HPSF THUMBNAIL HOW-TO

The VT_CF Format

Thumbnail information is stored as a VT_CF, or Thumbnail Variant. The Thumbnail Variant is used to store various types of information in a clipboard. The VT_CF can store information in formats for the Macintosh or Windows clipboard.

There are many types of data that can be copied to the clipboard, but the only types of information needed for thumbnail manipulation are the image formats.

The VT_CF structure looks like this:

Element:  Clipboard Size  Clipboard Format Tag  Clipboard Data 
Size:  32 bit unsigned integer (DWord)  32 bit signed integer (DWord)  variable length (byte array) 

The Clipboard Size refers to the size (in bytes) of Clipboard Data (variable size) plus the Clipboard Format (four bytes).

Clipboard Format Tag has four possible values:

Value  Identifier  Description 
-1L  CFTAG_WINDOWS  a built-in Windows© clipboard format value 
-2L  CFTAG_MACINTOSH  a Macintosh clipboard format value 
-3L  CFTAG_FMTID  a format identifier (FMTID) This is rarely used. 
0L  CFTAG_NODATA  No data This is rarely used. 

Windows Clipboard Data

Windows clipboard data has four image formats for thumbnails:

Value  Identifier  Description 
CF_METAFILEPICT  Windows metafile format - recommended 
CF_DIB  Device Independent Bitmap 
14  CF_ENHMETAFILE  Enhanced Windows metafile format 
CF_BITMAP  Bitmap - Obsolete - Use CF_DIB instead 

Windows Metafile Format

The most common format for thumbnails on the Windows platform is the Windows metafile format. The Clipboard places and extra header in front of a the standard Windows Metafile Format data.

The Clipboard Data byte array looks like this when an image is stored in Windows' Clipboard WMF format.

Identifier  CF_METAFILEPICT  mm  width  height  handle  WMF data 
Size  32 bit unsigned int  16 bit unsigned(?) int  16 bit unsigned(?) int  16 bit unsigned(?) int  16 bit unsigned(?) int  byte array - variable length 
Description  Clipboard WMF  Mapping Mode  Image Width  Image Height  handle to the WMF data array in memory, or 0  standard WMF byte stream 

Device Independent Bitmap

FIXME: Describe the Device Independent Bitmap format!

Macintosh Clipboard Data

FIXME: Describe the Macintosh clipboard formats!

by Drew Varner
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