Thread: Win Api Basics...

>i think the above posts explain it pretty well...
>hDC = GetDC(hwnd);

I tried all that.

I also searched for example code written by other people, but nothing worked. What I ever I did, it would fail one in 20 times or so.

I do read the manuals. Whatever was wrong with my code, it was something subtle that I couldn't figure out.

what do you mean it would fail one in 20 times?

does it compile?

>does it compile?

Of course. And it ran. And it worked - 1 in 20 times. But when it failed, GetDIBits would return invalid colour values in around half the image.

I tried other's people code which was doing exactly the same thing I was, and that had the same problem. I have no issues with stability on my machine (i.e. no graphic card problems).

Withouta really proper understanding of what was going on, and how the API was being used (which I have been unable to get from MSDN), I was unable to find a solution.

In the end, I resolved my problem by calling a third party API, which I guess just wrapped GetDIBits, but it worked. So it must have been something wrong with what I was doing.

Here was my code. Although I got my application to work now, I would still be interested to know what I did wrong.

Code:

int Gfx::create(Gfx& dst, void* src, int y, int cnt)
{
  // (this code DOES NOT seem to work reliably)
  // Creates gfx data and fills it with contents of bitmap from
  // platform specific bitmap object src. At the moment, this method
  // only works under windows. Does nothing under any other platform.
  // If this method fails, rslt.data is NULL(0) & returns
  // platform specific error code. Error is -1 if operation
  // is not supported on platform.
  int err;
  if (y < 0) y = 0;

  // Ensure existing is destroyed.
  destroy(dst);

#ifdef AIDAWIN
  // WINDOWS IMPLEMENTATION
  // src must not be 'selected into a device'
  err = 0;
  HDC hdc = 0;
  BITMAPINFO bmi;
  memset(&bmi, 0, sizeof(BITMAPINFO));
  bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);

  try
  {
    // Make temporary compatible device
    hdc = GetDC(0); // CreateCompatibleDC
    GdiFlush();

    // Get dimensions first
    GetDIBits(hdc, (HBITMAP)src, 0, 0, 0, &bmi, DIB_RGB_COLORS);

    // We always want positive height (bottom up bmp)
    bmi.bmiHeader.biHeight = abs(bmi.bmiHeader.biHeight);

    // Work out full size count if needed
    if (cnt <= 0) cnt = bmi.bmiHeader.biHeight - y;

    // Fill data
    dst.pDim.x = bmi.bmiHeader.biWidth;
    dst.pDim.y = cnt;

    // Check we can handle range
    err = ERROR_INVALID_PARAMETER;

    if (y + cnt <= bmi.bmiHeader.biHeight && cnt > 0)
    {
      // Set pixel format to be retrieved
      bmi.bmiHeader.biBitCount = 24;
      bmi.bmiHeader.biCompression = BI_RGB;

** HERE sz includes packing bytes
      int sz = dst.rowsz() * dst.pDim.y;

      if (sz > 0)
      {
        // Create memory array
        dst.pData = new char[sz];

** HERE  GetDiBits
        err = 0;
        if (GetDIBits(hdc, src, y, cnt, dst.pData,
          &bmi, DIB_RGB_COLORS) == 0)
        {
          // Error - destroy memory and
          // error code will be set below
          destroy(dst);
ShowMessage("Y " + IntToStr(y));
ShowMessage("CNT : " +IntToStr(cnt));

        throw Exception("Error " + IntToStr(GetLastError()));
        }
      }
    }

    // If no data - set error code
    if (dst.pData == 0)
    {
      dst.pDim = Axy();
      if (err == 0) err = GetLastError();
    }

    // Free temporary device context
    ReleaseDC(0, hdc);
  }
  catch(...)
  {
    // Clean-up
    destroy(dst);
    ReleaseDC(hdc);
    throw;
  }
#else
  // NOT SUPPORTED
  err = -1;
#endif

  return err;
}

Davros: I can't explain your problem, and I have no intention of reading through your code since I don't remember what parameters do what, and have no desire to read the documentations again But I did use GetDIBits once when I was trying to generate rotated bitmaps for my GDI graphics library, and never had any problems with it; if you care, you can sift through this and see if anything helps:

Code:

HBITMAP GFX::getRotatedBmp(HBITMAP orig, double angle, int rPointX, int rPointY)
{
	if(orig == NULL)
		return NULL;

	HBITMAP img;
	HDC dcSrc = CreateCompatibleDC(dcFront);

	RECT bounds = getBmpDimensions(orig);
	int width = bounds.right + 1;
	int height = bounds.bottom + 1;

	img = CreateCompatibleBitmap(dcFront, width, height);

	BITMAPINFO bi = {0};

	bi.bmiHeader.biSize = sizeof(bi.bmiHeader);
	bi.bmiHeader.biWidth = width;
	bi.bmiHeader.biHeight = height;
	bi.bmiHeader.biPlanes = 1;
	bi.bmiHeader.biBitCount = 32;
	bi.bmiHeader.biCompression = BI_RGB;

	unsigned char* srcBits = new unsigned char[4 * width * height];
	if(!GetDIBits(dcSrc, orig, 0, height, srcBits, &bi, DIB_RGB_COLORS))
	{
		delete[] srcBits;
		return NULL;
	}
	unsigned char* destBits = new unsigned char[4 * width * height];

	float sine = (float)sin(angle), cosine = (float)cos(angle); //Store result to save time
	float cX = (float)rPointX, cY = (float)(bounds.bottom - rPointY); //Typecast & convert from normal coords
	float tempX, tempY; //Offset on source to centre the image

	tempX = (cX*(cosine-1) - cY*sine);
	tempY = (cY*(cosine-1) + cX*sine);

	int origX, origY;

	unsigned char* destLine = destBits;
	for(int y = 0; y < height; ++y)
	{
		for(int x = 0; x < width; ++x)
		{
			origX = (int)(((float)x + tempX)*cosine + ((float)y + tempY)*sine);
			origY = (int)(((float)y + tempY)*cosine - ((float)x + tempX)*sine);

			if(origX >= 0 && origY >= 0 && origX < bounds.right && origY < bounds.bottom)
			{
				destLine[(x << 2)    ] = srcBits[((origX + width*origY) << 2)    ];
				destLine[(x << 2) + 1] = srcBits[((origX + width*origY) << 2) + 1];
				destLine[(x << 2) + 2] = srcBits[((origX + width*origY) << 2) + 2];
			}
			else
			{
				destLine[(x << 2)    ] = 0;
				destLine[(x << 2) + 1] = 0;
				destLine[(x << 2) + 2] = 0;
			}
		}
		destLine += (width << 2);
	}

	SetDIBits(dcSrc, img, 0, height, destBits, &bi, DIB_RGB_COLORS);

	DeleteDC(dcSrc);

	delete[] srcBits;
	delete[] destBits;

	return img;
}

>I have no intention of reading through your code

I can understand that. I did try simplifying my code in a previous post, but found I made so many typo errors in the simplification, that it made the effort pointless.

>if you care, you can sift through this and see if anything helps

Thanks. I will do so.

Cheers for the reply.