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//将Bitmap位图转化为base64字符串
function BitmapToString(img:TBitmap):string ;
var
  ms:TMemoryStream;
  ss:TStringStream;
  s:string;
begin
    ms := TMemoryStream.Create;
    img.SaveToStream(ms);
    ss := TStringStream.Create('');
    ms.Position:=0;
    EncodeStream(ms,ss);//将内存流编码为base64字符流
    s:=ss.DataString;
    ms.Free;
    ss.Free;
    result:=s; 
end;

///将base64字符串转化为Bitmap位图
function StringToBitmap(imgStr:string):TBitmap;
var ss:TStringStream;
    ms:TMemoryStream;
    bitmap:TBitmap;
begin
    ss := TStringStream.Create(imgStr);
    ms := TMemoryStream.Create;
    DecodeStream(ss,ms);//将base64字符流还原为内存流
    ms.Position:=0;
    bitmap := TBitmap.Create;
    bitmap.LoadFromStream(ms);
    ss.Free;
    ms.Free;
    result :=bitmap;
end;