Multi-Color Pen Plotting: Split a Photo into Pen Layers

Split a photo into CMYK, RGB or custom pen layers for multi-colour plotting, starting with a downloadable colour test chart to see what your pens can do.

By DrawingBots · Published

How colour separation works

A pen plotter holds one pen at a time. To plot a photo in colour you split it into one drawing per pen, plot them one after another on the same sheet, and let the lines overlap and mix on paper. Image to SVG does the splitting. Pick a mode under Colour and every algorithm produces one layer per pen instead of a single black drawing.

  • Output picks the pen set. Monochrome is one pen. CMYK separates into cyan, magenta, yellow and black, the printing primaries; turn Include K (black) off for a three-pen plot. RGB puts ink where a channel is strong, which is meant for red, green and blue pens on black paper. On white paper it draws the background. Custom lets you enter the pens you actually own.
  • Method decides how pixels are assigned. Classic split separates ink channels directly, so a purple pixel contributes to both the cyan and the magenta layer. Colour match gives each pixel to the single nearest palette colour, which suits a custom set of pens that do not mix like inks.
  • Tuning hides two sliders. Ink gamma is the response curve from tone to line density; a higher value keeps light areas cleaner. Distance power (custom mode with colour match) controls how strongly a pixel prefers its nearest colour over the runner-up.

The downloaded SVG has one <g> per pen, with ids such as cyan, magenta, yellow, black or custom-1, and every path inside carries the pen’s stroke colour. Plotting software that reads layers or groups by colour (Inkscape with a plotter extension, vpype, the AxiDraw and NextDraw tools) can plot them one at a time with a pen change in between.

A colour test chart to plot first

Before you spend an evening on a portrait, plot a chart that shows how your pens and settings behave. The one below is generated rather than photographed, so each region tests one thing.

Colour test chart with primary blocks, a hue sweep, a grey ramp, pastel gradients, fine stripes and simple shapes
color-test-chart.png, 800 × 800 pixels. Download image.
  • Six solid blocks (cyan, magenta, yellow, red, green, blue): does each land on one pen, or on two?
  • Hue sweep: where do the transitions between pens fall?
  • Grey ramp: how does line density track tone, and where does the black pen take over from mixed colours?
  • Soft pastel discs: how are gradual mixes handled? This is what skin and sky look like.
  • Stripes and checkerboard: the finest detail a given line count can resolve.
  • Star, ring and triangle: clean edges and a thick coloured outline.

Plot it once with your real pens and keep it. It tells you which colours your set can make and which it cannot.

CMYK versus RGB versus a custom palette

I converted the chart four times with Squiggle at default settings, changing only the colour mode.

Chart split into cyan, magenta, yellow and black squiggle layers
CMYK, classic split: 4 layers, 200 paths. Red becomes magenta + yellow, green becomes cyan + yellow, and the grey ramp goes to the black pen.
Chart split into red, green and blue layers with ink on the white areas
RGB, classic split: 3 layers, 150 paths. Ink lands where each channel is bright, so white paper is covered; use this mode for light pens on dark paper.
Chart matched to four pastel pens
Custom, colour match, 4 pens (pink, mint, violet, gold): 200 paths. Every pixel gets a pen, so the white background is drawn by the nearest colour, gold.
Chart matched to four pastel pens plus white
The same palette plus #ffffff: 5 layers, 250 paths. The background now goes to the white layer, which you just leave unplotted.

Two rules of thumb come out of this. Use CMYK when your pens are close to printing primaries, which many fineliner and gel sets are. For any other set use Custom with Colour match, and put your paper colour in the palette so the background lands on a layer you can delete instead of being forced onto a pen.

Open this design in Image to SVG · settings: color-test-chart.png · Squiggle · Output Custom · Method Colour match · palette #ff5fa2 #22c1a6 #7b61ff #ffb000 #ffffff

A real photo in colour

Photos are more forgiving than the chart because their colours are already mixed, but a light, pastel image like the corgi shows the same trade-offs.

Corgi photo split into CMYK squiggle layers
CMYK Squiggle: 200 paths. Most of the image is yellow and black; the pastel background barely inks.
Corgi photo matched to orange, pink, blue, dark grey and white pens
Custom Squiggle, colour match, palette orange, pink, blue, dark grey and white: 250 paths. The fur goes to the orange pen and the background to the discarded white layer.
Corgi photo as stipples in four pen colours
Custom Stipples, four pens without white: 8,000 dots, 2,000 per layer. The dark pen dominates because every layer gets the same dot budget.

For a custom palette, sample the photo. Pick the two or three colours that cover most of it, add a dark pen for eyes and shadows, and finish with the paper colour. Keep the pen count small. Every extra pen is another pen change and another chance for the paper to shift, and colour match spreads the ink thinner across layers.

Dot algorithms share their Max Stipples budget evenly across layers, so a colour that covers a small area still gets a full set of dots and looks heavy. Line algorithms such as Squiggle scale with the area each colour covers, which makes them the easier place to start.

Pens, plotting order and registration

  • Plot light to dark. For CMYK that usually means yellow, then cyan, then magenta, then black. A dark pen drawn first smears into the lighter ones and shows through thin lines.
  • Do not move the paper. Tape it down or use a registration frame, and keep the machine powered between pen changes so it does not lose its home position. A corner mark drawn on a scrap sheet lets you check that nothing shifted.
  • Match the pen width to the line spacing. Layers add up: four overlapping squiggle layers at 50 lines each put four times the ink on the page. Raise Ink gamma or lower the line count for multi-pen plots.
  • Let each layer dry before the next pen, especially with gel and paint pens.
  • Check the layers before plotting. Open the SVG in Inkscape and look at the Objects or Layers panel. Each pen is a group. Delete the white layer from a custom palette and confirm the groups carry the colours you expect. Plot Preview shows how the combined drawing sits on your paper.

Troubleshooting

  • A pen draws the whole background. You used RGB mode on white paper, or colour match without the paper colour in the palette. Switch to CMYK, or add white to the custom palette and skip that layer.
  • Colours look muddy. Too many pens or too much ink. Reduce the line count per layer, raise Ink gamma, and plot light colours first.
  • A colour is missing from the plot. Its layer may be very sparse. Check the group in the SVG. With colour match, lower Distance power so pixels are shared more generously.
  • One layer is much denser than the others. With dot algorithms this is the shared dot budget. Use a line algorithm or reduce Max Stipples.
  • Layers do not line up. The paper or the home position moved between pens. Re-home, tape the paper, and plot a small test first.

The figures on this page are browser exports rasterised for the web. The pen advice is ordinary plotting practice, but I have not plotted these particular files. For the single-pen basics, start with the image-to-SVG guide. Doodle Mosaic takes a different route to multi-pen work: it can split a mosaic into pen layers by tone or by category.