An outline effect.

import { Outline } from '@react-three/postprocessing'
import { BlendFunction, Resizer, KernelSize } from 'postprocessing'

return (
    selection={[meshRef1, meshRef2]} // selection of objects that will be outlined
    selectionLayer={10} // selection layer
    blendFunction={BlendFunction.SCREEN} // set this to BlendFunction.ALPHA for dark outlines
    patternTexture={null} // a pattern texture
    edgeStrength={2.5} // the edge strength
    pulseSpeed={0.0} // a pulse speed. A value of zero disables the pulse effect
    visibleEdgeColor={0xffffff} // the color of visible edges
    hiddenEdgeColor={0x22090a} // the color of hidden edges
    width={Resizer.AUTO_SIZE} // render width
    height={Resizer.AUTO_SIZE} // render height
    kernelSize={KernelSize.LARGE} // blur kernel size
    blur={false} // whether the outline should be blurred
    xRay={true} // indicates whether X-Ray outlines are enabled


selectionObjectsSelection of objects that will be outlined
blendFunctionBlendFunctionBlendFunction.SCREENThe blend function of this effect.
widthNumberResizer.AUTO_SIZEThe render width.
heightNumberResizer.AUTO_SIZEThe render height.
selectionLayerNumberThe selection layer
patternTextureNumbernullA pattern texture
edgeStrengthNumber1The edge strength
pulseSpeedNumber0The pulse speed. A value of zero disables the pulse effect.
visibleEdgeColorNumber0xffffffThe color of visible edges.
hiddenEdgeColorNumber0x22090aThe color of hidden edges.
kernelSizeKernelSizeKernelSize.VERY_SMALLThe blur kernel size.
blurBooleanfalseWhether the outline should be blurred.
xrayBooleantrueWhether occluded parts of selected objects should be visible.