32bitmap To 8bit PNG 만들기 삽질중!

[1단계]
encoder 이용하여 colorDepth 를 통해 눌러주기 ----> 실패
System.Drawing.Imaging.ImageCodecInfo encoder2[] = System.Drawing.Imaging.ImageCodecInfo.GetImageDecoders();
System.Drawing.Imaging.ImageCodecInfo encoder = GetEncoderInfo("image/png");
System.Drawing.Imaging.Encoder myEncoder = System.Drawing.Imaging.Encoder.ColorDepth;
System.Drawing.Imaging.EncoderParameters myEncoderParameters = new System.Drawing.Imaging.EncoderParameters(1);
System.Drawing.Imaging.EncoderParameter myEncoderParameter[] = new System.Drawing.Imaging.EncoderParameter[1];
myEncoderParameter[0] = new System.Drawing.Imaging.EncoderParameter(myEncoder, 8L);
// 이 옵션 안먹는다. tiff 파일만 먹는듯.
myEncoderParameters.set_Param(myEncoderParameter);
String fileName = ((i < 10) ? "0" : "") + i;
bmpInfo[i].img.Save(path + "\\" + fileName + ".png", encoder, myEncoderParameters);


private static ImageCodecInfo GetEncoderInfo(String mimeType)
{

int j;

ImageCodecInfo encoders[];

encoders = ImageCodecInfo.GetImageEncoders();
for (j = 0; j < encoders.length; j++)
{

if (encoders[j].get_MimeType().Equals(mimeType))

{

return encoders[j];

}

}
return null;

} //GetEncoderInfo