求助：ISaveAs 保存为png时 nodata区域变黑

ISaveAs 保存为png时 nodata区域变黑 在arcmap中再次打开变成了RGB0，0，0。ISaveAs2同样问题。
IWorkspace pWorkSpace = pWorkspaceFactory.OpenFromFile(strTempFolder, 0);
ISaveAs2 psaveAs = (ISaveAs2)bmpRaster;
var dataset = psaveAs.SaveAs(tempPngPicFullFileName, pWorkSpace, "PNG");