问如何用画布绘制梯形图形

EN

function drawTrapezoid(ctx, img, x, y, w, h, factor) {

    var startPoint = x + w * 0.5 * (factor*0.01), // calculate top x
        xi, yi, scale = img.height / h,           // used for interpolation/scale
        startLine = y,                            // used for interpolation
        endLine = y + h;                          // abs. end line (y)

    for(; y < endLine; y++) {

        // get x position based on line (y)
        xi = interpolate(startPoint, y, x, endLine, (y - startLine) / h);

        // get scaled y position for source image
        yi = (y * scale + 0.5)|0;

        // draw the slice
        ctx.drawImage(img, 0, yi, img.width, 1,       // source line
                           xi.x, y, w - xi.x * 2, 1); // output line
    }

    // sub-function doing the interpolation        
    function interpolate(x1, y1, x2, y2, t) {
        return {
            x: x1 + (x2 - x1) * t,
            y: y1 + (y2 - y1) * t
        };
    }
}