MapUnitSquareToGivenPoints.cs

private static SKMatrix MapUnitSquareToGivenPoints(SKPoint topLeft, SKPoint topRight, SKPoint bottomLeft, SKPoint bottomRight)
{
    var rightDiff = bottomRight - topRight;
    var bottomDiff = bottomRight - bottomLeft;

    var determinant = rightDiff.X * bottomDiff.Y - bottomDiff.X * rightDiff.Y;

    var topDiff = topRight - topLeft;
    var leftDiff = bottomLeft - topLeft;

    var persp0 = -(bottomDiff.X * topDiff.Y - bottomDiff.Y * topDiff.X) / determinant;
    var persp1 = (rightDiff.X * leftDiff.Y - rightDiff.Y * leftDiff.X) / determinant;

    return new SKMatrix
    {
        ScaleX = persp0 * topRight.X + topDiff.X,
        SkewX = persp1 * bottomLeft.X + leftDiff.X,
        TransX = topLeft.X,
        SkewY = persp0 * topRight.Y + topDiff.Y,
        ScaleY = persp1 * bottomLeft.Y + leftDiff.Y,
        TransY = topLeft.Y,
        Persp0 = persp0,
        Persp1 = persp1,
        Persp2 = 1,
    };
}