Set and Check True Size

What you are trying to accomplish

Tell PatternForge how the photographed pattern relates to real measurements, correct the camera angle when needed, and confirm the result against a known length.

“Scale set” and “True size — ruler measurement recorded” are not the same claim. Setting creates a scale. The second label means the printed reference was measured with a physical ruler and the matching line in the photo passed PatternForge's check.

Why it matters

Tracing can be visually perfect and still print at the wrong size when the photo scale is wrong. A single measured line can set overall scale, but it does not remove perspective distortion from a tilted photo. Four-corner correction handles the camera angle; the verification measurement checks the resulting size.

For a pattern started blank with no photo, PatternForge draws directly in real measurements, so photo calibration is not required.

Photo distance

Known real size

Step by step

1. Choose Set & Check Size in the workflow ribbon. In the first-pattern guide, Set and check size opens the dependable manual wizard directly so there is only one recommended next action.
2. Outside the guide, the Scale panel keeps both choices available. Use SET & CHECK SIZE BY HAND as the dependable path. Try automatic card finding is a secondary Beta assist.
3. In Check your printed card, choose the sizing card you actually printed. Measure its labelled line with a physical ruler, then type the printed length and what your ruler reads. A custom reference must be at least 50 mm (about 2 in) long.
4. In Correct perspective, use the on-screen order guide to place points on the four printed ring targets: top-left, top-right, bottom-right, then bottom-left. The rings on the printed card are not numbered. Zoom in and refine each point before continuing.
5. Choose Preview correction and inspect the straightened image.
6. In Set the true scale, tap both ends of the same printed verification line. Choose Use this checked size only when both the ruler entry and photo check pass.
7. If the result shows Caution, refine the points, check that the correct card was selected, and repeat. Do not turn a caution into approval by guessing.
8. If the photo was already taken straight down, you can open Flat-on photo? Set the scale by hand instead. Choose the card, confirm the printed line with a ruler, use Use Measure tool, enter the True length, and choose Set scale.
9. After scale is set, you can measure a second known length, enter Expected, and choose Check measurement. Use a reference at least 50 mm (about 2 in) long; it must be within both 1.5 mm and 1% to pass.

Try automatic card finding is a Beta assist. It can find the printed card and straighten the photo, but you still review its result and record the physical-ruler measurement. If it declines or produces a caution, return to the by-hand path.

What success looks like

• The rail says True size — ruler measurement recorded, not Set — tap to verify.
• The corrected photo looks square to the pattern surface.
• The verification result passes.
• A separate known distance agrees when measured in PatternForge.
• No newer visible photo sits outside the scale you verified.

Common problems and recovery

• The selected card does not match the project paper preference: Use the card that is physically in the photo. Later, choose the printer paper separately in Arrange for Print.
• The printed line is wrong: Reprint the sizing card at 100% / Actual Size. Do not calibrate around a misprinted card.
• The four points are hard to place: Zoom in, refine each target, and preview again.
• Scale is set but the ruler-backed check has not passed: Open the scale row and choose Re-check. Until it passes, Pattern Check and clean export will continue to treat size as untrusted.
• You choose Use photo without checked size: The photo can remain usable for drafting, but it is not safe evidence for final-size output. Any bypassed export remains an explicitly marked draft.
• You add another photo later: Recheck the scale with the new photo in place. A check tied to one photo does not automatically prove another photo.
• Existing pieces were traced before recalibration: Read the rescale choice carefully. Keeping old pieces without scaling leaves their size suspect; compare or retrace them before final output.

Quick safety check

Do not continue to final tracing based only on a straight-looking image. Confirm all three:

• the physical sizing card passed its ruler check;
• perspective correction was reviewed; and
• PatternForge reports that the ruler measurement and photo-line check passed.

Related tools and next step

Review Print and Check a Sizing Card if the physical sheet is uncertain. When the ruler-backed size check passes, continue to Trace a Pattern Piece.