How to threshold an image?

[cmk]

On a tangent : How did you come up with RGB(64,128,255) as a B&W threshold color ? I would have expected to take the pixel color, get its grey value, and check that against a threshold value of 128 (or whatever). e.g.

byte grey = (byte)(
(R * 0.299) +
(G * 0.587) +
(B * 0.114)
);

...cmk Save the whales - collect the whole set

[uus99]

John >>Can you get the hardware to give you a pointer to the image data instead of ploting it to a dc? The pointer to the image buffer is (void *)ImaqBuffer, however, i do not know the structure of data in it. The only way i can access it is by plotting to DC. >>I would have expected to take the pixel color, get its grey value, and check that against a threshold value of 128 (or whatever). I know, my comparison is just too crude. Thanks. I'll try that, but i still have to check all the pixels using SetPixel & GetPixel which is too slow. I need it fast. >>Haven't messed with bitmaps in a while, but try GetBitmapBits(), which will give you an array of bytes containing the bitmap. Then run through this array changing values. Then call SetBitmapBits() with the modified array. This should be much faster. I havent try this, but i will. I'll see how fast it is. Btw, i've tried this. 1. Open a color bmp from disk and change it to B&W using LR_MONOCHROME 2. Open a color bmp, load into DC and do setpixel and getpixel. The first method is way faster.. How come? I need a way to convert that fast. But, i want to do everything in memory to overcome disk read overhead. Although, in this case, the 1 method is still faster, although i'm reading from disk.

[Ryan Binns]

Try creating a monochrome bitmap, and do a BitBlt() from the colour one to the monochrome one - let Windows do all the work for you. It doesn't give you control over the threshold, but might be adequate for what you need.

Ryan

"Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"

[uus99]

Hi there, can you give me an example of using the GetBitmapBits()? I cant find any example on it.

[J Eric Vaughan]

This is copied from a test app I wrote (real quick, so don't be too critical - it works).

    CPaintDC dc(this);
    CRect cRect; GetClientRect( cRect );
    int nWidth = cRect.Width();
    int nHeight = cRect.Height();
    CBitmap cBitmap;

    cBitmap.LoadBitmap( IDB\_BITMAP1 );

    DWORD\* pBuffer = new DWORD\[ nWidth \* nHeight \];


    DWORD dwSize = cBitmap.GetBitmapBits( nWidth \* nHeight,
                                        pBuffer );
    for ( int y = 0 ; y < nHeight ; y++ )
    {
        for ( int x = 0; x < nWidth ; x++ )
        {
            if ( \*(pBuffer+x+nWidth\*y) < RGB( 64, 128 , 256 ) )
            {
                \*(pBuffer+x+nWidth\*y) = 0; //black
            }
            else
            {
                \*(pBuffer+x+nWidth\*y) = 0x00FFFFFF; //white
            }
        }
    }

    cBitmap.SetBitmapBits( nWidth \* nHeight ,
                           pBuffer );

    CDC cMemDC;
    cMemDC.CreateCompatibleDC( &dc );

    cMemDC.SelectObject( &cBitmap );

    dc.BitBlt( 0,
               0,
               nWidth,
               nHeight,
               &cMemDC,
               0,
               0,
               SRCCOPY );


    delete \[\] pBuffer;

[uus99]

Thanks a lot for the help. Looking at the code you gave me, seems that it would be faster than the one i just figured out, :P i havent try your code yet, coz i just got wrote myself a working code. Here is the code that i wrote, //the test image "bmptest.bmp" is a color bmp 360x360 pixels ------------------------------------------ CDC *pDC; CDC memDCLoad; CBitmap m_bmpBitmap; pDC = this->GetDC(); CString m_sBitmap; m_sBitmap="bmptest.bmp"; memDCLoad.CreateCompatibleDC(pDC); HBITMAP hBitmap =(HBITMAP)::LoadImage(AfxGetInstanceHandle(),m_sBitmap,IMAGE_BITMAP,0,0, LR_LOADFROMFILE|LR_DEFAULTCOLOR); if(hBitmap) { if(m_bmpBitmap.DeleteObject()) m_bmpBitmap.Detach(); m_bmpBitmap.Attach(hBitmap); } memDCLoad.SelectObject(&m_bmpBitmap); pDC->BitBlt(0,0,360,360,&memDCLoad,0,0,SRCCOPY); BYTE test[(4*360*360)]; void* pbBits = test;// One bit per pixel m_bmpBitmap.GetBitmapBits(4*360*360, &pbBits); for(int i=0;i<360*360;i++) { //BGR=000 //black //BGR=255,255,255//white int temp=i*4; //test[temp+0]=0;//blue //test[temp+1]=0;//green //test[temp+2]=0;//red //test[temp+3]=255;//padding BYTE gray= (byte)(test[temp+2]*0.299 +test[temp+1]*0.587 +test[temp+0]*0.114); if(gray<=230) { test[temp+0]=0;//blue test[temp+1]=0;//green test[temp+2]=0;//red } else { test[temp+0]=255;//blue test[temp+1]=255;//green test[temp+2]=255;//red } } m_bmpBitmap.SetBitmapBits(4*360*360, &pbBits); pDC->BitBlt(360,0,360,360,&memDCLoad,0,0,SRCCOPY);

[J Eric Vaughan]

My inner loop could be much simpler and faster:

   for ( int x = 0 ; x < dwSize ; x++ ) 
   {
        if ( pBuffer\[x\] < RGB( 64, 128 , 256 ) )
        {
            pBuffer\[x\] = 0; //black
        }
        else
        {
            pBuffer\[x\] = 0x00FFFFFF; //white
        }
    }

I was thinking two-dimensions, but it's really just one one big buffer :doh:

[uus99]

The function for thresholding works extremely fast compared by using SetPixel and GetPixel. However, i got another problem now. My camera (frame grabber) returns the captured image in a CDC object only. And not in a Cbitmap. A CDC object does not have SetBitmapbits and GetBitmapBits members, so i cant call those. I can only access pixels in CDC using GetPixel and SetPixel. (Correct me if i'm wrong). And using Get & SetPixel is too slow, hence i needed a faster algorithm which you came up with. To use getbitmapbits, the object must be a CBItmap. However, my question is now, how to copy contents of a CDC/DC buffer or object (already contains data) to a CBitmap object for processing, and then back to CDC object? //------------custom hardware function that draws image from camera to DC-----------// imgPlot ((GUIHNDL)MemDC.GetSafeHdc(), (void *)ImaqBuffer, 0, 0, Big, Big, 410, 10, plotFlag)); //------------------------------------------- MemDC is a DC. Now contains an image copied from the camera buffer (ImaqBuffer) using a driver function. How to copy MemDC to a CBitmap and do the thresholding with the function you came up with? After finishing, how to copy the CBitmap back to MemDC for plotting on window etc. THanks for the great help. :)

[J Eric Vaughan]

try using MemDC.GetCurrentBitmap() which returns a pointer to a CBitmap. Then you can do the thresholding with it.

[uus99]

Thanks, i tried that (GetCurrentBitmap), and it is exactly what i needded. Thanks a lot! You're a genius! Guess it works! Thanks a lot. I'll find u againg, if i've got problem.. haha :P