OpenCV: assertation failed ((img.depth() == CV_8U || img.depth() == CV_32F) && img.type() == templ.type())

Question

I get this error from above and don't know how to avoid it. My aim is to get a screenshot and then perform a template matching on it to see if an icon is shown on the screen at this moment. Till now it's just the position of the icon. My code:

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/core/core.hpp"
#include <Windows.h>
#include <iostream>
#include <stdio.h>
#include <string>


using namespace std;
using namespace cv;

Mat hwnd2mat();

/// Global Variables
Mat img; Mat templ; Mat result;

int main()
{
  /// Load image and template
  templ = imread( "Template.bmp",1);
  templ.convertTo(templ, CV_8U);

  //img = imread( "Image.jpg", 1 );
  img = hwnd2mat();


  /// Create the result matrix
  int result_cols =  img.cols - templ.cols + 1;
  int result_rows = img.rows - templ.rows + 1;

  result.create( result_cols, result_rows, CV_8U);

  /// Do the Matching and Normalize
  matchTemplate( img, templ, result, CV_TM_SQDIFF );
  normalize( result, result, 0, 1, NORM_MINMAX, -1, Mat() );

  /// Localizing the best match with minMaxLoc
  double minVal; double maxVal; Point minLoc; Point maxLoc;
  Point matchLoc;

  minMaxLoc( result, &minVal, &maxVal, &minLoc, &maxLoc, Mat() );

  /// show best position
  matchLoc = minLoc;
  cout<<matchLoc<<" is best position"<<endl;

  waitKey(0);
  return 0;
}

Mat hwnd2mat(){

    HWND hwnd = GetDesktopWindow();
    HDC hwindowDC,hwindowCompatibleDC;

    int height,width,srcheight,srcwidth;
    HBITMAP hbwindow;
    Mat src;
    BITMAPINFOHEADER  bi;

    hwindowDC=GetDC(hwnd);
    hwindowCompatibleDC=CreateCompatibleDC(hwindowDC);
    SetStretchBltMode(hwindowCompatibleDC,COLORONCOLOR);  

    RECT windowsize;    // get the height and width of the screen
    GetClientRect(hwnd, &windowsize);

    srcheight = windowsize.bottom;
    srcwidth = windowsize.right;
    height = windowsize.bottom/1;  //change this to whatever size you want to resize to
    width = windowsize.right/1;

    src.create(height,width,CV_8U);

    // create a bitmap
    hbwindow = CreateCompatibleBitmap( hwindowDC, width, height);
    bi.biSize = sizeof(BITMAPINFOHEADER);
    bi.biWidth = width;    
    bi.biHeight = -height;  //this is the line that makes it draw upside down or not
    bi.biPlanes = 1;    
    bi.biBitCount = 32;    
    bi.biCompression = BI_RGB;    
    bi.biSizeImage = 0;  
    bi.biXPelsPerMeter = 0;    
    bi.biYPelsPerMeter = 0;    
    bi.biClrUsed = 0;    
    bi.biClrImportant = 0;

    // use the previously created device context with the bitmap
    SelectObject(hwindowCompatibleDC, hbwindow);
    // copy from the window device context to the bitmap device context
    StretchBlt( hwindowCompatibleDC, 0,0, width, height, hwindowDC, 0, 0,srcwidth,srcheight, SRCCOPY); //change SRCCOPY to NOTSRCCOPY for wacky colors !
    GetDIBits(hwindowCompatibleDC,hbwindow,0,height,src.data,(BITMAPINFO *)&bi,DIB_RGB_COLORS);  //copy from hwindowCompatibleDC to hbwindow

    DeleteObject(hbwindow);
    DeleteDC(hwindowCompatibleDC);
    ReleaseDC(hwnd, hwindowDC);

    return src;
}

The function with the screenshot is not my own work, i got it from here

Any Ideas what to do?

Thanks for help, best regards!

Answer 1

The problem is that the function hwnd2mat is returning a grayscale image of type CV_8UC1 while templ is a color image of type CV_8UC3. So the assertion is failing on the function matchTemplate due to the failing condition img.type() == templ.type(). You may load the image as grayscale to avoid the error.

templ = imread( "Template.bmp",CV_LOAD_IMAGE_GRAYSCALE);

UPDATE:

It is worth noting that the function hwnd2mat is not working in its current form and it returns an invalid image. The original code creates the output image of type CV_8UC4 which is the correct approach.

src.create(height,width,CV_8UC4);

Either you may convert src to grayscale before returning from hwnd2mat, or you may convert templ to 4 channel image. Anyway, the point is that both image must have same type for matchTemplate to work.