Geometry APIs

The Geometry APIs allow you to create interactive tools such as erasers and selection mechanisms.

To illustrate practical application of the Geometry APIs, explore the following eraser implementation example.

Whole stroke eraser

fun eraseWholeStrokes(
    eraserBox: ImmutableBox,
    finishedStrokesState: MutableState<Set<Stroke>>,
) {
    val threshold = 0.1f

    val strokesToErase = finishedStrokesState.value.filter { stroke ->
        stroke.shape.computeCoverageIsGreaterThan(
            box = eraserBox,
            coverageThreshold = threshold,
        )
    }
    if (strokesToErase.isNotEmpty()) {
        Snapshot.withMutableSnapshot {
            finishedStrokesState.value -= strokesToErase
        }
    }
}

For a Compose implementation, make sure to trigger a recomposition, so the strokes are effectively removed. For example, an approach would be to use rememberCoroutineScope in your composable and pass the coroutine scope to your touch listener, allowing you to modify finishedStrokesState within the scope of Compose.

Note: An eraser that only removes the parts of the strokes it touches can be implemented by seeing if a stroke intersects with individual line segments of a StrokeInputBatch and creating new StrokeInputBatch and Stroke objects out of the line segments that aren't intersected.