Draw A Filled Polygon Using Scanline Loop

February 26, 2023 Post a Comment

I'm trying to draw a filled polygon using individual pixels in a scanline loop (so no lineTo or fill Canvas methods). I was able to achieve a triangle in this method (example below

Solution 1:

Scanline for non self intersecting polygons.

Draw any concave or convex polygon with 3 or more sides using scanline method.

Is the easiest, also slowest.
Polygon is a set of lines with start and end points.
Polygon lines can be unordered
Polygon must be closed
None of polygon lines can cross any of the other polygon lines.

Steps

Find bounding box of lines. top, left, right, bottom.
Set x, y to  top left of bounding box.
while y is less than bottom.
    Find all lines that will cross the line from left, y to right, y
    Sort lines in distance from x to point where above line crossed
    while there are sorted lines
        shift two lines from sorted lines and scan the pixels between
    add 1 to y

An implementation.

Function scanlinePoly(lines, col) draw the pixels. lines is create with createLines which is an array of lines with helper functions to add lines, find lines, sort lines, and get bounds.

Helper functions

createStar(x, y, r1, r2, points) will create a lines array for a star, x, y center of star, r1 radius, r2 second radius, points number of points
P2(x, y) returns 2D point
L2(p1, p2) returns 2D line that includes the slope of the line. p1, p2 are points as created by P2
atLineLevelY(y) returns true if line crosses scan line at y

const scanlinePoly = (lines, col) => {
    const b = lines.getBounds();
    var x, y, xx;
    ctx.fillStyle = col;
    b.left = Math.floor(b.left);
    b.top = Math.floor(b.top);
    for (y = b.top; y <= b.bottom; y ++) {
        // update 
        // old line was const ly = lines.getLinesAtY(y).sortLeftToRightAtY(y);
        // changed to
        const ly = lines.getLinesAtY(y + 0.5).sortLeftToRightAtY(y + 0.5);
        x = b.left - 1; 
        while(x <= b.right) {
            const nx1 = ly.nextLineFromX(x);
            if (nx1 !== undefined) {
                const nx2 = ly.nextLineFromX(nx1);
                if (nx2 !== undefined) {
                    const xS = Math.floor(nx1); 
                    const xE = Math.floor(nx2); 
                    for (xx = xS; xx < xE; xx++) {
                        ctx.fillRect(xx, y, 1, 1);
                    }
                    x = nx2;
                } else { break }
            } else { break }
        }
    }
}

function createLines(linesArray = []) {
     return   Object.assign(linesArray, {
        addLine(l) { this.push(l) },
        getLinesAtY(y) { return createLines(this.filter(l => atLineLevelY(y, l))) },
        sortLeftToRightAtY(y) {
            for (const l of this) { l.dist = l.p1.x + l.slope * (y - l.p1.y) }
            this.sort((a,b) => a.dist - b.dist);
            return this;
        },
        nextLineFromX(x) { // only when sorted
            const line = this.find(l => l.dist > x);
            return line ? line.dist : undefined;
        },
        getBounds() {
            var top = Infinity, left = Infinity;
            var right = -Infinity,  bottom = -Infinity;
            for (const l of this) {
                top = Math.min(top, l.p1.y, l.p2.y);
                left = Math.min(left, l.p1.x, l.p2.x);
                right = Math.max(right, l.p1.x, l.p2.x);
                bottom = Math.max(bottom, l.p1.y, l.p2.y);
            }
            return {top, left, right, bottom};
        },
    });
}

const createStar = (x, y, r1, r2, points) => {
    var i = 0, pFirst, p1, p2;
    const lines = createLines()
    while (i < points * 2) {
        const r = i % 2 ? r1 : r2;
        const ang = (i / (points * 2)) * Math.PI * 2;
        p2 = P2(Math.cos(ang) * r + x, Math.sin(ang) * r + y);
        if (pFirst === undefined) { pFirst = p2 };
        if (p1 !== undefined) { lines.addLine(L2(p1, p2)) }
        p1 = p2;
        i++;
    }
    lines.addLine(L2(p2, pFirst));    
    return lines;
}
const ctx = canvas.getContext("2d");

const P2 = (x = 0,y = 0) => ({x, y});
const L2 = (p1 = P2(), p2 = P2()) => ({p1, p2, slope: (p2.x - p1.x) / (p2.y - p1.y)});
const atLineLevelY = (y, l) => l.p1.y < l.p2.y && (y >= l.p1.y && y <= l.p2.y) || (y >= l.p2.y && y <= l.p1.y);

canvas.addEventListener("click", () => {
    ctx.clearRect(0,0,200,200);
    const star = createStar(
        100, 90, 
        Math.random() * 80 + 10,
        Math.random() * 80 + 10,
        Math.random() * 20 + 2 | 0
    );
    scanlinePoly(star, "#F00")


})


const star = createStar(100, 90, 90, 40, 10);
scanlinePoly(star, "#F00")

canvas {border: 1px solid black;}

<canvas id="canvas" width="200" height="180"></canvas>Click for rand star

Note that inner loop

for (xx = xS; xx < xE; xx++) {
    ctx.fillRect(xx, y, 1, 1);
}

Can be replaced with ctx.fillRect(xS, y, xE - xS, 1) to greatly improve performance.

UPDATE

Looking at my answer again to see if it could be improved I noticed a problem that resulted in lines incorrectly rendered.

To fix the the first line inside the outer loop of function scanlinePoly needs to be changed from.

const ly = lines.getLinesAtY(y).sortLeftToRightAtY(y);

const ly = lines.getLinesAtY(y + 0.5).sortLeftToRightAtY(y + 0.5);

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