Saving+a+GIF+image

//by Richard Russell, December 2006//

The main BBC BASIC for Windows documentation explains how to [|display a GIF image] and elsewhere you can find out how to load a GIF image for use in your program. This article describes how to **save** an image as a GIF file. It relies on the presence of the //GDI Plus// library so will work only on Windows XP (or later) or if you have specifically installed **GDIPLUS.DLL** on the target computer. Microsoft permits you to redistribute this file so you can include it with your program if necessary.

To begin with you need to have a **handle** to the bitmap you want to save (see below if instead the bitmap is in the form of a **DIB**). One way of obtaining a handle is to load the image from a file (e.g. a BMP file):

code format="bb4w" bmpfile$ = "\Windows\Soap Bubbles.bmp" SYS "LoadImage", 0, bmpfile$, 0, 0, 0, 16 TO hbitmap% code Here the image is loaded at its original size. You can alternatively scale the image to different dimensions:

code format="bb4w" bmpfile$ = "\Windows\Soap Bubbles.bmp" SYS "LoadImage", 0, bmpfile$, 0, dx%, dy%, 16 TO hbitmap% code Where **dx%** and **dy%** are the wanted width and height of the image respectively (the scaling quality is not particularly good so for best results you might prefer to scale the image using a third-party program).

There are a number of other ways in which you might obtain a bitmap handle, which are outside the scope of this article. You can easily obtain a handle to whatever is displayed in your program's output window:

code format="bb4w" SYS "GetCurrentObject", @memhdc%, 7 TO hbitmap% code but this ordinarily returns the //entire// 1920 x 1440 bitmap which is probably not what you want. To save just a region of your program's output window the easiest way is probably to save it first as a BMP file (using ***GSAVE**) then load it using LoadImage as shown above.

Note that a GIF file has a maximum of 256 different colours, and if you use the above methods a colour palette consisting of a standard selection of colours will be used. The palette will **not** be optimised to the actual set of colours present in the source image. This can result in the quality of the GIF image being significantly worse than it needs to be.

If this is important you should first convert the image to an 8-bit (256-colour) BMP file, using an appropriate image editor capable of synthesising an optimised custom palette (for example using Heckbert quantisation). You should then load that BMP file as follows (using LoadImage will //not// preserve the palette):

code format="bb4w" bmpfile$ = "tcf.bmp" bmpfile% = OPENIN(bmpfile$) IF bmpfile% = 0 ERROR 100, "Couldn't open file "+bmpfile$ size% = EXT#bmpfile% CLOSE #bmpfile% DIM bmpfile% size%-1 OSCLI "LOAD """+bmpfile$+""" "+STR$~bmpfile%

SYS "CreateDIBSection", @memhdc%, bmpfile%+14, 0, ^vbits%, 0, 0 TO hbitmap% IF hbitmap% = 0 ERROR 100, "Couldn't create DIBSection" O% = bmpfile%!10 FOR I% = 0 TO size%-O%-4 STEP 4 vbits%!I% = bmpfile%!(I%+O%) NEXT code Once you've got a handle to the bitmap you simply save it as a GIF file as follows:

code format="bb4w" PROCsavegif(hbitmap%, filename$) code Here **filename$** is the name of the GIF file to create.

One you've saved the file you should delete the bitmap handle:

code format="bb4w" SYS "DeleteObject", hbitmap% code Finally, here's the code for **PROCsavegif** itself:

code format="bb4w" DEF PROCsavegif(hbitmap%, filename$) LOCAL gdiplus%, ole32% SYS "LoadLibrary", "GDIPLUS.DLL" TO gdiplus% IF gdiplus% = 0 ERROR 100, "Couldn't load GDIPLUS.DLL" SYS "GetProcAddress", gdiplus%, "GdiplusStartup" TO `GdiplusStartup` SYS "GetProcAddress", gdiplus%, "GdiplusShutdown" TO `GdiplusShutdown` SYS "GetProcAddress", gdiplus%, "GdipCreateBitmapFromHBITMAP" TO `GdipCreateBitmapFromHBITMAP` SYS "GetProcAddress", gdiplus%, "GdipDisposeImage" TO `GdipDisposeImage` SYS "GetProcAddress", gdiplus%, "GdipSaveImageToFile" TO `GdipSaveImageToFile` SYS "LoadLibrary", "OLE32.DLL" TO ole32% SYS "GetProcAddress", ole32%, "CLSIDFromString" TO `CLSIDFromString` LOCAL tSI{}, lRes%, lGDIP%, lBitmap%, tGifEncoder%, guid%, filename% DIM tGifEncoder% LOCAL 15, guid% LOCAL 79, filename% LOCAL 2*LEN(filename$)+1 DIM tSI{GdiplusVersion%, DebugEventCallback%, \ \      SuppressBackgroundThread%, SuppressExternalCodecs%} REM Initialize GDI+ tSI.GdiplusVersion% = 1 SYS `GdiplusStartup`, ^lGDIP%, tSI{}, 0 TO lRes% IF lRes% ERROR 100, "GDI+ error "+STR$(lRes%) REM Create the GDI+ bitmap from the image handle SYS `GdipCreateBitmapFromHBITMAP`, hbitmap%, 0, ^lBitmap% TO lRes% IF lRes% ERROR 100, "GDI+ error "+STR$(lRes%) REM Initialize the encoder GUID SYS "MultiByteToWideChar", 0, 0, "{557CF402-1A04-11D3-9A73-0000F81EF32E}", -1, guid%, 40 SYS `CLSIDFromString`, guid%, tGifEncoder% REM Save the image SYS "MultiByteToWideChar", 0, 0, filename$, -1, filename%, LEN(filename$)+1 SYS `GdipSaveImageToFile`, lBitmap%, filename%, tGifEncoder%, 0 TO lRes% IF lRes% ERROR 100, "GDI+ error "+STR$(lRes%) REM Destroy the bitmap SYS `GdipDisposeImage`, lBitmap% REM Shutdown GDI+ SYS `GdiplusShutdown`, lGDIP% SYS "FreeLibrary", gdiplus% ENDPROC code

If, instead of a bitmap **handle**, you have a bitmap (DIB) stored in memory you can use this alternative routine: code format="bb4w" DEF PROCsavegifdib(dib%, bmi%, filename$) LOCAL gdiplus%, ole32% SYS "LoadLibrary", "GDIPLUS.DLL" TO gdiplus% IF gdiplus% = 0 ERROR 100, "Couldn't load GDIPLUS.DLL" SYS "GetProcAddress", gdiplus%, "GdiplusStartup" TO `GdiplusStartup` SYS "GetProcAddress", gdiplus%, "GdiplusShutdown" TO `GdiplusShutdown` SYS "GetProcAddress", gdiplus%, "GdipCreateBitmapFromGdiDib" TO `GdipCreateBitmapFromGdiDib` SYS "GetProcAddress", gdiplus%, "GdipDisposeImage" TO `GdipDisposeImage` SYS "GetProcAddress", gdiplus%, "GdipSaveImageToFile" TO `GdipSaveImageToFile` SYS "LoadLibrary", "OLE32.DLL" TO ole32% SYS "GetProcAddress", ole32%, "CLSIDFromString" TO `CLSIDFromString` LOCAL tSI{}, lRes%, lGDIP%, lBitmap%, tGifEncoder%, guid%, filename% DIM tGifEncoder% LOCAL 15, guid% LOCAL 79, filename% LOCAL 2*LEN(filename$)+1 DIM tSI{GdiplusVersion%, DebugEventCallback%, \ \      SuppressBackgroundThread%, SuppressExternalCodecs%} REM Initialize GDI+ tSI.GdiplusVersion% = 1 SYS `GdiplusStartup`, ^lGDIP%, tSI{}, 0 TO lRes% IF lRes% ERROR 100, "GDI+ error "+STR$(lRes%) REM Create the GDI+ bitmap from the DIB SYS `GdipCreateBitmapFromGdiDib`, bmi%, dib%, ^lBitmap% TO lRes% IF lRes% ERROR 100, "GDI+ error "+STR$(lRes%) REM Initialize the encoder GUID SYS "MultiByteToWideChar", 0, 0, "{557CF402-1A04-11D3-9A73-0000F81EF32E}", -1, guid%, 40 SYS `CLSIDFromString`, guid%, tGifEncoder% REM Save the image SYS "MultiByteToWideChar", 0, 0, filename$, -1, filename%, LEN(filename$)+1 SYS `GdipSaveImageToFile`, lBitmap%, filename%, tGifEncoder%, 0 TO lRes% IF lRes% ERROR 100, "GDI+ error "+STR$(lRes%) REM Destroy the bitmap SYS `GdipDisposeImage`, lBitmap% REM Shutdown GDI+ SYS `GdiplusShutdown`, lGDIP% SYS "FreeLibrary", gdiplus% ENDPROC code

You would call it as follows: code format="bb4w" PROCsavegifdib(dibits%, bmi{}, filename$) code where **dibits%** is the address of the bitmap data and **bmi{}** is a BITMAPINFO structure containing the dimensions etc. and (optionally) colour palette for the bitmap.