48h.arts.ens.fr/js/triangles-mask.js

(function () {
    /*--------------------------------------------------------*/
    /* INITIALISATION
    /*--------------------------------------------------------*/

    const canvas = document.getElementById("main-header-background");
    const context = canvas.getContext("2d");

    // Get the node dimensions and enforce the canvas width and height
    let WIDTH = canvas.clientWidth;
    let HEIGHT = canvas.clientHeight;

    canvas.setAttribute("width", WIDTH);
    canvas.setAttribute("height", HEIGHT);

    // Delay between point positions' updates
    const POINT_POSITIONS_UPDATE_DELAY = 50; // ms

    // Number of points and initial position parameters
    const NB_POINTS_PER_ROW = 4 + Math.round(WIDTH / 200);
    const NB_POINTS_PER_COL = 5;

    const INIT_ROW_SPACING = HEIGHT / (NB_POINTS_PER_COL - 1);
    const INIT_COL_SPACING = WIDTH / (NB_POINTS_PER_ROW - 3);

    const X_ORIGIN = -INIT_COL_SPACING;
    const Y_ORIGIN = -100;

    const INIT_MAX_X_SHIFT = INIT_ROW_SPACING / 3;
    const INIT_MAX_Y_SHIFT = INIT_COL_SPACING / 4;

    // Movement of the points
    const POINT_MIN_X_SPEED = 0.15;
    const POINT_MAX_X_SPEED = 0.3;

    const POINT_MIN_Y_SPEED = 0.15;
    const POINT_MAX_Y_SPEED = 0.3;

    const POINT_MIN_X_RADIUS = 1;
    const POINT_MAX_X_RADIUS = 2;

    const POINT_MIN_Y_RADIUS = 1;
    const POINT_MAX_Y_RADIUS = 2;

    // Canvas update halting
    const NB_CANVAS_UPDATES_DELAYS_TO_RECORD = 100;
    const ANIM_HALT_TIME_THRESHOLD = 100; // ms




    /*--------------------------------------------------------*/
    /* GRID OF POINTS
    /*--------------------------------------------------------*/

    const points = [];

    // Create all points
    // Each point is defined by its position
    // plus several parameters which control its movement
    function createAllPoints () {
        for (let row = 0; row < NB_POINTS_PER_COL; row++) {
            for (let col = 0; col < NB_POINTS_PER_ROW; col++) {
                points.push({
                    // Initial position
                    x: X_ORIGIN + col * INIT_COL_SPACING + ((row % 2 === 1) ? INIT_COL_SPACING : 0),
                    y: Y_ORIGIN + row * INIT_ROW_SPACING,

                    // Parameters of the movement
                    xDirection: (Math.random() > 0.5 ? 1 : -1),
                    yDirection: (Math.random() > 0.5 ? 1 : -1),

                    xSpeed: (Math.random() * (POINT_MAX_X_SPEED - POINT_MIN_X_SPEED)) + POINT_MIN_X_SPEED,
                    ySpeed: (Math.random() * (POINT_MAX_Y_SPEED - POINT_MIN_Y_SPEED)) + POINT_MIN_Y_SPEED,

                    xRadius: (Math.random() * (POINT_MAX_X_RADIUS - POINT_MIN_X_RADIUS)) + POINT_MIN_X_RADIUS,
                    yRadius: (Math.random() * (POINT_MAX_Y_RADIUS - POINT_MIN_Y_RADIUS)) + POINT_MIN_Y_RADIUS
                });
            }
        }

        // Move each point a little bit
        // This will make the triangular structure look more random
        for (let point of points) {
            point.x += (2 * Math.random() * INIT_MAX_X_SHIFT) - INIT_MAX_X_SHIFT;
            point.y += (2 * Math.random() * INIT_MAX_Y_SHIFT) - INIT_MAX_Y_SHIFT;
        }
    }

    createAllPoints();



    /*--------------------------------------------------------*/
    /* TRIANGLE DRAWING
    /*--------------------------------------------------------*/

    // Draw a line between two points
    function drawLineBetweenPoints (from, to) {
        context.beginPath();
        context.moveTo(from.x, from.y);
        context.lineTo(to.x, to.y);
        context.stroke();
    }


    // Draw white triangles with transparent contours
    function drawTriangles () {
        // Style of the drawing
        context.fillStyle = "#FFFFFF";
        context.lineWidth = 3;

        // Fill the canvas with a white background...
        context.globalCompositeOperation = "source-over";
        context.fillRect(0, 0, WIDTH, HEIGHT);

        // ...and erase the contours of the triangles
        context.globalCompositeOperation = "destination-out";

        for (let row = 0; row < NB_POINTS_PER_COL; row++) {
            for (let col = 0; col < NB_POINTS_PER_ROW; col++) {
                // Nothing to do with the last point of each row
                if (col == NB_POINTS_PER_ROW - 1) {
                    continue;
                }

                // Get the current index and point
                const index = row * NB_POINTS_PER_ROW + col;
                const point = points[index];

                // When applicable, draw lines to
                // - the next point on the same row
                // - the aligned point on the previous row
                // - the aligned point on the next row
                if (! (row % 2 === 1 && col === NB_POINTS_PER_ROW - 2))
                    drawLineBetweenPoints(point, points[index + 1]);

                if (row > 0)
                    drawLineBetweenPoints(point, points[index - NB_POINTS_PER_ROW]);
                if (row < NB_POINTS_PER_COL - 1)
                    drawLineBetweenPoints(point, points[index + NB_POINTS_PER_ROW]);

                // If the row number is odd, when applicable, also draw lines to
                // - the next point on the previous row
                // - the next point on the next row
                if (row % 2 === 1) {
                    if (row > 0)
                        drawLineBetweenPoints(point, points[index - NB_POINTS_PER_ROW + 1]);
                    if (row < NB_POINTS_PER_COL - 1)
                        drawLineBetweenPoints(point, points[index + NB_POINTS_PER_ROW + 1]);
                }

            }
        }
    }




    /*--------------------------------------------------------*/
    /* POINT POSITIONS UPDATE
    /*--------------------------------------------------------*/

    // N° of the current position update iteration
    let positionUpdateIter = 0;

    
    function updatePointPositions () {
        const nbPoints = points.length;

        for (let i = 0; i < nbPoints; i++) {
            const point = points[i];

            point.x += point.xDirection * Math.cos(positionUpdateIter * point.xSpeed) * point.xRadius;
            point.y += point.yDirection * Math.sin(positionUpdateIter * point.ySpeed) * point.yRadius;
        }

        positionUpdateIter++;
    }

    window.setInterval(updatePointPositions, POINT_POSITIONS_UPDATE_DELAY);



    
    /*--------------------------------------------------------*/
    /* CANVAS UPDATE
    /*--------------------------------------------------------*/

    // Array of times between canvas updates
    // It is used to compute whether the drawing process should be halted
    // (if the device is too slow for the animation to look fluid)
    const canvasUpdateDelays = new Array(NB_CANVAS_UPDATES_DELAYS_TO_RECORD);
    canvasUpdateDelays.fill(0);

    // Last recorded time (it is set for the first time a bit later)
    let lastRecordedTime = 0;

    // Flag indicating whether the canvas update has been halted or not
    let animationHasBeenHalted = false;


    function resetHaltingMechanism () {
        canvasUpdateDelays.fill(0);
        lastRecordedTime = performance.now();
    }

    function recordTimeSinceLastCanvasUpdate (time) {
        canvasUpdateDelays.shift();
        canvasUpdateDelays.push(time - lastRecordedTime);

        lastRecordedTime = time;
    }


    function animationShouldBeHalted () {
        let sumOfTimes = canvasUpdateDelays.reduce((sumOfTimes, time) => {
            return sumOfTimes + time;
        }, 0);

        // console.log(sumOfTimes / NB_CANVAS_UPDATES_DELAYS_TO_RECORD);
        return (sumOfTimes / NB_CANVAS_UPDATES_DELAYS_TO_RECORD) > ANIM_HALT_TIME_THRESHOLD;
    }


    function updateCanvas (currentTime) {
        // Record the time since last update, and halt the animation if required
        recordTimeSinceLastCanvasUpdate(currentTime);
        if (animationShouldBeHalted()) {
            animationHasBeenHalted = true;
            return;
        }

        // Repeat the drawing process
        drawTriangles();

        window.requestAnimationFrame(updateCanvas);
    }

    resetHaltingMechanism();
    updateCanvas(lastRecordedTime);

    /*
     * If the page is hidden (e.g. on tab change), the web browser
     * is likely to stop calling requestAnimationFrame (to save ressources).

     * This may halt the animation when the user displays the page again,
     * since the time since the last redraw will possibly be very high.
     * 
     * Therefore, on each visibility change, the halting mechanism must be reset!
     */
    document.addEventListener("visibilitychange", () => {
        resetHaltingMechanism();

        // In case the animation has been halted before this callback was executed,
        // the canvas update must be restarted
        if (animationHasBeenHalted)
            updateCanvas(lastRecordedTime);
    }, false);
})();