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OffscreenCanvas · 05 Coordinate Units
Series contents · Technology map
Stage: Control and the First Render · Editable example · Storage ID: offscreen-canvas-step-05
Goals and learning outcomes
- Learning outcome: explain the role of CSS size. Input cases: A 100 CSS px square.
- Deliverable: CSS·buffer sizes and shape width. Explain the rationale for handling DPR, linking it to the code.
Prerequisites
Prepare the experiment results for Context and Replacement Canvas from Unit 4, “Context Creation Failure”.
Key concepts and explanations
CSS size
CSS size determines the layout on screen. It differs from the size of the pixel backing store.
DPR
DPR is the ratio of backing-store pixels to CSS pixels. Rendering and input must use the same units.
Practice setup
First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. The existing offscreen-canvas-01·02·03 are starting examples for supported browsers. Implement the new units in main-thread and Worker files in a separate local HTTP project. Check API and context support, and provide a main-thread fallback on a new Canvas if transfer fails. Preserve CSP and isolation, and record performance measurements yourself.
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
- In src/day-5.js in a separate practice project, set the buffer size from the CSS size and DPR, and apply setTransform. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: A 100 CSS px square. Baseline: 1. Run under the baseline conditions, then record observations and errors in notes/day-5.md.
- Variable: DPR; changed value: 2. Run again through the same input path. Deliverable to save: CSS·buffer sizes and shape width. Hypothesis to check: The shape keeps the same size on screen while the backing-store resolution increases. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| DPR | 1 | 2 | The shape keeps the same size on screen while the backing-store resolution increases. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- Required deliverable: CSS·buffer sizes and shape width. Results are available for both the baseline and changed conditions, allowing comparison of 2 conditions; input values and the execution environment are recorded together.
- The explanation of differences between the two conditions connects both concepts: CSS size and DPR. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does CSS size affect in this exercise? Input cases: A 100 CSS px square.
- Variable: DPR. Hypothesis to check: The shape keeps the same size on screen while the backing-store resolution increases. Did the actual results agree with the hypothesis? Explain the evidence in terms of DPR.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Coordinate Units experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.