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OffscreenCanvas · 27 Summarizing Experiment Results
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Stage: Responsiveness Comparison Project · Editable example · Storage ID: offscreen-canvas-step-27
Goals and learning outcomes
- Learning outcome: explain the role of Distributions. Input cases: 3 repeated runs.
- Deliverable: A comparison table including conditions and raw values. Explain the rationale for handling Limitations, linking it to the code.
Prerequisites
Prepare the experiment results for Competing work and Fair comparison from Unit 26, “Main-Thread Contention Experiment”.
Key concepts and explanations
Distributions
An average alone does not reveal brief stalls. Summarize the median together with long frames.
Limitations
Device, browser, and power state affect results. Do not extend conclusions to environments you have not measured.
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-27.js in a separate practice project, collect result JSON and build a comparison table by mode·particle count·DPR. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: 3 repeated runs. Baseline: Main thread first. Run under the baseline conditions, then record observations and errors in notes/day-27.md.
- Variable: Run order; changed value: Worker first. Run again through the same input path. Deliverable to save: A comparison table including conditions and raw values. Hypothesis to check: Warm-up or ordering effects can be identified. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Run order | Main thread first | Worker first | Warm-up or ordering effects can be identified. |
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 comparison table including conditions and raw values. 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: Distributions and Limitations. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Distributions affect in this exercise? Input cases: 3 repeated runs.
- Variable: Run order. Hypothesis to check: Warm-up or ordering effects can be identified. Did the actual results agree with the hypothesis? Explain the evidence in terms of Limitations.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
The example shows request-to-acknowledgment time and sample count for one x patch. Visible-display latency, medians, long frames, three-run comparisons, and JSON export are separate assignments.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Main-Thread Contention Experiment · Next: Evaluating Rendering Separation