Enlarging illustration without melting the linework

Illustration fails differently from photography when you enlarge it. A photograph that loses a little sharpness still reads as a photograph; a drawing whose lines have gone soft reads as a mistake. Getting this right is mostly a matter of not using the settings that were designed for photographs.

Why line art is the harder case

A photograph is texture almost everywhere — skin, fabric, foliage, noise. A model that reconstructs texture convincingly will make it look good.

A drawing is the opposite: large flat areas, and thin high-contrast lines carrying nearly all of the information. Adding texture to a flat fill makes it look dirty. Softening a line by even a pixel changes the weight of the stroke, and on tapered line ends, it changes the drawing's character.

So the goal when enlarging illustration is not "add detail". It is keep the lines exactly as they are, at a larger size.

Use the illustration model

There is a model tuned for illustration alongside the one for photographs, and the difference is visible on any image with clean linework. On an Apple Silicon laptop, a 1024-pixel-square image at x4 finishes in about 3.9 seconds with it.

The photo model will produce something that looks acceptable at fit-to-window size and wrong at 100%: line ends slightly rounded, thin strokes fading, flat areas picking up a faint texture that was never in the drawing.

Keep the scale factor conservative

x4 asks the model to fill in four times as much area as x2. On texture that is usually fine. On a drawing, more invention means more opportunity to change a stroke.

Start at x2. Go to x4 only when the destination genuinely needs that many pixels — a large print, for instance — rather than as a default.

Be careful with noise reduction

Noise reduction helps compressed photographs. On clean line art it can flatten the subtle variation inside a stroke, which is what makes a hand-drawn line look hand-drawn.

If your drawing came out of a paint application and was saved once as PNG, there is no noise to remove. If it was saved as JPEG or passed through a chat app, then there is, and a moderate setting is worth trying.

What to look at when checking the result

Compare at 100%, not fit-to-window, and look specifically at:

• Line ends and tapers. These are where softening shows first.
• The thinnest strokes in the drawing. Hatching, eyelashes, hair strands.
• Flat fills. They should stay flat. Any texture that appears was invented.
• Colour boundaries. A halo along a hard colour edge means the setting was too aggressive.

What this will not do

It will not redraw anything. If the original is a small, heavily compressed thumbnail of a drawing, the linework in it is already damaged, and enlarging it produces a larger version of damaged linework rather than a clean one.

It also will not change the drawing. That is deliberate: the models here reconstruct what is present rather than generating what might plausibly be there, which is the property you want when the character has to stay on-model.

Processing runs entirely in your browser, so unreleased work never leaves your machine — which for illustration is usually the point.