Java读取DWG预览图实现方法

public void generateThumbnail(File input, File output) throws IOException, de.uni_siegen.wineme.come_in.thumbnailer.ThumbnailerException {
    //GENERATE FROM EXISTING BITMAP IN DWG
    byte[] outputByte = new byte[4096];
    FileInputStream fis = new FileInputStream(input);
    fis.skip(0x0D);
    fis.read(outputByte, 0, 4);
    int PosSentinel = (((outputByte[3])&0xFF)*256*256*256)+(((outputByte[2])&0xFF)*256*256)+(((outputByte[1])&0xFF)*256)+((outputByte[0])&0xFF);
    fis.skip(PosSentinel-0x0D-4+30);
    outputByte[1]=0;
    fis.read(outputByte, 0, 1);
    int TypePreview = ((outputByte[0])&0xFF);
    if (TypePreview==2) {
        fis.read(outputByte, 0, 4);
        int PosBMP = (((outputByte[3])&0xFF)*256*256*256)+(((outputByte[2])&0xFF)*256*256)+(((outputByte[1])&0xFF)*256)+((outputByte[0])&0xFF);
        fis.read(outputByte, 0, 4);
        int LenBMP = (((outputByte[3])&0xFF)*256*256*256)+(((outputByte[2])&0xFF)*256*256)+(((outputByte[1])&0xFF)*256)+((outputByte[0])&0xFF);
        fis.skip(PosBMP-(PosSentinel+30)-1-4-4+14);
        fis.read(outputByte, 0, 2);
        int biBitCount = (((outputByte[1])&0xFF)*256)+((outputByte[0])&0xFF);
        fis.skip(-16);
        int bisSize=0;
        int bfSize = 0;
        if (biBitCount<9)
            bfSize = 54 + 4 * ((int) (Math.pow(2,biBitCount))) + LenBMP ;
        else bfSize = 54 + LenBMP ;
        //WORD "BM"
        outputByte[0]=0x42;outputByte[1]=0x4D;
        //DWORD bfSize
        outputByte[2]=(byte)(bfSize&0xff);outputByte[3]=(byte)(bfSize>>8&0xff);
        outputByte[4]=(byte)(bfSize>>16&0xff);outputByte[5]=(byte)(bfSize>>>24);