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DirectDraw: Loading Jpg's and Gif's
By: Jack Hoxley
Written: May 2000


So far (if you're following my tutorials) you will have only been able to load bitmaps into a directdraw surface. Directdraw has no built-in support for loading any file type other than .bmp, this is fine to a certain point, but bitmaps tend to have large file sizes - something most people want to avoid when distributing their application. There are other, newer, file compression technologies available at the moment, using these can have a considerable file-size advantage over bitmap files.

For example, a 640x480 picture with 65536 colours in it.
Saved as a Bitmap the file size was:...............................900kb
Saved as a highy compressed .Jpg the file size was:..........3.47kb (0.38% of original size)
Saved as a low-compression .Jpg the file size was:............254kb (28% of original size)
Saved as a .Gif the file size was:....................................312kb (35% of original size)

As you can quite clearly see from the above figures, different file format can drastically reduce the size of files. One thing to bare in mind, the .Gif file format only allows 256 colours and the Jpg files have slightly poorer quality (The 3.47kb file was really really bad quality).

By preference I use low-compression Jpeg files for my game related graphics, 28% is a still very good; But, I still use bitmaps where quality is important. One place you'll notice quality is in transparent images, by rule of thumb, ALWAYS USE BITMAPS for sprites and other SMALL logos and pictures that need transparencies; by small I mean both dimensions being below 250. Using Jpg's for fullscreen pictures and background textures is a good idea - they add lots to a game, but no one's going to be incredibly bothered by a slight reduction in quality.

Now, onto the code.
This tutorial will show you how to make a simple procedure that automates loading of Jpegs into surfaces. The method I use is 100% garaunteed to work and is very very simple - there are other faster ways of doing it, but I find them to be incredibly complicated. This method is fine for when you load all your pictures at start-up, but if you're application relies on fast loading and swapping pictures around this will slow your application down considerably.

Public Function CreateASurface(DirectdrawObject As DirectDraw7, DDSurface As DirectDrawSurface7, Width As Long, Height As Long, SourceFile As String) As Boolean
On Error GoTo errhandle:
Dim ddsdF As DDSURFACEDESC2
Dim Surfpic As Picture

ddsdF.lFlags = DDSD_CAPS Or DDSD_HEIGHT Or DDSD_WIDTH
ddsdF.ddsCaps.lCaps = DDSCAPS_OFFSCREENPLAIN And DDSCAPS_VIDEOMEMORY
ddsdF.lWidth = Width
ddsdF.lHeight = Height
Set Surfpic = LoadPicture(SourceFile)
SavePicture Surfpic, App.path & "\TempFile.Bmp"
Set DDSurface = DirectdrawObject.CreateSurfaceFromFile(App.path & "\TempFile.bmp", ddsdF)
Kill App.path & "\TempFile.Bmp"
Set Surfpic = Nothing
CreateASurface = True

Exit Function
errhandle:
CreateASurface = False
End Function

As you can see, this is a simple function that returns true if it succeeded and false if it didn't. The actual internal code is almost identical to the normal loading of a surface, just with a converter in front. The function uses a visual basic variable (picture) to hold the image file, it then saves it to the hard drive as a bitmap - this bitmap can now be loaded straight into the directdraw surface. As soon as this is done it will delete the file from the hard drive and empty the surfpic variable - so that the file isn't still present in memory.

This procedure gets slower as the size of the file increases, I've found that it still takes under a second to load a 640x480 image - but that figure is based on the hardware that I have (strictly average - nothing special).

DirectX 4 VB 2000 Jack Hoxley. All rights reserved.
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