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OffscreenCanvas · 06 Frame Timing
Series contents · Technology map
Stage: Control and the First Render · Editable example · Storage ID: offscreen-canvas-step-06
Goals and learning outcomes
- Learning outcome: explain the role of Delta. Input cases: Normal frames and input with a long gap.
- Deliverable: A table of positions at the same elapsed time. Explain the rationale for handling Long gaps, linking it to the code.
Prerequisites
Prepare the experiment results for CSS size and DPR from Unit 5, “Coordinate Units”.
Key concepts and explanations
Delta
Moving by frame number alone makes speed depend on the refresh rate. Integrate position using elapsed time.
Long gaps
A large delta on return can cause a jump. Define a policy to clamp it or reset the simulation.
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-6.js in a separate practice project, check Worker requestAnimationFrame support and implement delta-based movement. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Normal frames and input with a long gap. Baseline: None. Run under the baseline conditions, then record observations and errors in notes/day-6.md.
- Variable: Delta limit; changed value: Apply an upper bound. Run again through the same input path. Deliverable to save: A table of positions at the same elapsed time. Hypothesis to check: Abrupt movement after a long gap is reduced. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Delta limit | None | Apply an upper bound | Abrupt movement after a long gap is reduced. |
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: A table of positions at the same elapsed time. 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: Delta and Long gaps. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Delta affect in this exercise? Input cases: Normal frames and input with a long gap.
- Variable: Delta limit. Hypothesis to check: Abrupt movement after a long gap is reduced. Did the actual results agree with the hypothesis? Explain the evidence in terms of Long gaps.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Frame Timing experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Coordinate Units · Next: Basic Rendering Validation