Full Res vs Thumbnails

.resizable() shrinks what you see. It does not shrink what you keep.

Twelve procedural Harbor tiles. Three modes. phys_footprint on screen so the still does not need Instruments.

Empty — the baseline

-mode empty. No bitmaps.

Naive — the control

-mode naive. Image(uiImage:) on twelve 1536×1536 bitmaps, drawn small in a grid. Accounted cost: 12 × 1536² × 4 = 108.0 MB. Decoded RGBA is also retained so pages stay dirty — otherwise the simulator under-reports.

Live sits above account because the bench holds both the UIImage and a retained RGBA buffer per tile — roughly two dirty copies plus the app baseline.

Thumb — downsample first

-mode thumb. Same twelve labels, generated at 96×96 before the view ever sees them.

Visually the same grid. Accounted cost drops from 108.0 to 0.4. Live falls from 236 back near the empty baseline (21 vs 18).

Mini-exercise

Change thumbSide from 96 to 256. Re-run -mode thumb and watch ACCOUNT become 12 × 256² × 4 ≈ 3.0 MB. Compare live to the 96² still.

Challenges

1. Replace procedural tiles with PhotosPicker assets and keep the same three modes.
2. Add a fourth mode that uses ImageRenderer / downsampled CGImage via CGImageSourceCreateThumbnailAtIndex.
3. Plot live footprint every 200 ms into an on-screen sparkline while cycling.
4. Purge with tiles = []; retainedRGBA = [] mid-cycle and confirm live returns near 18 MB.
5. Wire XCUITest to assert ACCOUNT text contains 108.0 under -mode naive.

Key Points

• Observed EMPTY: LIVE 18 MB, ACCOUNT 0.0.
• Observed NAIVE: LIVE 236 MB, ACCOUNT 108.0 (12×1536²×4).
• Observed THUMB: LIVE 21 MB, ACCOUNT 0.4 (12×96²×4).
• Naive Image(uiImage:) is the control — scaling on screen does not free pixels.
• Downsample before the view, not after.
• Live footprint needs resident pages; retain decoded buffers when demonstrating.
• Accounted bytes explain the still when Instruments is not open.

Next up: Core Image and filters — a live filter chain on a still, each stage as a strip.