Ring with rotating gap

This is a continuous version of the purple dots illusion. Keep staring at the crosshairs. Eventually, instead of a white gap rotating, you’ll start seeing a bright band of the opponent color.

Incidentally, you’ll notice that this opponent color is quite bright - it in fact appears brighter than the white background of your display! This is because you’re experiencing a Chimerical color.

Code

import { dom, time } from "/src/cscheid/cscheid.js";

{
    var canvasEl = d3.select("#main").append("canvas")
        .attr("width", 600)
        .attr("height", 600);

    var canvasCrosshair = d3.select("#main").append("canvas")
        .attr("width", 600)
        .attr("height", 600)
        .style("position", "relative")
        .style("top", "-600px");

    d3.select("#main").select("div").style("height", "600px");

    var ctx = dom.setupCanvas(canvasEl.node());
    var xhCtx = dom.setupCanvas(canvasCrosshair.node());

    // crosshairs
    xhCtx.fillStyle = "rgba(0,0,0,0)";
    xhCtx.fillRect(0,0,600,600);
    xhCtx.strokeStyle = "black";
    xhCtx.lineWidth = 1 * ctx.dpr;
    xhCtx.beginPath();
    xhCtx.moveTo(280, 300);
    xhCtx.lineTo(320, 300);
    xhCtx.moveTo(300, 280);
    xhCtx.lineTo(300, 320);
    xhCtx.stroke();

    canvasEl.node().style.filter = "blur(10px)";

    function tick()
    {
    var now = time.elapsed();

    // wipe
    ctx.fillStyle = "white";
    ctx.fillRect(0,0,600,600);

    ctx.lineWidth = 30 * ctx.dpr;

    var nSteps = ~~(100 * Math.PI * 2);
    var radius = 200;
    var xScale = d3.scaleLinear().domain([-1, 1]).range([50, 550]);
    var yScale = d3.scaleLinear().domain([-1, 1]).range([550, 50]);

    for (var i=0; i<nSteps; ++i) {
        var angleBeg = (i / nSteps) * Math.PI * 2;
        var angleEnd = ((i+1.5) / nSteps) * Math.PI * 2;
        var lerp = d3.interpolateLab(d3.hcl(angleBeg * 180 / Math.PI, 70, 50), "white");

        var gapCenterAngle = now * (Math.PI * 2) % (Math.PI * 2);
        var sigma2 = 1;
        var weight = Math.max(
        Math.exp(-Math.pow(gapCenterAngle - angleBeg, 2) / sigma2),
        Math.exp(-Math.pow((gapCenterAngle + 2 * Math.PI) - angleBeg, 2) / sigma2),
        Math.exp(-Math.pow((gapCenterAngle - 2 * Math.PI) - angleBeg, 2) / sigma2),
        );

        ctx.strokeStyle = String(lerp(weight));
        ctx.beginPath();
        ctx.moveTo(xScale(Math.cos(angleBeg)), yScale(Math.sin(angleBeg)));
        ctx.lineTo(xScale(Math.cos(angleEnd)), yScale(Math.sin(angleEnd)));
        ctx.stroke();
    }

    window.requestAnimationFrame(tick);
    }

    tick();
}

Acknowledgements

I’m sure this has been discovered before, but I came up with this particular version.