Wrong processing of GetDIBits() builder c++ - c++builder

Question

I use GetDIBits() to retrieve the bits of bitmap and copies them into a buffer. The function doesn't fail (the result is different to NULL), but I get wrong height of bitmap and erroneous buffer.
This is a part of my code:
HDC hdcMemDC = CreateCompatibleDC(hDC); // (hDC = hDC = BeginPaint(hwnd, &ps): i get it in WM_Paint)
int l_uiWidth = 400;
int l_uiHeight = 120;
HBITMAP hbmp = CreateCompatibleBitmap(hDC, l_uiWidth, l_uiHeight);
HGDIOBJ oldhbmp = SelectObject(hdcMemDC,hbmp);
BITMAPINFO bi;
bi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bi.bmiHeader.biWidth = l_uiWidth;
bi.bmiHeader.biHeight = l_uiHeight;
bi.bmiHeader.biPlanes = 1;
bi.bmiHeader.biBitCount = 8;
bi.bmiHeader.biCompression = BI_RGB;
bi.bmiHeader.biSizeImage = 0;
bi.bmiHeader.biXPelsPerMeter = 0;
bi.bmiHeader.biYPelsPerMeter = 0;
bi.bmiHeader.biClrUsed = 256;
bi.bmiHeader.biClrImportant = 0;
BYTE *l_ImagePDM = new BYTE[l_uiWidth * l_uiHeight];
GetDIBits(hdcMemDC,hbmp,0,l_uiHeight,l_Image,&bi,DIB_RGB_COLORS);
Please Help me!
What's wrong with my code ?

Answer 1

You have not drawn anything on the bitmap that you are querying the bits for. CreateCompatibleBitmap() does not make a copy of the pixels of the source HDC. It merely allocates an HBITMAP that is compatible with the specified HDC, which then allows you to select the HBITMAP into that HDC. But you still have to draw something onto the bitmap to make its content meaningful before you can then query its bits.

Answer 2

When you use C++ Builder you can use Graphics unit for processing graphics.
Example of copying something from window:
void __fastcall TForm1::Button1Click(TObject *Sender)
{
// Create a bitmap
Graphics::TBitmap* bit = new Graphics::TBitmap();
bit->PixelFormat = pf24bit;
bit->HandleType = bmDIB;
bit->Width = 200;
bit->Height = 200; // or bit->SetSize(200, 200) - newer versions C++ Builder
// Copy something from this Form (from window)
bit->Canvas->CopyRect(TRect(0, 0, 200, 200), this->Canvas, TRect(0, 0, 200, 200));
// Do something with bitmap data
RGBTRIPLE* line;
for (int i = 0; i < bit->Height; i++)
{
// Get memory address from line i
line = (RGBTRIPLE*) bit->ScanLine[i];
// Change 5'th pixel
line[5].rgbtRed = 255;
}
// Get whole bitmap memory. Bitmap data are stored upside down and size of each row is rounded
// up to a multiple of 4 bytes.
unsigned char* bitmem = (unsigned char*)(bit->ScanLine[ bit->Height-1 ]);
//...
// Draw bitmap on Form
Canvas->Draw(0, 200, bit);
delete bit;
}
When you have device context you can use TCanvas like this:
void __fastcall TForm1::Button2Click(TObject *Sender)
{
// Create device context
HDC hdc = GetDC(this->Handle); // or GetWindowDC - whole window with frame
// Create canvas and associate device context
TCanvas* canv = new TCanvas();
canv->Handle = hdc;
// Create a bitmap
Graphics::TBitmap* bit = new Graphics::TBitmap();
bit->PixelFormat = pf24bit;
bit->HandleType = bmDIB;
bit->Width = this->ClientWidth;
bit->Height = this->ClientHeight; // or bit->SetSize(w, h) - newer versions C++ Builder
// Copy window content
TRect r(0, 0, this->ClientWidth, this->ClientHeight);
bit->Canvas->CopyRect(r, canv, r); // USEING TCanvas
// Release context and delete canvas
ReleaseDC(this->Handle, hdc);
delete canv;
// Do something with the bitmap
bit->SaveToFile("screenshot.bmp");
delete bit;
}
You can also use TCanvas for device context from BeginPaint:
canv->Handle = BeginPaint(hwnd, &ps);