Create a Transparent Bitmap using BitBlt API

This example shows how to create a transparent bitmap. It starts with a bitmap of my son and one of a red "T" on a blue background. The "T" is superimposed over my son allowing his picture to show through its blue background. Each step along the way is show. The key to this is a number of GDI functions, most importantly, the BitBlt (Bit-Block Transfer) API.

Create a transparent bitmap with the BitBlt function
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First, we need to display the bitmaps and get a handle to the "T". Two variables are Dimmed as StdPicture. The StdPicture object lets you keep an image in memory without having a picturebox or image control on a form. It also provides a handle property which is what we need. The bitmaps are stored in the StdPicture objects using LoadPicture and then assigned to the picture property of the picturebox controls.

Most GDI (Graphic Device Interface) functions work with Device Contexts. A device context (DC) is a structure that defines a set of graphic objects and their attributes, and the graphic modes that affect output. Also, the DC refers to a physical output device — its name, device driver, and other attributes. GDI function parameters contain a handle to a DC to include the attributes of the specified device. There are four types of DC: display (supports drawing operations on a video display terminal), printer (supports drawing operations on a printer or plotter), memory (supports drawing operations on a bitmap), and information (supports retrieval of device data.) We are interested in memory DCs here.

From here on, I will refer to bitmap of my son as the bitmap and the image of the "T" as a sprite.

What we need to do is: mask out the background of the sprite, mask out the portion of the bitmap where the "T" will be placed, and merge the two images together.

The first step is to create a temporary Device Context compatible with the destination picturebox's DC using the CreateCompatibleDC function. CreateCompatibleDC creates a one monochrome pixel by one monochrome pixel memory DC. Before using this DC we must select the sprite into it with a call to SelectObject. At this point, we have a copy of the sprite in memory (see picture 3). A call to GetObject, passing the handle of the sprite, writes the sprite's dimensions to a Bitmap structure. This information is needed throughout.

To create the above mentioned masks, we need some temporary DCs to work with. Four calls to CreateCompatibleDC are made (picture 4 shows the output of this step). Before a DC can be used we must select a bitmap into it meaning we must create four bitmaps. We will need two monochrome and two color bitmaps. The monochrome bitmaps are created with the CreateBitmap API passing in the bitmap's dimensions, a 1 for the color depth and a 1 for the number of bits to use to identify a color. In a monochrome bitmap, zeros represent the foreground color and ones the background color.

The two color bitmaps are created by calling the CreateCompatibleBitmap function passing in the desired dimensions and the handle to the DC for the bitmap of my son. The result is two, one pixel by one pixel color bitmaps. Using SelectObject, these bitmaps are copied into the memory device contexts. Pictures 5 and 6 show the results.

Everything is now in place to begin the real work. The mapping mode, which defines the unit of measure applied to the sprite, is set to that of the destination picturebox via the GetMapMode and SetMapMode functions. The blue background of the sprite is the color we want to be transparent. Therefore, we need to set the background of the sprite's DC to blue. This is accomplished via the SetBkColor API specifying the DC to change and the color to change to.

We need to create two masks for the sprite using the monochrome bitmaps. Recall, in a monochrome bitmap, ones represent the background color (blue) and zeros the foreground color (i.e. the red "T"). In one mask, ones will represent the "T" and in the other, the background. To create a mask representing the background, the BitBlt (Bit-Block Transfer) function is employed to copy the colored sprite to a monochrome bitmap. Using the vbSrcCopy option instructs BitBlt to do a straight copy of the color data from one device context to another. To create the mask representing the "T", the first mask is inverted. This is done by BitBlt-ing the first mask to the second monochrome bitmap specifying the vbNotSrcCopy option. See pictures 7 and 8.

Almost done! The original bitmap is BitBlt to a color memory DC with the vbSrcCopy flag (picture 9). Next, its bits where the "T" is to be located are masked out by AND-ing in the mask with the zeros where the "T" is (picture 10). The background of the sprite is masked out using BitBlt and the mask with the zeros for the background (picture 11).

We are now left with two DCs: a color one of the bitmap with the "T" masked out and a color one of the sprite with only the "T". To produce the final image, these two DCs are combined by OR-ing their data together via a call to BitBlt with the vbSrcPaint flag (picture 12). All that remains is to delete the memory device contexts and clean up.


Download this project and copy the 2 bitmaps to the proper folder.

The images used by this program are expected to be in App.Path. That means if you compile this program it should work fine. However, if you step through the code, App.Path will point to the folder where VB is installed. Copy the bitmaps to the correct folder before running.

Also, when stepping through the code, make sure that the form is no obscured by any other windows in order for it to paint properly.

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