82 lines
2.7 KiB
Markdown
82 lines
2.7 KiB
Markdown
# Reduce repeated clip-space work in `draw_sprite`
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## Summary
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`draw_sprite` calls `to_clip` four times for an axis-aligned quad:
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```odin
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p0 := to_clip(x0_px, y0_px, sw, sh)
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p1 := to_clip(x1_px, y0_px, sw, sh)
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p2 := to_clip(x1_px, y1_px, sw, sh)
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p3 := to_clip(x0_px, y1_px, sw, sh)
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```
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This repeats the same divisions and converts duplicate x/y coordinates. An
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axis-aligned sprite has only two unique x values and two unique y values.
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This is a measurable optimization, but it is low priority at the current
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128-sprite limit because the absolute saving is small.
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## Evidence
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The checked-in unoptimized profiles report `engine::to_clip` at 5.33% and 5.51%
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self time. Those percentages are inflated by the unoptimized profiling build,
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so the change was also measured with `-debug -o:speed`.
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A temporary benchmark used the real baked toad metadata, changed sprite
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position on every iteration, preallocated the queue, and performed two million
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draws per mode over seven trials:
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| Mode | Median time per draw |
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| --- | ---: |
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| Current `draw_sprite` | 25.128 ns |
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| Precomputed clip scale and reused coordinates | 21.531 ns |
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| Same math plus cached `Frame_Def` | 21.778 ns |
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Simplifying the math improved isolated draw time by approximately **14.3%**.
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Caching the resolved frame did not provide an additional benefit and should not
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be included without new evidence.
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At `MAX_SPRITES == 128`, the measured math saving is only about 0.46
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microseconds per completely full frame. GPU/driver work dominated the
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end-to-end benchmark, so this should follow the profiling and deterministic
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benchmark improvements.
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Environment:
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- Odin `dev-2026-05-nightly:ea5175d`
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- Optimized with `-debug -o:speed`
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- Linux x86-64
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## Proposed change
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Compute clip scaling once and construct corners from the unique coordinates:
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```odin
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sx := 2.0 / f32(app.swapchain_w)
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sy := 2.0 / f32(app.swapchain_h)
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left := x0_px * sx - 1
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right := x1_px * sx - 1
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top := 1 - y0_px * sy
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bottom := 1 - y1_px * sy
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p0 := Vec2{left, top}
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p1 := Vec2{right, top}
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p2 := Vec2{right, bottom}
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p3 := Vec2{left, bottom}
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```
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Keep `to_clip` for general callers and its existing tests; this change only
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specializes quad construction inside `draw_sprite`.
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## Acceptance criteria
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- Existing sprite geometry, camera, UV, and batching tests pass.
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- Add or extend a test that compares all four generated corners against
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`to_clip` for representative viewport and sprite coordinates.
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- Flipped and unflipped sprites produce identical vertices to the current code.
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- An optimized deterministic benchmark shows at least a 10% improvement in
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isolated `draw_sprite` time under comparable conditions.
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- Do not add a per-sprite frame cache as part of this issue.
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