Display of transparent bitmap

 

There are many ways to draw a bitmap that contains transparent colors. The simplest method is to call a ready-made function:TransparentBlt can also implement functions similar to TransparentBlt through its own code. The implementation process also has two forms: one is to make a mask bitmap beforehand, and the other is to dynamically generate a mask bitmap. This article describes how to dynamically generate a mask bitmap to draw a transparent area bitmap.

I. Use of the TransparentBlt Function

The TransparentBlt function is run in Windows98/Windows2000 or later versions. The system must contain Msimg32.dll, which can be linked to Msimg32.lib for use.
TransparentBlt in Windows 98 may cause resource leakage. Therefore, we do not recommend that you use this function in WIN98.
The TransparentBlt function prototype is as follows:

BOOL TransparentBlt (
  

HDC hdcDest,// Target DC
  

Int nXOriginDest,// Target X offset
  

Int nYOriginDest,// Target Y offset
  

Int nWidthDest,// Target width
  

Int hHeightDest,// Target height
  

HDC hdcSrc,// Source DC
  

Int nXOriginSrc,// Source X start point
  

Int nYOriginSrc,// Source Y start point
  

Int nWidthSrc,// Source width
  

Int nHeightSrc,// Source height
  

UINT crTransparent// Transparent color, COLORREF type
  

);
  

Example:

CBitmap FootballBMP;
  

FootballBMP. LoadBitmap (IDB_FOOTBALLBMP );
  

CDC ImageDC;
  

ImageDC. CreateCompatibleDC (pDC );
  

CBitmap * pOldImageBMP = ImageDC. SelectObject (& FootballBMP );
  

TransparentBlt (pDC-> m_hDC, 0, 0,218,199, ImageDC. m_hDC, 0, 0,218,199, RGB (, 0xff ));
  

ImageDC. SelectObject (pOldImageBMP );
  

II. ImplementationTransparentBlt Function

To understand the process of drawing a transparent bitmap, we will build an experimental function with the same TransparentBlt function. It is called TransparentBlt2.

Experiment material: There are two bitmap: bk.bmp is the background image, football.bmp contains the transparent area, the transparent color is blue RGB (, 0xff)
Objective: To use bk.bmp as the background and draw football.bmp into the background to form the following final result.

2.1 Principle of transparent bitmap
Hypothetical football.bmp-> load HBITMAP hImageBMP-> select HDC hImageDC

2.1.1 generate a monochrome mask bitmap for football. The Transparent area is white (all 1) and the non-transparent area is black (all 0)

HBITMAP hMaskBMP = CreateBitmap (nWidthDest, nHeightDest, 1, 1, NULL); // create a monochrome bitmap
  

SetBkColor (hImageDC, RGB (0, 0, 0xff); // set the background color to blue.
  

BitBlt (hMaskDC, 0, 0, nWidthDest, nHeightDest, hImageDC, 0, 0, SRCCOPY); // copy to hMaskDC
  

In this way, the blue area of the football bitmap becomes white in the mask bitmap, and the other areas are black.HMaskBMP:
(Figure 1)

2.1.2 set the background color to black and the foreground color to white. The mask Bitmap (figure 1) is the same as the football bitmap "and"

SetBkColor (hImageDC, RGB (0, 0 ));
  

SetTextColor (hImageDC, RGB (1, 255,255,255 ));
  

BitBlt (hImageDC, 0, 0, nWidthDest, nHeightDest, hMaskDC, 0, 0, SRCAND );