JavaScript Rasterizer

// @CasualEffects & @pervognsen
var shift = ((border !== TRANSPARENT) && (border !== fill)) ? 0.5 : 0;
        
// For each non-horizontal edge, store:
//
//    [startX, startY, dx/dy slope, vertical height].
//
// These are the values needed for the edge-intersection test.  Add edges so that the
// start Y coordinate is less than the end one.
var edgeArray = [];        
var y0 = height, y1 = -1;
function addEdge(Sx, Sy, Ex, Ey) {
    if (Sy < Ey) {
        // Update bounding box
        if (Sy < y0) { y0 = Sy; }
        if (Ey > y1) { y1 = Ey; }
        edgeArray.push([Sx, Sy, (Ex - Sx) / (Ey - Sy), Ey - Sy]);
    } else if (Sy > Ey) {
        addEdge(Ex, Ey, Sx, Sy);
    }
}

addEdge(Ax, Ay, Bx, By);
addEdge(Bx, By, Cx, Cy);
addEdge(Cx, Cy, Ax, Ay);

// Intentionally left as a float to avoid float to int conversion for the inner loop
y0 = Math.max(0,  Math.floor(y0));
y1 = Math.min(height - 1, Math.floor(y1));
for (var y = y0; y <= y1; ++y) {
    // For this scanline, intersect the edge lines of the triangle.
    // As a convex polygon, we can simply intersect ALL edges and then
    // take the min and max intersections.
    var x0 = Infinity, x1 = -Infinity;
    for (var i = edgeArray.length - 1; i >= 0; --i) {
        var seg = edgeArray[i];

        var ry = y - seg[1];
        if ((ry >= 0) && (ry < seg[3])) {
            x = seg[0] + ry * seg[2];
            if (x < x0) { x0 = x; }
            if (x > x1) { x1 = x; }
        }
    }

    if ((x0 <= 63.5) && (x1 >= 0)) {
        _screen.fill(fill,
                     (Math.max(Math.round(x0 + shift) << 0,  0) + (y << logWidth)) | 0,
                     (Math.min(Math.round(x1 - shift) << 0, width - 1) + (y << logWidth) + 1) | 0);
    }
}