The precision of martin-tile-utils

[sharkAndshark]

Remember we have precision problem when running bbox to tile index or back.

Maybe we can get some inspiration from ogc tms spec, there is an epsilon:

In this pseudocode it is assumed that bBoxMinX, bBoxMinY, bBoxMaxX, bBoxMaxY, tileMatrixMinX, tileMatrixMinY, tileMatrixMinY, tileMatrixMaxY, tileSpanX and tileSpanY are floating point variables (IEEE-754) that have accuracy issues derived from the finite precision of the representation. These accuracy issues could be amplified in a typical floor() rounding down function that could return a value ±1 than what is expected. To overcome this issue this code uses a small value (epsilon) added or subtracted in a place that is not affected by CRS coordinate precision.

// to compensate for floating point computation inaccuracies
epsilon = 1e-6

let tileMinCol = ((bBoxMinX - tileMatrixMinX) / tileSpanX + epsilon).floor()
let tileMaxCol = ((bBoxMaxX - tileMatrixMinX) / tileSpanX - epsilon).floor()
let tileMinRow = ((tileMatrixMaxY - bBoxMaxY) / tileSpanY + epsilon).floor()
let tileMaxRow = ((tileMatrixMaxY - bBoxMinY) / tileSpanY - epsilon).floor()

// to avoid requesting out-of-range tiles
if tileMinCol < 0 { tileMinCol = 0; }
if tileMaxCol >= matrixWidth { tileMaxCol = matrixWidth-1; }
if tileMinRow < 0 { tileMinRow = 0
if tileMaxRow >= matrixHeight { tileMaxRow = matrixHeight-1; }

[nyurik]

cc: @CommanderStorm due to recent rtest changes

[CommanderStorm]

Jea, should probably have checked it more than just "yea, looks good to me".

Thinking about it:
A tile at z10 is 0.352°lng¹.
Needing ZERO precision is highly sus..

martin/martin-tile-utils/src/lib.rs

Lines 467 to 487 in 3bfc831

	#[rstest]
	#[case(0..11, 0)]
	#[case(11..14, 1)]
	#[case(14..17, 2)]
	#[case(17..21, 3)]
	#[case(21..24, 4)]
	#[case(24..27, 5)]
	#[case(27..30, 6)]
	fn test_get_zoom_precision(
	#[case] zoom: std::ops::Range<u8>,
	#[case] expected_precision: usize,
	) {
	for z in zoom {
	let actual_precision = get_zoom_precision(z);
	assert_eq!(
	actual_precision, expected_precision,
	"Zoom level {z} should have precision {expected_precision}, but was {actual_precision}"
	);
	}
	}
	}

Footnotes

1. https://wiki.openstreetmap.org/wiki/Zoom_levels ↩