Itext中设置透明度不起做用的问题

今天在设置画的线条透明度的时候，用baseColor设置透明度不起做用，最后发现原来要靠pdfGState来设置才行，具体代码以下：

        PdfGState gState = new PdfGState();
        if(alphas < 255){
            gState.setStrokeOpacity(0.5f);
        }else{
            gState.setStrokeOpacity(1f);
        }
        canvas.setGState(gState);

        Log.i(TAG, "red =="+red + ",green =="+green +",blue =="+blue+",alphas =="+alphas);

        BaseColor baseclor= new BaseColor(red, green, blue);

        canvas.setColorStroke(baseclor);