谁做过Twain开发

操作外围设备去取像,Twain接口如何设置dpi和图片大小等参数.我试了不行.
    TwCap[] tCap = new TwCap[3];
            tCap[0] = TwCap.XferCount;
            tCap[1] = TwCap.ICAP_XRESOLUTION;
            tCap[2] = TwCap.ICAP_YRESOLUTION;
            short[] sval = new short[3];
            sval[0]=(short)1;
            sval[1]=(short)200;
            sval[2]=(short)200;            TwCapability cap = new TwCapability(tCap, sval);            //设置DS参数
            rc = DScap(appid, srcds, TwDG.Control, TwDAT.Capability, TwMSG.Set, cap);  TwImageInfo iinf = new TwImageInfo();
                iinf.XResolution = 200;
                iinf.YResolution = 200;
                rc = DSiinf(appid, srcds, TwDG.Image, TwDAT.ImageInfo, TwMSG.Get, iinf);
                if (rc != TwRC.Success)
                {
                    CloseSrc();
                    return pics;
                }                //真正传输图像,，写入hbitmap指向的结构
                rc = DSixfer(appid, srcds, TwDG.Image, TwDAT.ImageNativeXfer, TwMSG.Get, ref hbitmap);
                if (rc != TwRC.XferDone)
                {
                    CloseSrc();
                    return pics;
                }                //结束传输
                rc = DSpxfer(appid, srcds, TwDG.Control, TwDAT.PendingXfers, TwMSG.EndXfer, pxfr);
                if (rc != TwRC.Success)
                {
                    CloseSrc();
                    return pics;
                }