What could be the reason for AlphaBlend to return 'false'

Question

I am trying to stretch an HBITMAP with alpha and draw it to anther hdc.

I'm using StretchDIBits and then AlphaBlend, as shown in the code below.

The problem is that AlphaBlend fails and returns false.
1. Does anyone knows what could be the reason? 2. Is there a better why to stretch and draw a transparent image?

void AnimationManager::Draw(HDC hBBDC, Instance sInstance,RECT sClientRect)
{
    // sClientRect is the hwnd rect

    int nID = GetId(sInstance.nAnemationId);
    int nFrameindex = sInstance.nFrameIndexs;

    HDC hdcScreen = GetDC(NULL);
    HDC hdcMem = CreateCompatibleDC(hdcScreen);

    BITMAP bmp;
    PBITMAPINFO pbmi;
    WORD cClrBits;

    ///******************* create PBITMAPINFO *********************///

    GetObject(m_pAnimations[nID]->m_pFramesArray[nFrameindex]->hBmp, sizeof(bmp), &bmp);
    cClrBits = (WORD)(bmp.bmPlanes * bmp.bmBitsPixel);
    if(cClrBits == 1)
        cClrBits = 1;
    else if(cClrBits <= 4)
            cClrBits = 4;
    else if(cClrBits <= 8)
            cClrBits = 8;
    else if(cClrBits <= 24)
            cClrBits = 24;
    else cClrBits = 32;

    if(cClrBits != 24)
        pbmi = (PBITMAPINFO)LocalAlloc(LPTR, sizeof(BITMAPINFOHEADER)+sizeof(RGBQUAD)*(1<<cClrBits));
    else
        pbmi = (PBITMAPINFO)LocalAlloc(LPTR, sizeof(BITMAPINFOHEADER));

    pbmi->bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    pbmi->bmiHeader.biWidth = bmp.bmWidth;
    pbmi->bmiHeader.biHeight = bmp.bmHeight;
    pbmi->bmiHeader.biPlanes = bmp.bmPlanes;
    pbmi->bmiHeader.biBitCount = bmp.bmBitsPixel;
    if(cClrBits < 24)
        pbmi->bmiHeader.biClrUsed = (1<<cClrBits);

    pbmi->bmiHeader.biCompression = BI_RGB;

    pbmi->bmiHeader.biSizeImage = ((pbmi->bmiHeader.biWidth * cClrBits + 31)& ~31)/8 * pbmi->bmiHeader.biHeight;
    pbmi->bmiHeader.biClrImportant = 0;


    ///**************************end PBITMAPINFO creation *****************************************///


    // I checked the code to this point and it seems to be fine. The problem stasrts here:

    ///create a pointer to the image bits
    LPVOID lpvBits = (LPVOID)LocalAlloc(GPTR,pbmi->bmiHeader.biSizeImage);
    GetBitmapBits(m_pAnimations[nID]->m_pFramesArray[nFrameindex]->hBmp, pbmi->bmiHeader.biSizeImage, lpvBits);

    ///stretch
    bool test = StretchDIBits(hdcMem,0,sClientRect.bottom,sClientRect.right, -1*sClientRect.bottom,0,0,pbmi->bmiHeader.biWidth,pbmi->bmiHeader.biHeight, lpvBits,pbmi,DIB_RGB_COLORS, SRCCOPY);

    ///blend
    test = AlphaBlend(hBBDC,0,0,sClientRect.right, sClientRect.bottom,hdcMem,0,0,sClientRect.right,sClientRect.bottom,m_sBlendfunc);


    test = DeleteDC(hdcMem);  // after CreateCompatibleDC
    test = ReleaseDC(NULL, hdcScreen); // after GetDC

    LocalFree(pbmi);
    LocalFree(lpvBits);
}

You won't know until you let Windows tell you. Use GetLastError(). — Hans Passant, Nov 24 '10 at 15:43
Unfortunately GetLastError is unhelpful, and always returns code 87 indicating an invalid parameter. — Graham Asher, Jul 02 '13 at 11:41
There's no call to SelectObject. I believe you need to use SelectObject to select the bitmap into your compatible DC. See the sample code in my answer below. — Graham Asher, Jul 02 '13 at 11:54

score 0 · Answer 1 · answered Jul 02 '13 at 11:48

I can't say exactly why the code is failing, but I know from recent experience that AlphaBlend is very picky about the bitmap formats it accepts. Luckily I can contribute some working code, which should be adaptable to other circumstances.

static void DrawBitmapWithAlpha(HDC aDeviceContext,const CartoType::TBitmap* aBitmap,const CartoType::TRect& aDestRect,const CartoType::TRect& aSourceRect)
{
HDC hdc = CreateCompatibleDC(aDeviceContext);
BITMAPINFOHEADER bitmap_header;
memset(&bitmap_header,0,sizeof(bitmap_header));
bitmap_header.biSize = sizeof(BITMAPINFOHEADER);
bitmap_header.biWidth = aBitmap->Width();
bitmap_header.biHeight = aBitmap->Height();
bitmap_header.biPlanes = 1;
bitmap_header.biBitCount = 32;
bitmap_header.biCompression = BI_RGB;
bitmap_header.biSizeImage = 0;
bitmap_header.biXPelsPerMeter = 1000;
bitmap_header.biYPelsPerMeter = 1000;
bitmap_header.biClrUsed = 0;
bitmap_header.biClrImportant = 0;

void* bitmap_data = NULL;
HBITMAP hbitmap = CreateDIBSection(hdc,(const BITMAPINFO*)&bitmap_header,DIB_RGB_COLORS,&bitmap_data,NULL,0);

// Reflect the data vertically and change from ABGR to BGRA.
unsigned char* dest_row = (unsigned char*)bitmap_data;
const unsigned char* source_row = aBitmap->Data() + (aBitmap->Height() - 1) * aBitmap->RowBytes();
for (int y = 0; y < aBitmap->Height(); y++, dest_row += aBitmap->RowBytes(), source_row -= aBitmap->RowBytes())
    {
    const unsigned char* p = source_row;
    unsigned char* q = dest_row;
    unsigned char* end = q + aBitmap->RowBytes();
    while (q < end)
        {
        *q++ = p[1];
        *q++ = p[2];
        *q++ = p[3];
        *q++ = p[0];
        p += 4;
        }
    }
SelectObject(hdc,hbitmap);

BLENDFUNCTION bf;
bf.BlendOp = AC_SRC_OVER;
bf.BlendFlags = 0;
bf.SourceConstantAlpha = 0xFF;
bf.AlphaFormat = AC_SRC_ALPHA;

AlphaBlend(aDeviceContext,aDestRect.Left(),aDestRect.Top(),aDestRect.Width(),aDestRect.Height(),
           hdc,aSourceRect.Left(),aSourceRect.Top(),aSourceRect.Width(),aSourceRect.Height(),
           bf);

DeleteObject(hbitmap);
DeleteDC(hdc);
}

The section starting 'reflect the data vertically' can easily be changed to do whatever is necessary to copy your bitmap data into the bitmap created by CreateDIBSection.

My bitmap format is compatible with Windows in that its number of row bytes will always be the same; if your bitmap format doesn't have that property you need to change the code to copy the data accordingly.

Another useful point: I couldn't get this working with bitfields; and I further suspect that you have to supply a 32bpp bitmap with compression set to BI_RGB.

score 0 · Accepted Answer · answered Nov 24 '10 at 15:39

0

Why using StretchDIBits if AlphaBlend also can stretch?

answered Nov 24 '10 at 15:39

valdo

12,632
2
37
67

What could be the reason for AlphaBlend to return 'false'

2 Answers2