photocraft/docs/quick-selection-performance.md

Quick Selection stroke performance

Measured 2026-10-10 on an AWS c7i.4xlarge, Intel Xeon Platinum 8488C, 16 vCPUs, 32 GiB RAM, Ubuntu 24.04, Rust 1.99.0. Release builds and the default native worker count. Builds and other tests were idle during timing. Algorithm commit: 78dca424510bbdecb01228a4a98a971fd070b658.

What changed

The fitted Quick Selection energy, brush footprint, costs, weights, working resolution, boundary bands and window expansion rules stay the same.

Large finite graphs with both terminal directions use FIFO push/relabel, implemented from Goldberg and Tarjan (JACM 1988). Local excess pushes replace repeated augmenting paths. Periodic global relabelling computes distances to the sink and, for the remaining vertices, back to the source. Returning unused source flow is necessary to retain a feasible residual graph and support another solve after adding capacities. The final source-reachability pass returns the smallest source-side cut. Graphs below 4096 nodes, one-terminal graphs, and graphs with nonfinite or negative edge capacities keep the existing Boykov-Kolmogorov implementation. Both solvers use the existing f32 residual capacities and saturation epsilon. The new solver stores f32 reverse source capacities and f64 excess, 12 bytes per free graph node, and reuses the graph's height, current-arc and active arrays.

Graph construction skips weights between two fixed pixels. With fewer than one free pixel in eight, a five-way merge enumerates just the canonical edge sources incident to free pixels, in their original raster order. This makes edge enumeration and weight evaluation O(free pixels), while the existing indexing and terminal setup remain linear in the working image. Preserving edge order also preserves f32 accumulation into fixed-neighbour terminals.

Raster surface strokes

The timed call is the full quick_select CPU pipeline through SurfaceSampler, as used by select.quick: raster reading and depth conversion, RGB sampling, window expansion, footprint and distance costs, coarse-to-fine cuts, mask upsampling and region trimming. Document construction is outside the timer. The stroke paints three rows across a 2400 x 1200 textured object with a 180 px brush. The fixture is deterministic synthetic RGB with a contrasting background and a small texture component.

One warm pair, then five alternating before/after pairs per depth and document. The table reports medians. Every run asserts identical bounds and every mask byte.

Document	Depth	Before (ms)	After (ms)	Speedup
6000x4000	8-bit	747.60	201.96	3.70x
6000x4000	16-bit	747.36	208.29	3.59x
6000x4000	32f	738.22	211.35	3.49x
7200x5000	8-bit	721.61	207.24	3.48x
7200x5000	16-bit	719.26	207.96	3.46x
7200x5000	32f	732.08	205.96	3.55x

Other complete CPU strokes

These use ImageSampler to isolate the same complete stroke pipeline from raster storage conversion. One warm pair and three alternating measured pairs. The click uses a 30 px brush on a 220 x 220 object; the long stroke uses a 256 px brush across 2000 px on a 2400 x 1000 object; paint-object is the three-row stroke above. All 24 MP and 36 MP fixtures retain normal working resolution and automatic window expansion.

Document	Stroke	Before (ms)	After (ms)	Speedup
6000x4000	click	113.60	39.90	2.85x
6000x4000	long-stroke	1199.74	360.31	3.33x
6000x4000	paint-object	736.28	205.92	3.58x
7200x5000	click	121.53	33.09	3.67x
7200x5000	long-stroke	1168.25	353.19	3.31x
7200x5000	paint-object	725.02	201.60	3.60x

Correctness and regression checks

• 1170 integer-capacity graphs compare flow and all labels against BK, then solve again.
• 600 arbitrary f32-capacity graphs compare flows and all labels against BK.
• 240 fractional graphs enumerate every possible cut, checking optimal energy and the intersection of all optimal source sets independently of BK.
• A 5000-node disconnected chain checks return-to-source progress, a repeated solve, and adding a link before another solve.
• Exhaustive small constraint masks and edge-source masks cover borders, diagonals, degenerate dimensions and tied cuts.
• 36 full Quick Selection comparisons cover texture, low contrast, gradients, noise, thin bars, discs, uniform areas and canvas-border strokes, with direct and coarse-to-fine cuts.
• Every measured workflow compares full region bounds and mask bytes.

Reproduction

cargo test --release -p photocraft-algo quick_selection_surface_release_comparison -- --ignored --nocapture --test-threads=1
cargo test --release -p photocraft-algo quick_selection_workflow_release_comparison -- --ignored --nocapture --test-threads=1
cargo test --release -p photocraft-algo solver_workload_release_comparison -- --ignored --nocapture --test-threads=1

The private const test path keeps the previous dense weight scan and the unchanged BK solver, sharing the rest of the full stroke pipeline. Solver-only fixtures additionally cover chains, one-terminal grids and dense terminal grids at 1024, 4096, 16384 and 65536 nodes. Their numbers measure just the solve, with graph cloning outside the timer.

Raw samples

SURFACE 6000x4000 format=PixelFormat { mode: Rgb, sample: U8, alpha: true } old_ms=[745.840809, 746.246292, 747.604875, 747.82731, 748.878614] new_ms=[201.579251, 201.723446, 201.956769, 202.38253, 202.948418] speedup=3.70
SURFACE 6000x4000 format=PixelFormat { mode: Rgb, sample: U16, alpha: true } old_ms=[745.974128, 746.216757, 747.364691, 748.216512, 748.33632] new_ms=[206.63495600000002, 206.932412, 208.287491, 209.847055, 210.480741] speedup=3.59
SURFACE 6000x4000 format=PixelFormat { mode: Rgb, sample: F32, alpha: true } old_ms=[737.480891, 737.6333950000001, 738.216963, 738.513631, 739.73055] new_ms=[209.130586, 209.490552, 211.347315, 211.520884, 213.142088] speedup=3.49
SURFACE 7200x5000 format=PixelFormat { mode: Rgb, sample: U8, alpha: true } old_ms=[720.72951, 721.2813970000001, 721.6096229999999, 722.4278710000001, 723.3628600000001] new_ms=[206.039241, 207.077233, 207.241128, 207.246913, 210.42549] speedup=3.48
SURFACE 7200x5000 format=PixelFormat { mode: Rgb, sample: U16, alpha: true } old_ms=[718.551252, 718.5636020000001, 719.25546, 719.810975, 719.868964] new_ms=[203.704922, 205.042543, 207.958082, 208.070805, 208.25623900000002] speedup=3.46
SURFACE 7200x5000 format=PixelFormat { mode: Rgb, sample: F32, alpha: true } old_ms=[730.028451, 731.879452, 732.082273, 733.348433, 734.600343] new_ms=[205.59655800000002, 205.75180600000002, 205.957811, 206.02760800000001, 207.490222] speedup=3.55
QUICK 6000x4000 case=click brush=30 old_ms=[113.313339, 113.600531, 114.41872000000001] new_ms=[39.298109, 39.903393, 39.940391] speedup=2.85 bbox=Rect { x0: 2891, y0: 1891, x1: 3111, y1: 2111 }
QUICK 6000x4000 case=long-stroke brush=256 old_ms=[1184.445199, 1199.7410909999999, 1199.926849] new_ms=[352.47967, 360.312007, 367.333399] speedup=3.33 bbox=Rect { x0: 1801, y0: 1501, x1: 4201, y1: 2501 }
QUICK 6000x4000 case=paint-object brush=180 old_ms=[732.409731, 736.284987, 736.734234] new_ms=[204.055566, 205.92469899999998, 208.78787100000002] speedup=3.58 bbox=Rect { x0: 1801, y0: 1401, x1: 4201, y1: 2601 }
QUICK 7200x5000 case=click brush=30 old_ms=[121.405737, 121.530837, 122.215665] new_ms=[32.693926, 33.087069, 33.834208] speedup=3.67 bbox=Rect { x0: 3491, y0: 2391, x1: 3711, y1: 2611 }
QUICK 7200x5000 case=long-stroke brush=256 old_ms=[1166.053819, 1168.25045, 1169.190979] new_ms=[352.533248, 353.18776299999996, 355.84682899999996] speedup=3.31 bbox=Rect { x0: 2401, y0: 2001, x1: 4801, y1: 3001 }
QUICK 7200x5000 case=paint-object brush=180 old_ms=[723.5930350000001, 725.024707, 726.1529770000001] new_ms=[200.803006, 201.595578, 202.331318] speedup=3.60 bbox=Rect { x0: 2401, y0: 1901, x1: 4801, y1: 3101 }
SOLVER width=32 n=1024 case=chain old_ms=[0.026896, 0.027003, 0.02743] new_ms=[0.02681, 0.027076999999999997, 0.033721] speedup=1.00
SOLVER width=32 n=1024 case=all-source old_ms=[0.016474, 0.01651, 0.024678] new_ms=[0.016201999999999998, 0.016773999999999997, 0.017098000000000002] speedup=0.98
SOLVER width=32 n=1024 case=dense-grid old_ms=[0.5345420000000001, 0.541424, 0.546977] new_ms=[0.519929, 0.543284, 0.566567] speedup=1.00
SOLVER width=64 n=4096 case=chain old_ms=[0.118043, 0.118185, 0.120604] new_ms=[0.106363, 0.106517, 0.114974] speedup=1.11
SOLVER width=64 n=4096 case=all-source old_ms=[0.065567, 0.06612499999999999, 0.067655] new_ms=[0.071212, 0.07142100000000001, 0.073924] speedup=0.93
SOLVER width=64 n=4096 case=dense-grid old_ms=[3.866533, 3.8864490000000003, 3.922701] new_ms=[1.256155, 1.319511, 1.3287730000000002] speedup=2.95
SOLVER width=128 n=16384 case=chain old_ms=[0.439333, 0.43940500000000005, 0.44905] new_ms=[0.38673599999999997, 0.395393, 0.475274] speedup=1.11
SOLVER width=128 n=16384 case=all-source old_ms=[0.26877999999999996, 0.28474099999999997, 0.295766] new_ms=[0.285945, 0.301348, 0.305836] speedup=0.94
SOLVER width=128 n=16384 case=dense-grid old_ms=[13.136574999999999, 13.181371, 13.289321999999999] new_ms=[1.110547, 1.204107, 1.2151900000000002] speedup=10.95
SOLVER width=256 n=65536 case=chain old_ms=[1.83223, 1.855307, 1.887005] new_ms=[1.5852430000000002, 1.624603, 1.795397] speedup=1.14
SOLVER width=256 n=65536 case=all-source old_ms=[1.075863, 1.0908529999999999, 1.178408] new_ms=[1.16852, 1.184938, 1.224871] speedup=0.92
SOLVER width=256 n=65536 case=dense-grid old_ms=[33.334272999999996, 33.502357, 33.531151] new_ms=[4.31308, 4.609208, 4.618033] speedup=7.27