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OffscreenCanvas · 22 Transferring ImageBitmap Frames
Series contents · Technology map
Stage: Responsiveness Comparison Project · Editable example · Storage ID: offscreen-canvas-step-22
Goals and learning outcomes
- Learning outcome: explain the role of Frame transfer. Input cases: 30 consecutive frames.
- Deliverable: Counts of displayed·discarded bitmaps and close calls. Explain the rationale for handling Resource lifetime, linking it to the code.
Prerequisites
Prepare the experiment results for Distinguishing errors and Fallback from Unit 21, “Recovering from Worker Failure”.
Key concepts and explanations
Frame transfer
OffscreenCanvas results can be passed as an ImageBitmap. This is a different output method from transferring control directly.
Resource lifetime
Close a bitmap after displaying it if it is no longer needed. Bound the queue if production outpaces display.
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-22.js in a separate practice project, implement Worker transferToImageBitmap and a main-thread bitmap display path. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: 30 consecutive frames. Baseline: Immediate. Run under the baseline conditions, then record observations and errors in notes/day-22.md.
- Variable: Consumption speed; changed value: Slow. Run again through the same input path. Deliverable to save: Counts of displayed·discarded bitmaps and close calls. Hypothesis to check: The number of pending bitmaps stays bounded even with slow consumption. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Consumption speed | Immediate | Slow | The number of pending bitmaps stays bounded even with slow consumption. |
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: Counts of displayed·discarded bitmaps and close calls. 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: Frame transfer and Resource lifetime. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Frame transfer affect in this exercise? Input cases: 30 consecutive frames.
- Variable: Consumption speed. Hypothesis to check: The number of pending bitmaps stays bounded even with slow consumption. Did the actual results agree with the hypothesis? Explain the evidence in terms of Resource lifetime.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
The button requests one ImageBitmap. The main thread draws it into a new preview Canvas and calls close. A continuous frame stream and bounded queue are not implemented.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Recovering from Worker Failure · Next: Canvas Context Loss