weidagang2046的專欄

Introduction

I finally managed to get transparent drawing working and made a few routines that makes it a snap. I adapted this code from an example Chris Becke's Bitmap Basics - A GDI tutorial.

The situation

I have a master picture with lots of images in it, the transparent areas are all in purple. At runtime I pick parts of this picture and BitBlt it on the screen while preserving the transparency.

Or maybe you have a bitmap that you wish to show transparently, just set the transparent areas to a unique color and use the routines below.

First the routines, at the end an example that puts it all together.

				//**----------------------------------------------------------
				

				//** STEP 1: Load the bitmap into a CBitmap Object
				

				//**----------------------------------------------------------
				
BOOL CMyDlg::LoadFileBitmap(CBitmap* pBmp, LPCTSTR szFilename)
{
?? pBmp->DeleteObject();
?? return pBmp->Attach(LoadImage(NULL, szFilename, IMAGE_BITMAP, 0, 0,
????????????????????? LR_LOADFROMFILE | LR_DEFAULTSIZE));
}


//**----------------------------------------------------------
//**STEP 2: Create the Mask and dump it into a CBitmap Object
//**----------------------------------------------------------
void CMyDlg::PrepareMask( CBitmap* pBmpSource,
????????????????????????? CBitmap* pBmpMask,
????????????????????????? COLORREF clrpTransColor, // Pass null if unknownint iTransPixelX,????? // = 0int iTransPixelY?????? // = 0
??????????????????????? )
{
?? BITMAP bm;

?? // Get the dimensions of the source bitmap
?? pBmpSource->GetObject(sizeof(BITMAP), &bm);

?? // Create the mask bitmap
?? pBmpMask->DeleteObject();
?? pBmpMask->CreateBitmap( bm.bmWidth, bm.bmHeight, 1, 1, NULL);

?? // We will need two DCs to work with. One to hold the Image
???// (the source), and one to hold the mask (destination).
???// When blitting onto a monochrome bitmap from a color, pixels
???// in the source color bitmap that are equal to the background
???// color are blitted as white. All the remaining pixels are
???// blitted as black.

?? CDC hdcSrc, hdcDst;

?? hdcSrc.CreateCompatibleDC(NULL);
?? hdcDst.CreateCompatibleDC(NULL);

?? // Load the bitmaps into memory DC
?? CBitmap* hbmSrcT = (CBitmap*) hdcSrc.SelectObject(pBmpSource);
?? CBitmap* hbmDstT = (CBitmap*) hdcDst.SelectObject(pBmpMask);

?? // Dynamically get the transparent color
?? COLORREF clrTrans;
?? if (clrpTransColor == NULL)
?? {
????? // User did not specify trans color so get it from bmp
????? clrTrans = hdcSrc.GetPixel(iTransPixelX, iTransPixelY);
?? }
?? else
?? {
????? clrTrans = clrpTransColor;
?? }


?? // Change the background to trans color
?? COLORREF clrSaveBk? = hdcSrc.SetBkColor(clrTrans);

?? // This call sets up the mask bitmap.
?? hdcDst.BitBlt(0,0,bm.bmWidth, bm.bmHeight, &hdcSrc,0,0,SRCCOPY);

?? // Now, we need to paint onto the original image, making
???// sure that the "transparent" area is set to black. What
???// we do is AND the monochrome image onto the color Image
???// first. When blitting from mono to color, the monochrome
???// pixel is first transformed as follows:
???// if? 1 (black) it is mapped to the color set by SetTextColor().
???// if? 0 (white) is is mapped to the color set by SetBkColor().
???// Only then is the raster operation performed.

?? COLORREF clrSaveDstText = hdcSrc.SetTextColor(RGB(255,255,255));
?? hdcSrc.SetBkColor(RGB(0,0,0));

?? hdcSrc.BitBlt(0,0,bm.bmWidth, bm.bmHeight, &hdcDst,0,0,SRCAND);

?? // Clean up by deselecting any objects, and delete the
???// DC's.
?? hdcDst.SetTextColor(clrSaveDstText);

?? hdcSrc.SetBkColor(clrSaveBk);
?? hdcSrc.SelectObject(hbmSrcT);
?? hdcDst.SelectObject(hbmDstT);

?? hdcSrc.DeleteDC();
?? hdcDst.DeleteDC();
}


//**----------------------------------------------------------
//** STEP 3: Drawing with Transparency. Call from OnPaint
//**----------------------------------------------------------
void CMyDlg::DrawTransparentBitmap(CMemDC* pDC,
?????????????????????????????????? int xStart,? int yStart,
?????????????????????????????????? int wWidth,? int wHeight,
?????????????????????????????????? CDC* pTmpDC,
?????????????????????????????????? int xSource, // = 0
????????????????????????????????????int ySource? // = 0)
{

?? // We are going to paint the two DDB's in sequence to the destination.
???// 1st the monochrome bitmap will be blitted using an AND operation to
???// cut a hole in the destination. The color image will then be ORed
???// with the destination, filling it into the hole, but leaving the
???// surrounding area untouched.

?? CDC hdcMem;
?? hdcMem.CreateCompatibleDC(NULL);

?? CBitmap* hbmT = hdcMem.SelectObject(&m_bmpMask);

?? pDC->BitBlt( xStart, yStart, wWidth, wHeight, &hdcMem,
??????????????? xSource, ySource, SRCAND);

?? // Also note the use of SRCPAINT rather than SRCCOPY.

?? pDC->BitBlt(xStart, yStart, wWidth, wHeight, pTmpDC,
?????????????? xSource, ySource,SRCPAINT);

?? // Now, clean up.
?? hdcMem.SelectObject(hbmT);
?? hdcMem.DeleteDC();
}

It is that simple. MSDN examples are very confusing. Chris Becke's examples are a lot better but in Win32. :) 

So here is a example on how to put it together in a real life situation.

				//**------------------------------------
				//** EXAMPLE: Drawing with Transparency
				//**------------------------------------
				//**------------------------------------------
				//** EXP STEP 1: Declarations & variables
				//**------------------------------------------
In YourDlg.h, add the following

CBitmap m_bmpPlmain;
CBitmap m_bmpMask;

BOOL LoadFileBitmap(CBitmap* pBmp, LPCTSTR szFilename);

void PrepareMask( CBitmap* pBmpSource,
                  CBitmap* pBmpMask,
                  COLORREF clrpTransColor,
                  int iTransPixelX = 0,
                  int iTransPixelY = 0 );

void DrawTransparentBitmap (CMemDC* pDC,
                            int xStart, int yStart,
                            int wWidth, int wHeight,
                            CDC* pTmpDC,
                            int xSource = 0,
                            int ySource = 0);


//**------------------------------------------
//** EXP STEP 2: Load and Prepare the bitmaps
//**------------------------------------------
CYourDlg::OnInitDialog()
{
  ...
  ...
  ...

  // I'm loading from a bitmap file but this can come from resource as well
??if (!LoadFileBitmap(&m_bmpPLMain, "BmpWithHoles.bmp"))
  {
    // Opps ... where did the picture go ??
  }

  // Third param is NULL because I don't know the transparent color
??// but I know the trans color is at pixel 200, 50// or is you know the trans color(RED) then do the following//  PrepareMask( &m_bmpPLMain, &m_bmpMask, RGB(255, 0, 0));

  PrepareMask( &m_bmpPLMain, &m_bmpMask, NULL, 200, 50);

}

//**---------------------------------
//** EXP STEP 3: Drawing the bitmaps
//**---------------------------------
CYourDlg::OnPaint()
{

  CPaintDC dc(this);              // device context for painting
  CDC dcMem;                  // memory device context
  dcMem.CreateCompatibleDC(&dc);

  // Select the bmp into the tmp memory DC
  CBitmap* pOldBmp = (CBitmap*) dcMem.SelectObject(&m_bmpPLMain);

  DrawTransparentBitmap( &dc,           // The destination DC.
                         POP_X,         // Where to draw
                         POP_Y,
                         POP_WIDTH,     // Width & Height
                         POP_HEIGHT,
                         &dcMem,        // the DC holding the bmp
                         POP_BMP_X_OFF, // x & y pos in master bmp
                         POP_BMP_Y_OFF);
  ....
  ....
  // Do whatever you want to do ...

  dcMem->SelectObject(pOldBmp);
}

//**-----------------------------
//** EXP STEP 4: Cleaning Up ..
//**-----------------------------
CYourDlg::OnDestroy()
{
   // Dont forget to delete the bmp's
   m_bmpPLMain.DeleteObject();
   m_bmpMask.DeleteObject();
}

That's about it ...

Any mistakes, additions or optimizations, please feel free to point them out. Just send the comments to windev and/or raja@gelife.com.my

Raja Segar