How to draw rounded corner polygons in Jetpack Compose Canvas?

Question

I'm trying to create a rounded triangle using Canvas in Jetpack Compose.

I try this code for drawing triangle:

@Composable
fun RoundedTriangle() {
    Canvas(modifier = Modifier.size(500.dp)) {
        val trianglePath = Path().apply {
            val height = size.height
            val width = size.width
            moveTo(width / 2.0f, 0f)
            lineTo(width, height)
            lineTo(0f, height)
        }
            drawPath(trianglePath, color = Color.Blue)
    }
}

But I don't know how to round the triangle corners. I also tried to use arcTo, but I was unable to get a suitable result.

How can I draw something like the figure below?

Answer 1

For Stroke you can specify rounding like this:

drawPath(
    ...
    style = Stroke(
        width = 2.dp.toPx(),
        pathEffect = PathEffect.cornerPathEffect(4.dp.toPx())
    )
)

Yet Fill seems lack of support rounding. I've created a feature request, please star it.

But Canvas has drawOutline function, which accepts both Outline, which can wrap a Path, and Paint, for which you can specify pathEffect:

Canvas(modifier = Modifier.fillMaxWidth().aspectRatio(1f)) {
    val rect = Rect(Offset.Zero, size)
    val trianglePath = Path().apply {
        moveTo(rect.topCenter)
        lineTo(rect.bottomRight)
        lineTo(rect.bottomLeft)
        close()
    }

    drawIntoCanvas { canvas ->
        canvas.drawOutline(
            outline = Outline.Generic(trianglePath),
            paint = Paint().apply {
                color = Color.Black
                pathEffect = PathEffect.cornerPathEffect(rect.maxDimension / 3)
            }
        )
    }
}

Path helpers:

fun Path.moveTo(offset: Offset) = moveTo(offset.x, offset.y)
fun Path.lineTo(offset: Offset) = lineTo(offset.x, offset.y)

Result: