Reading the preview

The G-code preview shows exactly what the nozzle will do — not a render of your model, but the actual toolpaths. Two minutes here saves a failed print.

Press P to switch between the model and the preview. A slice you press brings you here by itself; a re-slice the slicer ran on its own leaves you where you were. Change that under Whether the view follows the slice.

Colour by

The Colour by dropdown repaints the whole preview. Each mode answers a different question.

Mode	Answers
Role (default)	What is each line for — wall, infill, support, bridge?
Speed	Where does it slow down? Sudden slow patches often mean cooling limits.
Flow	How much plastic per second — the real load on your hotend.
Line Width	Where beads get thin or fat. Useful for checking thin walls.
Layer Height	Varies only if you're doing something clever with layers.
Acceleration	Where the machine is being pushed. Ringing usually starts here.
Fan Speed	Cooling, per layer.
Temperature	Per-layer nozzle temperature.
Layer Time	Which layers are so quick they won't have cooled.

In Role mode the legend doubles as a filter. Click a role to hide it — hiding infill to inspect the walls underneath is the most common move. Travel moves and seams are hidden by default; turn them on to hunt down stringing or a seam landing somewhere visible.

Roles are grouped so you can toggle a whole family at once: Shell, Fill & Surfaces, Support & Adhesion, Movement & Markers.

Moving through the print

• Layer slider — or ↑ / ↓ for one layer at a time.
• Show all layers vs current layer only — the stack, or one slice in isolation. Isolation is better for looking at a specific problem.
• Progress slider — scrub within a layer. → / ← step move by move, so you can follow the nozzle through a tricky bit. The count beside it is how far into the layer you are — move 137 of 138, say — so you can tell whether the bit you're looking at is near the start of the layer or the end of it.
• On a touchscreen, arrow buttons flank both sliders, since there are no arrow keys to press. They step by exactly the same amount, and holding one runs through the layers — faster the longer you hold — the same way holding + on a number field does.
• Hover any line for its role, layer, Z height, width, height and speed.

Reading the file itself

The </> button in the toolbar docks the sliced file's text beside the scene, in a column of its own — the plate narrows rather than being covered.

The two views stay on the same move. Drag the layer or progress slider and the text scrolls to the line that emitted the move you are looking at, with that line marked. Click a line in the text and the preview jumps to it: the layer changes, the progress slider lands on that move, and the nozzle marker moves with it. A line that commands no movement — a comment, a temperature change — has no move of its own, so the nearest one above it is used.

This is the fastest way to answer "why is it doing that": find the odd-looking bead in the preview, and read the line that produced it.

The text is read-only. It is the slicer's output, so an edit would be thrown away by the next slice.

The column is hidden on a phone, where showing the plate and the file side by side leaves no room for either.

Pausing mid-print

Insert a nut, drop in a magnet, or swap filament for a two-colour print: pick the layer on the slider and use the buttons beside it.

• ⏸ Pause stops the print at that layer so you can reach in.
• 🎨 Colour change stops and asks the printer for a filament change.
• − removes whatever is set on the selected layer.

Each trigger shows as a mark on the slider — click one to jump to its layer. The pause happens at the start of that layer, before anything is printed on it, so set it to the first layer that would cover what you need to reach.

The command sent depends on your printer's firmware (Marlin M0 / M600, Klipper PAUSE, RepRap M226), and adding or removing a trigger means the model is re-sliced.

Folding it away

The legend and its controls are a tall panel sitting over the plate. Tap the G-code header at the top of the panel to fold it down to a single row — the layer counter stays visible, and the Slice button stays where it was.

Unfolded, the panel takes as much room as the screen has, so on a tablet or a desktop the whole legend and both sliders are visible at once. It only scrolls when the window is genuinely too short — and even then the Slice button stays put.

On a tablet or phone it starts folded, since the panel would otherwise cover most of what you're trying to look at. On a large screen it starts open. Either way, once you fold or unfold it yourself the slicer remembers, and stops guessing.

What to look for

Before your first print of a model:

• The first layer covers what it should and has your skirt or brim.
• Bridges actually span a gap rather than hanging in air — switch to Role and look for bridge-coloured lines.
• Supports touch what needs supporting and nothing that doesn't.
• The top surface is continuous, without stray dashes or gaps.

When something printed badly:

• Blobs or zits → look at seam placement.
• Stringing → turn on travel and look for long moves across open space.
• Poor overhangs → check fan speed and layer time on those layers.
• Ringing → check acceleration and speed around corners.

Moves that leave the bed

Anything the preview draws in bright red is a move the printer cannot make, because one end of it is off the bed. It stays red in every colour mode, so switching to Speed or Fan won't hide it, and a note at the top of the viewer counts how many there are.

The usual culprit is not the model at all — it's a skirt, a brim or a prime line with enough distance or loops to push it over the edge of a full plate. Reduce the skirt distance or loop count, shrink the brim, or move the model inwards.

The stale-preview rule

The Re-Slice button turns amber the moment you change a setting or move a model. The preview you're looking at is from before that change. Re-slice before you trust it.

Most of the time it re-slices itself a second later and the amber clears on its own — that's the flash button beside the model / preview toggle, and it holds off automatically on plates slow enough that you'd rather choose the moment. See Re-slicing on its own.