Extracting JPEG preview image from DNG file

Question

My company is using the DNG SDK to support raw or DNG files for users of our digital asset management software. We extract a jpeg preview/thumbnail from ALL file types we support.
My question is: how can we extract a jpeg preview from a DNG file, using the latest version of the DNG SDK?
Currently, our software uses the SDK to read a DNG file and write a tiff file, and passes the tiff to imagemagick to get a resized jpeg we can use for a preview or thumbnail.
When we extract the tiff from the customer's file, we get a base version of the image that does not show subsequent colour adjustments made in (eg) CS3; the customer would like to get a preview that shows (especially) the colour adjustments that have been made.
I know the jpeg preview exists inside the DNG, I used exiftools to extract it,and it does show the colour adjustments made.
How do I get that preview image out of the DNG file using the DNG SDK? Does the DNG SDK support that operation?
The SDK has a lot of API, but not much documentation or samples to show usage.
Thanks for any help anyone can provide.

Answer 1

Maybe it isn't the smartest way, but this should work:<br /><br />... call info.Parse (host, stream) etc here - the usual stuff<br />{<br />     // find biggest jpeg image for preview<br />     unsigned int subMaxW=0;<br />     int subMaxI=-1;<br />     for(unsigned int i=0;i<info.fIFDCount;i++)<br />     {<br />          dng_ifd* pIFD=info.fIFD[i].Get();<br />          if (ccJPEG==pIFD->fCompression && 1==pIFD->fNewSubFileType)<br />          {<br />               if (pIFD->fImageWidth>subMaxW)<br />               {<br />                    subMaxW=pIFD->fImageWidth;<br />                    subMaxI=i;<br />               }<br />          }<br />     }<br />     if (subMaxI>=0) // load desired jpeg preview<br />     {<br />          unsigned int i=subMaxI;<br /><br />          dng_ifd* pIFD=info.fIFD[i].Get();<br />          if (ccJPEG==pIFD->fCompression && 1==pIFD->fNewSubFileType)<br />          {<br />               // seek to pIFD->fTileOffset[0] and load jpeg<br />          }               <br />     }<br />}