BBC BASIC Programmers' Reference

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=====Extracting a file's thumbnail=====

//by Richard Russell, July 2007//\\ \\  Several file types can have associated thumbnail images, which are displayed (for example) in Windows Explorer's **Thumbnails** view. Examples of such file types are image files (e.g. BMP, GIF, JPEG), PowerPoint files, Word and Excel files (when the **Save preview picture** option is enabled) and folders.\\ \\  Displaying such thumbnails in your own program, should you want to, is quite tricky, but is made simple using the **FN_thumbnail** function listed in this article. This function returns a //handle// to the thumbnail image, which you can use as you would any other image handle. For example you can use it to display the image in a **static control**, either in a dialogue box or on your main output window.\\ \\  The code below will display, in a static control, the thumbnail for the file **CLOCK.JPG**:

<code bb4w> 
        INSTALL @lib$+"WINLIB5"

        xpos% = 100
        ypos% = 100
        cx% = 128
        cy% = 128

        _WS_BORDER = &800000
        _SS_BITMAP = &0E
        _STM_SETIMAGE = &172

        file$ = "C:\Program Files\BBC BASIC for Windows\EXAMPLES\GRAPHICS\CLOCK.JPG"
        hsb% = FN_staticbox("", xpos%, ypos%, cx%, cy%, 0, _WS_BORDER OR _SS_BITMAP)
        hbm% = FN_thumbnail(file$, cx%, cy%)
        SYS "SendMessage", hsb%, _STM_SETIMAGE, 0, hbm%
</code>

Here **xpos%** and **ypos%** are the position where you want the static control to be displayed (pixels from the top left-hand corner) and **cx%** and **cy%** are the desired width and height in pixels. If you want to display the thumbnail for a folder **don't** include a trailing backslash (**\**) in the pathname you specify.\\ \\  When you have finished with the image you should delete its handle as follows:

<code bb4w>
        SYS "DeleteObject", hbm%
</code>

Don't do this whilst the image is still displayed, since it will not refresh correctly when uncovered, or when restored after being minimised.\\ \\  The **FN_thumbnail** function is listed below. Its parameters are the file or folder whose thumbnail is wanted and the preferred width and height of the thumbnail in pixels. It returns a handle to the thumbnail image, or zero if the file does not have a thumbnail. An error will result if the file/folder does not exist or the file is an unsuitable type.

<code bb4w>
        DEF FN_thumbnail(file$, cx%, cy%)
        LOCAL patha%, pathw%, name%, ole32%, shfroot%, shf%, pidl%, iei%, flags%, hbm%
        LOCAL IShellFolder{}, IExtractImage{}, IID{}, size{}

        DIM patha% LOCAL 260, pathw% LOCAL 521
        DIM IShellFolder{QueryInterface%, AddRef%, Release%, ParseDisplayName%, \
        \                EnumObjects%, BindToObject%, BindToStorage%, CompareIDs%, \
        \                CreateViewObject%, GetAttributesOf%, GetUIObjectOf%, \
        \                GetDisplayNameOf%, SetNameOf%}
        DIM IExtractImage{QueryInterface%, AddRef%, Release%, GetLocation%, Extract%}
        DIM IID{a%,b%,c%,d%}, size{cx%,cy%}

        SYS "LoadLibrary", "OLE32.DLL" TO ole32%
        SYS "GetProcAddress", ole32%, "CoInitialize" TO `CoInitialize`
        SYS "GetProcAddress", ole32%, "CoUninitialize" TO `CoUninitialize`
        SYS `CoInitialize`, 0

        SYS "GetFullPathName", file$, 260, patha%, ^name%
        SYS "MultiByteToWideChar", 0, 0, patha%, -1, pathw%, 260

        SYS "SHGetDesktopFolder", ^shfroot%
        IF shfroot%=0 ERROR 100, "SHGetDesktopFolder failed"
        !(^IShellFolder{}+4) = !shfroot%

        SYS IShellFolder.ParseDisplayName%, shfroot%, 0, 0, pathw%, 0, ^pidl%, 0
        IF pidl%=0 ERROR 100, "IShellFolder::ParseDisplayName failed (1)"

        IID.a% = &000214E6 : REM IID_IShellFolder
        IID.b% = &00000000
        IID.c% = &000000C0
        IID.d% = &46000000

        SYS "SHBindToParent", pidl%, IID{}, ^shf%, 0
        IF shf%=0 ERROR 100, "SHBindToParent failed"
        SYS IShellFolder.Release%, shfroot%
        !(^IShellFolder{}+4) = !shf%

        SYS "MultiByteToWideChar", 0, 0, name%, -1, pathw%, 260

        SYS IShellFolder.ParseDisplayName%, shf%, 0, 0, pathw%, 0, ^pidl%, 0
        IF pidl%=0 ERROR 100, "IShellFolder::ParseDisplayName failed (2)"

        IID.a% = &BB2E617C : REM IID_IExtractImage
        IID.b% = &11D10920
        IID.c% = &C0000B9A
        IID.d% = &C1D6C24F

        SYS IShellFolder.GetUIObjectOf%, shf%, 0, 1, ^pidl%, IID{}, 0, ^iei%
        IF iei%=0 ERROR 100, "IShellFolder::GetUIObjectOf failed"
        !(^IExtractImage{}+4) = !iei%

        size.cx% = cx%
        size.cy% = cy%
        _IEIFLAG_ORIGSIZE = &40
        _IEIFLAG_QUALITY = &200
        flags% = _IEIFLAG_ORIGSIZE OR _IEIFLAG_QUALITY

        SYS IExtractImage.GetLocation%, iei%, pathw%, 260, 0, size{}, 24, ^flags%
        SYS IExtractImage.Extract%, iei%, ^hbm%

        SYS IShellFolder.Release%, shf%
        SYS IExtractImage.Release%, iei%
        SYS `CoUninitialize`
        = hbm%
</code>