Drawing

End If

If Stretch < 32 Then Shrinking = False
If Stretch > MaxStretch Then Shrinking = True

''''Stretch the sprite onto the form
StretchBlt Me.hdc, X, Y, Stretch, Stretch, picMask.hdc, 0, 0, _
SpriteWidth, SpriteHeight, vbSrcAnd

StretchBlt Me.hdc, X, Y, Stretch, Stretch, picSprite.hdc, 0, 0, _
SpriteWidth, SpriteHeight, vbSrcPaint

X = (X Mod Me.ScaleWidth) 2
Y = (Y Mod Me.ScaleHeight) 2

. Force update of the form
Me.Refresh

End Sub

The first thing that is done in code is to check the stretching variable. This variable can be in one of two states, Shrinking or Stretching. To identify the different states we''''ll create a Boolean variable named Shrinking and set it to either True of False, depending on the desired state. The Stretch variable is allowed to be either 32 pixels less or more than the original sprite size, if they get out of this range, the state is changed, and the opposite stretch action will be used.

The sprite is drawn in the usual way, first the mask and then the sprite. But instead of using the BitBlt function we call the StretchBlt function, and set the destination width and height to the value of the stretch variable.

As you can observe, the StretchBlt can be a useful function for doing simple tricks on a sprite. You should use it cautiously though, since it may be a little slower than the ordinary BitBlt function, since some cycles are used when it stretches or shrinks an image.

Using and creating memory DCs

So far we have been using picture boxes as storage areas for the sprites. This does not come without a price since the picture box adds additional time and resource overhead to the animating scheme. This problem can be overcome by simply creating our own Device Contexts to hold the bitmaps. These Device Contexts reside in memory and allow us to perform the required operations without resorting to PictureBoxes. Before doing anything, some more specific information is required on what a Device Context is, and what can be done with it.

A Device Context is a Windows structure, with several important and useful attributes. Each of these attributes has a default value, which is set when the device context is created. The most important attribute of device context to us, is the Bitmap attribute. This attribute is initially set to nothing (meaning there is no bitmap associated with the device context). This attribute can be set by using the SelectObject API function.

To create a memory device context we use the API function CreateCompatibleDC, which returns a memory DC (a long value). In order to select the bitmap into the device context we need a handle to the specific bitmap. This handle can be obtained by using the LoadImage function. This function can load a bitmap from file, and return a handle to the loaded bitmap. The last thing to do is to select the handle of the loaded bitmap into our newly created memory DC, and we now have a useable device context for blitting.

The MEMORYDC sample, in the MEMORYDC sub-directory of the Chap1 directory, demonstrates the steps we have just outlined and will be exploring next, in creating a compatible device context and selecting a bitmap.

Let''''s put all of the required code into a reusable function which will take care of the creation of a memory device context for us and just requires a filename to the actual bitmap. The function will either return a device context (long value) or 0 if something went wrong.

The code for our function looks like this:

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