Grabbing a window into an image, cropping

[Kybo Ren]

Hey all, this bit of code is driving me insane!

I'm trying to get a window, whose HWND I know, grab it's DC, and essentially take a screenshot of it (into an HBITMAP) and then crop a section of it out.

However, the code I've had for this is giving me an access denied 0xC00005 error because SOME pointer is NULL...
If it helps, the debugger breaks in strlen.c... but I don't use strlen anywhere, so... yeah.

Here's all my relevant code:

Code: Select all

RGBQUAD* CaptureScreen(HWND window)
{
   HDC WinDC;
   HDC CopyDC;
   HBITMAP hBitmap;
   ULONG bWidth, bHeight;

   WinDC = GetDC (window);
   CopyDC = CreateCompatibleDC (WinDC);

   RECT rect;
   GetWindowRect(window, &rect);

   bWidth =  abs(rect.right-rect.left);
   bHeight = abs(rect.bottom - rect.top);

   hBitmap = CreateCompatibleBitmap(WinDC, bWidth, bHeight);
   SelectObject(CopyDC, hBitmap);
   BitBlt(CopyDC, 0, 0, bWidth, bHeight, WinDC, 0, 0, SRCCOPY);
              
   ReleaseDC(window, WinDC);
   DeleteDC(CopyDC);

   RGBQUAD *image;
   try{
      image = new RGBQUAD[bWidth*bHeight];
   }
   catch(std::bad_alloc)
   {
      return NULL;
   }

   GetBitmapBits(hBitmap, bWidth*bHeight*4, image);

   DeleteObject(hBitmap);

   return image;
}//I know this isn't returning NULL because I can see the pointer value in my debugger

RECT GetTheBox(HWND hwnd)
{
   RECT returnval;
   POINT pt1, pt2;
   
   pt2.x = pt1.x = 0;//rect.left;
   pt2.y = pt1.y = 0;//rect.top;

   pt1.x += 450;
   pt1.y += 319;

   pt2.x += 558;
   pt2.y += 338;

   returnval.bottom = pt2.y;
   returnval.left = pt1.x;
   returnval.right = pt2.x;
   returnval.top = pt1.y;

   return returnval;
}


RGBTRIPLE *Crop(RGBQUAD *image, unsigned int width, unsigned int height, RECT crop)
{
    RGBTRIPLE *newimage;

   try{
      newimage = new RGBTRIPLE[abs(crop.bottom-crop.top)*abs(crop.right-crop.left) + 4];
   }
   catch(std::bad_alloc)
   {
      return NULL;
   }
   unsigned int j = 0;

   for(unsigned int i = 0; i < width*height; ++i)
   {
       int curx, cury;
      curx = i % width;
      cury = i / width;

      if(curx >= crop.left && curx <= crop.right)
      {
         if(cury >= crop.top && cury <= crop.bottom)
         {
            newimage[j].rgbtBlue = image[i].rgbBlue;
            newimage[j].rgbtRed = image[i].rgbRed;
            newimage[j].rgbtGreen = image[i].rgbGreen;
            ++j;
         }
      }
   }

   return newimage;

}

RGBTRIPLE Average(RGBTRIPLE *image, unsigned int height, unsigned int width)
{
   unsigned int red = 0, green = 0, blue = 0;
   for(unsigned int i = 0; i < height*width; ++i)
   {
      red += image[i].rgbtRed;
      green += image[i].rgbtGreen;
      blue += image[i].rgbtBlue;
   }

   red /= (height*width);
   green /= (height*width);
   blue /= (height*width);

   RGBTRIPLE rgbt;
   rgbt.rgbtRed = red;
   rgbt.rgbtBlue = blue;
   rgbt.rgbtGreen = green;

   return rgbt;
}

RGBTRIPLE GetAverageBoxColor(HWND hwnd)
{
   RGBTRIPLE *rgbq;
   RGBTRIPLE *rgbt;
   RGBTRIPLE avg;
   
   rgbq = CaptureScreen(hwnd);
   rgbt = Crop(rgbq, 1024, 715, GetMyBox(hwnd));
   avg = Average(rgbt, 19, 108);

    delete [] rgbq;
   delete [] rgbt;
   std::cout << "Average: (" << (int)avg.rgbtRed << ", " << (int)avg.rgbtGreen << ", " << (int)avg.rgbtBlue << ")." << std::endl;

   return avg;
}


Any help?
Thanks!

[Darryl]

Offhand I don't see anything wrong. Internally windows uses strings for resources, window text, wndclass names, so I would check them out if you are using any of that.

Are you trying to write to a file anywhere, maybe you have a bad filename?

Are you using Drawtext() anywhere? Maybe to put a caption?

[Darryl]

this might not be the problem but...

Code: Select all

if(curx >= crop.left && curx <= crop.right)
      {
         if(cury >= crop.top && cury <= crop.bottom)
         {
            newimage[j].rgbtBlue = image[i].rgbBlue;
            newimage[j].rgbtRed = image[i].rgbRed;
            newimage[j].rgbtGreen = image[i].rgbGreen;
            ++j;
         }
      }


look at you top and bottom, left and right. Above this code you used abs so order didn't matter but in your getthebox code, the left > right and bottom > top so your assignments here are never done

[Kybo Ren]

this might not be the problem but...

Code: Select all

if(curx >= crop.left && curx <= crop.right)
      {
         if(cury >= crop.top && cury <= crop.bottom)
         {
            newimage[j].rgbtBlue = image[i].rgbBlue;
            newimage[j].rgbtRed = image[i].rgbRed;
            newimage[j].rgbtGreen = image[i].rgbGreen;
            ++j;
         }
      }


look at you top and bottom, left and right. Above this code you used abs so order didn't matter but in your getthebox code, the left > right and bottom > top so your assignments here are never done

Yeah, I thought the same thing, so I checked and everything was the code works exactly as intended.


I WAS trying to write to a file, but I removed it because I didn't think it was causing the issue. I'll try again without the file code.

[Kybo Ren]

Well, it doesn't do that anymore, but it does give another access error on the first pixel the Crop function would crop.
Anyone have any idea why? Crop IS returning a valid pointer to memory allocated, so...

[MXP]

Which line is it crashing on?