# VectorPaint cylinder-side unfolding: distinguish circumference, diameter and seam allowance

Language: en
Canonical: [VectorPaint cylinder-side unfolding: distinguish circumference, diameter and seam allowance](<https://allluckly.cn/en/guides/vectorpaint-cylinder-side-unfolding/>)
中文：[中文 HTML](<https://allluckly.cn/guides/vectorpaint-cylinder-side-unfolding/>)
English: [English HTML](<https://allluckly.cn/en/guides/vectorpaint-cylinder-side-unfolding/>)
Published: 2026-11-08T00:00:00+08:00
Modified: 2026-11-08T09:34:15+08:00
Author: Bison

Match an original cylinder to its side rectangle, checking direction, circumference and height while identifying approximations and additional seam allowance.

English translation of the [Chinese original](<https://allluckly.cn/guides/vectorpaint-cylinder-side-unfolding/>), translated on 2026-11-08. Original publication dates, attribution, code and version notes are retained.

The width of an unfolded cylinder side is not the base diameter. Use an invented cylinder to connect its circumference, side surface and one seam. This geometric diagram provides no precision guarantee for paper fabrication or container production. VectorPaint supplies manually drawn shapes and labels; you verify calculations and correspondence.

## State radius, height and approximation conventions

Use a landscape canvas with a cylinder on the left and space for the unfolded rectangle on the right. Set an invented base radius of 60 units and height of 200, so diameter is 120. Record circumference as 120 times pi, approximately 376.991. At one pixel per exercise unit, draw the rectangle about 377 pixels wide and 200 high, retaining an approximation mark in its width label. These units are task settings, not printed millimetres. A rounded integer drawing is not an exact seamless unfolding.

## Do not measure circumference from the drawn ellipse

Represent the cylinder with top/bottom ellipses and two vertical sides. The ellipse is a view representation, not a physically flattened base with a redefined radius. Distinguish front and rear lower arcs using either a dashed rear arc or reduced visibility, choosing and explaining one convention. Its drawn width is not the unfolded width; a rectangle copied from the ellipse’s bounding width does not establish unfolding. Radius 60 still refers to the geometric circular base. If resizing the left view for composition, explicitly identify its different display scale.

## Assign one geometric quantity to each rectangle direction

Draw the right rectangle approximately 377 wide and 200 high. Label the horizontal side “circumference approximately 377” and the vertical side “height 200”, rather than leaving two unexplained numbers. Top and bottom rectangle edges correspond to the upper and lower circular boundaries, not to the circular base surfaces themselves. A short leader can explain a complete circuit from a chosen start point, represented by left-to-right travel across the rectangle. These arrows express correspondence, not automatic path measurement. Check the blank rectangle before adding patterns or colour regions.

## Mark one seam through the pair of rectangle edges

Choose one vertical generator on the cylinder as the cut position and label it seam A. Label both vertical rectangle edges seam A, because rolling them back identifies the same location; do not give them unrelated interface names. Matching small marks may be added without counting them as extra length. Trace the rectangle boundary to verify that top/bottom are circumferential boundaries and left/right are the cut edges. If rotating the rectangle ninety degrees later, update direction labels rather than substituting page-horizontal/vertical for their geometric roles.

## Keep seam allowance outside the side-surface width

Make a separate diagram adding a pale strip 20 units wide at the rectangle’s right, labelled “additional seam allowance illustration”. The side surface remains approximately 377 wide and the overall outline approximately 397. Do not silently incorporate 20 into the circumference label or derive it from radius. Retain the original right boundary so side surface and additional strip remain distinguishable. This is not a recommendation for actual bonding length or material technique; the number distinguishes geometry from allowance. Keep the original allowance-free diagram for comparison.

## Update only the affected quantities when radius changes

Duplicate the set, changing radius from 60 to 30 while retaining height 200. Diameter becomes 60 and circumference 60 pi, approximately 188.496; rectangle width is about 188.5 while height remains unchanged. If retaining an illustrative 20-unit allowance, overall width is about 208.5, but this allowance is a deliberate choice, not something that must halve with radius. Compare both sets for correct changes in base size and circumferential width. Do not halve a whole group containing height and labels, which would incorrectly change the 200-unit height.

## Inspect labels and geometry together in output

Save the editable source with calculations, export PNG or PDF and open it separately to inspect radius, diameter, circumference, height and allowance. If the approximation symbol becomes a missing-glyph box, replace it with the word “approximately” rather than deleting it and implying exactness. For printing, verify output size and scaling on a proof rather than inferring paper circumference from screen length. Keep seam marks and side boundaries visible, without hiding the actual seam when removing references. A graphical PDF does not establish compatibility with cutting equipment or material assembly.

## Check diameter-versus-circumference confusion first

For an overly narrow unfolding, check whether 120 was mistaken for circumference. For a halved height, check whole-group scaling. For a 20-unit width discrepancy, check double-counted allowance. If seam labels disagree, return to cutting along one generator rather than renaming two edges to conceal the conflict. Retain original-radius and half-radius sets, and allowance-free versus allowance-added extents. Every number should trace to a definition and calculation. Completion means clear quantity and boundary relationships, not resemblance to a production-ready packaging template.

## Related tutorials and sources

[VectorPaint six-face box nets: verify shared edges and opposite faces](<https://allluckly.cn/en/guides/vectorpaint-six-face-box-net/>), [VectorPaint canvas sizes: distinguish px, pt, mm and exported pixels](<https://allluckly.cn/en/guides/vectorpaint-canvas-units/>).

Sources checked on November 8, 2026: VectorPaint public version 1.0.8, requiring iOS / iPadOS 17.0 or later. Capability scope follows the [official App Store description](<https://apps.apple.com/cn/app/id1587272880>) and [Apple public version lookup](<https://itunes.apple.com/lookup?id=1587272880&country=cn>). This original exercise can be performed independently; use the controls in your installed version. It does not claim completed physical-device, private-file or all-export-environment testing. Manual checks and external records are not newly promised automatic analysis features.