draw_contours=2)
(This is in fact what
This will generate two things:
a list of
lines, each of which is a list of points
an SVG file as described above, to give you an idea of the vectorised representation
See vectorise() for full details of the parameters it takes.
Visualise how the plotter will draw the lines using
draw() takes a set of lines (as generated by
vectorise()) and uses the Python turtle graphics module to draw
them, sequentially. It’s fairly slow - but faster than the actual plotter.