TPicture from MyBase Xml file

Question

Using Delphi 10.2 I right-click on a TClientDataSet and choose 'Save to MyBase Xml UTF-8 table.' I now have an XML file with this format:

<ROW NAME="Angel Fish" SIZE="2" WEIGHT="2" AREA="Computer Aquariums" BMP="AQAAAfY+AABCTfY+AAAAAAAAdgAAACgAAADIAAAAoAAAAAEA ...

The BMP field is defined as:

<FIELD attrname="BMP" fieldtype="bin.hex" SUBTYPE="TypedBinary" WIDTH="1"/>

I'm trying to convert what I believe to be a Base64 string representation of a binary bitmap file into a picture. So far I've got this:

uses 
  XMLDoc, Vcl.ComCtrls, XMLIntf, IdCoder, IdCoderMIME, IdGlobal,
  Vcl.ExtCtrls;

procedure TForm2.Button1Click(Sender: TObject);
var Doc:TXMLDocument;
First:IXMLNode;
Str:String;
Bytes: TIdBytes;
Pic:TPicture;
Stream:TMemoryStream;
Writer: TBinaryWriter;

begin
Doc:=TXMLDocument.Create(Self);
Doc.FileName:='D:\temp\ClientDataSet2.xml';
Doc.Active:=true;

First:=Doc.DocumentElement.ChildNodes['ROWDATA'].ChildNodes.First;
Str:=First.Attributes['BMP'];

Bytes:=TIdDecoderMIME.DecodeBytes(Str);
Stream:=TMemoryStream.Create;
Writer:=TBinaryWriter.Create(Stream);

Writer.Write(TBytes(Bytes));
Stream.Position:=0;

Pic:=TPicture.Create;
Pic.LoadFromStream(Stream);

Image1.Picture:=Pic;

RichEdit1.Text:=Str;
end;

However, TPicture.LoadFromStream throws this exception:

First chance exception at $74DCCBB2. Exception class EInvalidGraphic with message 'Unsupported stream format'.

Could anybody please tell me what I'm doing wrong? Many thanks.

Why are you reading the raw XML and not just loading it back into a TClientDataset and accessing it through the TField? — Ken White, Dec 29 '17 at 01:55
Because I want to reuse this XML file in C#, which I can do minus the image. Having not been able to import the image in C# I thought I'd go back to Delphi to work out the issues. — Shunyata Kharg, Dec 29 '17 at 07:55

Remy Lebeau · Accepted Answer · 2017-12-29T11:34:33.003

The base64 string you showed decodes to binary data that begins with 8 bytes (0x01 0x00 0x00 0x01 0xF6 0x3E 0x00 0x00) before the actual BMP data (0x42 0x4D ...) . The subtype of the BMP attribute is TypedBinary, so this is likely some kind of metadata header to indicate the data is a BMP image. You need to omit those beginning bytes before loading the stream data into TPicture.

BTW, TPicture.LoadFromStream() just calls TPicture.Bitmap.LoadFromStream(), so you should replace TPicture with TBitmap instead:

Bmp := TBitmap.Create;
try
  Bmp.LoadFromStream(Stream); 
  Image1.Picture.Assign(Bmp);
finally
  Bmp.Free;
end;

I'm supposed to avoid saying thank you, but thank you anyhow, that was the solution. — Shunyata Kharg, Dec 29 '17 at 11:35

TPicture from MyBase Xml file

1 Answers1