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Canvas 2D · 02 Paths and Line State

Series contents · Technology map

Stage: Pixels and Drawing State · Editable example · Storage ID: canvas-2d-step-02

Goals and learning outcomes

Prerequisites

Prerequisite concepts: CSS and the Buffer, Resetting State. Prepare the result files and observation records from Unit 1.

Key concepts and explanations

Path Lifetime

beginPath clears the current path. If an earlier path remains, stroke for a new shape redraws the earlier lines too.

Line Boundaries

lineCap and lineJoin determine the shapes of endpoints and joins. Lines of equal thickness can occupy different areas at corners.

Practice setup

First check the execution scope guidance on the example screen. The following describes preparation for extending the assignment in a separate project. Implement this yourself in an HTML and ES module project on a separate local HTTP development server. Record the browser version, viewport, and DPR.

The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.

Practice tasks

  1. Prepare day-02.js and index.html in the external practice project. Draw three bent lines with beginPath,moveTo,lineTo.
  2. Connect lineCap and lineJoin to input select controls and show a comparison drawing without path resets alongside them.
  3. Save the output with the input condition miter, then run again with round. Expected: Sharp outer corners become rounded. Compare this with the actual screen and logs, and record the actual results and reasons for differences in day-02-notes.md.

Variables to observe

VariableBaselineChangeExpected change / hypothesis
lineJoinmiterroundSharp outer corners become rounded

Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.

Completion criteria

Self-check

  1. How does closePath differ from beginPath?
  2. Which corners need miterLimit?

Official references

Linked example and execution scope

Example source · Runtime environments and limits

This is an editable browser experiment. Measurements vary with the current device and browser.

Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.

Next learning

Previous: Coordinates and Pixels · Next: Shape Functions and State Isolation

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