How to update the internal thmbnail of a jpg image ?

[neoneuf]

I use the Bitmap class of GDI+ to access and modify a jpeg image.
However after saving, the internal thumbnail is not updated.
How can I update it ?
I think I should use PropretyItem but don't know how exactly.
Does anyone has a sample code to do this ?

[neoneuf]

nobody knows ?

[neoneuf]

Finally I found how to do it.
The idea is to create a new bitmap (jpeg here) using the Clone method.
When the clone bitmap is created the metadata (and so the thumbnail) are not copied.
So first I save the metadata of the original bitmap.
Then I create the clone.
Then I copy the metadata except the thumbnail to the clone bitmap.
When I save the clone bitmap, it creates automatically a new thumbnail and add it to the metadata of the clone bitmap.
Here is the code if someone needs to do the same thing.

BTW, if someone knows how to configure the encoder (for example to choose the compression ratio of the jpeg) I'm interested...

// Save the metadata
UINT totalBufferSize;
UINT numProperties;
orginalBMP.GetPropertySize(&totalBufferSize, &numProperties);
PropertyItem* pAllItems = (PropertyItem*)malloc(totalBufferSize);
orginalBMP.GetAllPropertyItems(totalBufferSize, numProperties, pAllItems);

// Create the clone
Bitmap * pCloneBMP=orginalBMP.Clone(0,0,pres.GetWidth(),pres.GetHeight(),pres.GetPixelFormat());

//Copy all the metadata except the thumbnail
for(UINT j = 0; j < numProperties; ++j)
{
       if (pAllItems[j].id==0x501B) continue; //avoid copying the existing thumbnail

	pCloneBMP->SetPropertyItem(&pAllItems[j]);
}

free(pAllItems);	

// Save the clone : this will create a new thumbnail
CLSID Clsid;
GetEncoderClsid(L"image/jpeg", &Clsid);
pCloneBMP->Save(FileName.AllocSysString(), &Clsid, NULL); //FileName is a CString

[boybawang]

The thumbnail is part of the EXIF profile. ImageMagick only updates the resolution of the EXIF profile. For the thumbnail you'll need another utility..