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OffscreenCanvas · 11 Transforming Pointer Coordinates
Series contents · Technology map
Stage: Synchronizing Size and Input · Editable example · Storage ID: offscreen-canvas-step-11
Goals and learning outcomes
- Learning outcome: explain the role of Local coordinates. Input cases: Clicks at the center and all four corners.
- Deliverable: Input coordinates and the selected particle ID. Explain the rationale for handling Matching scales, linking it to the code.
Prerequisites
Prepare the experiment results for Environment changes and Resolution cap from Unit 10, “Detecting DPR Changes”.
Key concepts and explanations
Local coordinates
clientX/Y are relative to the viewport. Subtract the Canvas rect position to obtain local CSS coordinates.
Matching scales
If rendering uses CSS units, do not multiply pointer coordinates by DPR again. Applying the scale twice offsets the selection.
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-11.js in a separate practice project, transform coordinates with getBoundingClientRect and send pointer messages to the Worker. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Clicks at the center and all four corners. Baseline: 1. Run under the baseline conditions, then record observations and errors in notes/day-11.md.
- Variable: DPR; changed value: 2. Run again through the same input path. Deliverable to save: Input coordinates and the selected particle ID. Hypothesis to check: The same screen position selects the same particle. 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 same screen position selects the same particle. |
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: Input coordinates and the selected particle ID. 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: Local coordinates and Matching scales. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Local coordinates affect in this exercise? Input cases: Clicks at the center and all four corners.
- Variable: DPR. Hypothesis to check: The same screen position selects the same particle. Did the actual results agree with the hypothesis? Explain the evidence in terms of Matching scales.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Transforming Pointer Coordinates experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.