This section discusses some Windows related specific that we need to know to create Hide To Desktop. These are namely some essentials, so necessary for any at least a bit advanced graphic operations, that you must be very concentrated and try to remember everything:

Each window is assigned a window handle when it is initially created and this handle is used by Windows to refer to this window any time some kind of operation has to be performed over it. This handle is a Long VB type number, unique for the system. To get the handle of your form (window), you use the Hwnd property.

Each window outlook is represented by a Device Context (DC) attached to it on its creation. Any drawing operations on a given window are performed using its DC as identifier. The DC is also a Long VB type number. You may retrieve it either by the VB property hDC or by using the API function GetDC specifying the window handle as a parameter.

The DC of each window is actually holding the image displayed on the screen at the position of this window. If your window is partly overlapped by another one, the DC of your window will contain also the part of the other window. Also, if your window is partly or completely hidden off the screen, these hidden parts are represented on the DC with pure black color.

The Windows desktop is nothing else, but a window. And as the DC contains everything that's over it, the DC of the Desktop window is actually a snapshot of your screen, updated 50-100 times per second. So, you can count on it to get whatever part of the screen you need.

There is such thing called a compatible (sometimes called memory) DC. This is a virtual DC created in memory and often used as an auxiliary drawing area. By the way, it's much faster to draw on such DC and then transfer its contents at once onto a visual DC, than just drawing on this visual DC. A compatible DC is called "compatible", because it must be compatible with some existing visual DC. And again, "compatible" means it will have the same color setting and other attributes as the visual DC. And you don't need to care much about these attributes.

To create a compatible DC, you call CreateCompatibleDC API function and specify the visual DC as a parameter.

A surprise! - a DC is not just a canvas to draw on. It is actually a set of objects needed for drawing. These objects include a pen, a brush, a bitmap and some more. It is actually the bitmap you draw on when using the DC. And if you draw a circle on a DC, you will draw it on the DC's bitmap, using the DC's pen as a drawing tool specifying the color and width of the circle's stroke. You will only need the bitmap object for this example, as there is no drawing in it.

Now, if you create a DC using CreateCompatibleDC, it will have no bitmap loaded into it. So, you cannot use it. And it means you need to load a bitmap in it. But where to get the bitmap from?

Ok, there is such thing called compatible (memory) bitmap. It is, as the compatible DC is, a virtual representation of a bitmap. And "compatible" means the same as above. To create a compatible bitmap, you call CreateCompatibleBitmap API and specify a DC that the bitmap will be compatible with. You should avoid using a compatible DC as a parameter, instead, use the DC you specified as a parameter when creating the compatible DC.

When you create a compatible bitmap, it is not yet loaded into any DC. To do so, you call SelectObject API (also used for loading other objects, like pens, into a DC). You specify the DC as a first parameter and the handle of the object as second.

Because it takes some time for a window to be redrawn (Windows does not keep a copy of the old desktop contents in memory, instead it just sends a message to the programs covering it that they should redraw their windows where needed), we need to call RedrawWindow API function to make sure the desktop area we need will be redraw. This is actually the weakest point of this sample - if you run it over Photoshop picture, the window will desappear, then Photoshop will take some time to draw its picture and then the sample window will appear again and "fade out".

And lastly, you use BitBlt function to copy a given part of a DC onto another DC. It is maybe the most used function for drawing and if you are not familiar with it yet, you will surely be soon. Ok, it is a fast and easy function.

Sorry for the above paragraph isn't the last (I lied to you). After you have finished using a given compatible DC, you should free the resources it uses by calling DeleteDC API function. It takes just the DC handle as a parameter. The DC of your window is automatically deleted by VB itself.

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Copyright (c) 1999, Billy&George Software and Peter Dimitrov
Revised May 1999