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 has four image formats for thumbnails:
| Value | Identifier | Description |
|---|---|---|
| 3 | CF_METAFILEPICT |
Windows metafile format - recommended |
| 8 | CF_DIB |
Device Independent Bitmap |
| 14 | CF_ENHMETAFILE |
Enhanced Windows metafile format |
| 2 | CF_BITMAP |
Bitmap - Obsolete - Use CF_DIB instead |
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 |
FIXME: Describe the Device Independent Bitmap format!
FIXME: Describe the Macintosh clipboard formats!